2025
Efstratios M. Kritikos, William A. Goddard, Paul M. Bellan
Can electrostatic stresses affect charged water structures in weakly ionized plasmas? Journal Article
In: Physics of Plasmas, vol. 32, no. 6, pp. 063702, 2025, ISSN: 1070-664X, 1089-7674.
@article{kritikos_can_2025,
title = {Can electrostatic stresses affect charged water structures in weakly ionized plasmas?},
author = {Efstratios M. Kritikos and William A. Goddard and Paul M. Bellan},
url = {https://pubs.aip.org/pop/article/32/6/063702/3349362/Can-electrostatic-stresses-affect-charged-water},
doi = {10.1063/5.0270908},
issn = {1070-664X, 1089-7674},
year = {2025},
date = {2025-06-01},
urldate = {2025-08-21},
journal = {Physics of Plasmas},
volume = {32},
number = {6},
pages = {063702},
abstract = {This theoretical and numerical study investigates the impact of electrostatic stresses on the shape of charged water structures (grains) in weakly ionized plasmas. We developed an analytic model to predict the conditions under which a grain in a plasma is deformed. We find that electrostatic stresses can overcome the opposing surface tension stresses on nanometer-scale grains, causing initially spherical clusters to elongate and become ellipsoidal. The exact size limit of the grain for which electrostatic stress will dominate depends on the floating potential, surface tension, and local radius of curvature. Clusters larger than this limit are not affected by electrostatic stresses due to an insufficient number of electrons on the surface. The model is compared to Molecular Dynamics (MD) simulations performed with a calculated solvated electron potential on initially spherical grains of 2.5 nm radius charged with 0.5%–1% electrons. We find excellent agreement between MD simulations and the analytic theory. We also carried out Quantum Mechanics (QM) computations showing that the surface tension increases with decreasing size of the water molecule cluster and increases even more with the addition of solvated electrons. This increase in surface tension can hinder the elongation of the grains. Our QM computations also show that on the nanosecond time scale, the binding force of electrons to water molecule clusters is stronger than the electrostatic repulsion between adjacent electrons and thus the cluster behaves as an insulator. However, consideration of the very small conductivity of ice shows that on time scales of a fraction of a second, ice clusters behave as conductors, so their surface may be considered to be at an equipotential.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
This theoretical and numerical study investigates the impact of electrostatic stresses on the shape of charged water structures (grains) in weakly ionized plasmas. We developed an analytic model to predict the conditions under which a grain in a plasma is deformed. We find that electrostatic stresses can overcome the opposing surface tension stresses on nanometer-scale grains, causing initially spherical clusters to elongate and become ellipsoidal. The exact size limit of the grain for which electrostatic stress will dominate depends on the floating potential, surface tension, and local radius of curvature. Clusters larger than this limit are not affected by electrostatic stresses due to an insufficient number of electrons on the surface. The model is compared to Molecular Dynamics (MD) simulations performed with a calculated solvated electron potential on initially spherical grains of 2.5 nm radius charged with 0.5%–1% electrons. We find excellent agreement between MD simulations and the analytic theory. We also carried out Quantum Mechanics (QM) computations showing that the surface tension increases with decreasing size of the water molecule cluster and increases even more with the addition of solvated electrons. This increase in surface tension can hinder the elongation of the grains. Our QM computations also show that on the nanosecond time scale, the binding force of electrons to water molecule clusters is stronger than the electrostatic repulsion between adjacent electrons and thus the cluster behaves as an insulator. However, consideration of the very small conductivity of ice shows that on time scales of a fraction of a second, ice clusters behave as conductors, so their surface may be considered to be at an equipotential.
Efstratios M. Kritikos, Stewart Cant, Andrea Giusti
A computational approach for the study of electromagnetic interactions in reacting flows Miscellaneous
2025, (arXiv:2505.06433 [physics]).
@misc{kritikos_computational_2025,
title = {A computational approach for the study of electromagnetic interactions in reacting flows},
author = {Efstratios M. Kritikos and Stewart Cant and Andrea Giusti},
url = {https://www.semanticscholar.org/paper/841cadf2b83262d99ae86435cc2dea45e81ba5ab},
doi = {10.48550/arXiv.2505.06433},
year = {2025},
date = {2025-05-01},
urldate = {2025-08-21},
publisher = {arXiv},
abstract = {A computational fluid dynamics methodology for the simulation of electromagnetic interactions in compressible reacting flows has been formulated. The developed code, named EMI, is based on the SENGA Direct Numerical Simulation (DNS) software. Static electric and magnetic fields are solved using Gauss's laws of Maxwell's equations. Electromagnetic wave propagation is solved by discretizing Ampere's and Faraday's equations using the explicit Finite-Difference Time-Domain (FDTD) method. The equations for the electromagnetic fields are fully coupled with the Navier-Stokes equations, such that interactions between the electromagnetic fields and the fluid are included in the formulation. The interaction terms include the Lorentz, polarization, and magnetization forces. These forces determine volume forces that affect the transport of momentum, the diffusion velocity, and the energy conservation equations. In addition, the medium's properties affect the propagation of the electromagnetic fields via electrical permittivity and conductivity, charge density, and magnetic permeability. The solution of electromagnetic fields is validated against analytical and numerical solutions. The implementation of the coupling between electromagnetic fields and conservation equations for species, energy, and momentum is validated with laminar reacting flow numerical solutions from the literature. The capabilities of the formulation are investigated for a range of laminar methane-air computations under electrostatic, magnetostatic, and high-frequency electromagnetic waves. The validity of the electrostatic formulation in the presence of currents related to the movement of charged species is also assessed. Results demonstrate that EMI-SENGA can capture the fundamental effects of electromagnetic fields on reacting flows and the dynamics of charged species and their effect on flame shape and reactivity.},
note = {arXiv:2505.06433 [physics]},
keywords = {},
pubstate = {published},
tppubtype = {misc}
}
A computational fluid dynamics methodology for the simulation of electromagnetic interactions in compressible reacting flows has been formulated. The developed code, named EMI, is based on the SENGA Direct Numerical Simulation (DNS) software. Static electric and magnetic fields are solved using Gauss’s laws of Maxwell’s equations. Electromagnetic wave propagation is solved by discretizing Ampere’s and Faraday’s equations using the explicit Finite-Difference Time-Domain (FDTD) method. The equations for the electromagnetic fields are fully coupled with the Navier-Stokes equations, such that interactions between the electromagnetic fields and the fluid are included in the formulation. The interaction terms include the Lorentz, polarization, and magnetization forces. These forces determine volume forces that affect the transport of momentum, the diffusion velocity, and the energy conservation equations. In addition, the medium’s properties affect the propagation of the electromagnetic fields via electrical permittivity and conductivity, charge density, and magnetic permeability. The solution of electromagnetic fields is validated against analytical and numerical solutions. The implementation of the coupling between electromagnetic fields and conservation equations for species, energy, and momentum is validated with laminar reacting flow numerical solutions from the literature. The capabilities of the formulation are investigated for a range of laminar methane-air computations under electrostatic, magnetostatic, and high-frequency electromagnetic waves. The validity of the electrostatic formulation in the presence of currents related to the movement of charged species is also assessed. Results demonstrate that EMI-SENGA can capture the fundamental effects of electromagnetic fields on reacting flows and the dynamics of charged species and their effect on flame shape and reactivity.
Andre Nicolov, Murthy Gudipati, Seth Pree, Efstratios Kritikos, Paul Bellan
How Fractal Grains of Water-Ice Nucleate, Interact, and Evolve in a Cryogenic Laboratory Plasma Proceedings Article
In: American Astronomical Society Meeting Abstracts, pp. 315–01, 2025.
@inproceedings{nicolov2025fractal,
title = {How Fractal Grains of Water-Ice Nucleate, Interact, and Evolve in a Cryogenic Laboratory Plasma},
author = {Andre Nicolov and Murthy Gudipati and Seth Pree and Efstratios Kritikos and Paul Bellan},
year = {2025},
date = {2025-01-01},
booktitle = {American Astronomical Society Meeting Abstracts},
volume = {246},
pages = {315–01},
keywords = {},
pubstate = {published},
tppubtype = {inproceedings}
}
2024
Zhaoran Zhu, James P. Ewen, Efstratios M. Kritikos, Andrea Giusti, Daniele Dini
Effect of electric fields on the decomposition of phosphate esters Journal Article
In: Journal of Physical Chemistry C, vol. 128, no. 38, pp. 15959–15973, 2024, ISSN: 1932-7447, (Publisher: American Chemical Society).
@article{zhu_effect_2024,
title = {Effect of electric fields on the decomposition of phosphate esters},
author = {Zhaoran Zhu and James P. Ewen and Efstratios M. Kritikos and Andrea Giusti and Daniele Dini},
url = {https://doi.org/10.1021/acs.jpcc.4c04412},
doi = {10.1021/acs.jpcc.4c04412},
issn = {1932-7447},
year = {2024},
date = {2024-09-01},
urldate = {2025-08-21},
journal = {Journal of Physical Chemistry C},
volume = {128},
number = {38},
pages = {15959–15973},
abstract = {Phosphate esters decompose on metal surfaces and form protective polyphosphate films. For many applications, such as in lubricants for electric vehicles and wind turbines, an understanding of the effect of electric fields on molecular decomposition is urgently required. Experimental investigations have yielded contradictory results, with some suggesting that electric fields improve tribological performance, while others have reported the opposite effect. Here, we use nonequilibrium molecular dynamics (NEMD) simulations to study the decomposition of tri-n-butyl phosphate (TNBP) molecules nanoconfined between ferrous surfaces (iron and iron oxide) under electrostatic fields. The reactive force field (ReaxFF) method is used to model the effects of chemical bonding and molecular dissociation. We show that the charge transfer with the polarization current equalization (QTPIE) method gives more realistic behavior compared to the standard charge equilibration (QEq) method under applied electrostatic fields. The rate of TNBP decomposition via carbon–oxygen bond dissociation is faster in the nanoconfined systems than that in the bulk due to the catalytic action of the surfaces. In all cases, the application of an electric field accelerates TNBP decomposition. When electric fields are applied to the confined systems, the phosphate anions are pulled toward the surface with high electric potential, while the alkyl cations are pulled to the surface with lower potential, leading to asymmetric film growth. Analysis of the temperature- and electric field strength-dependent dissociation rate constants using the Arrhenius equation suggests that, on reactive iron surfaces, the increased reactivity under an applied electric field is driven mostly by an increase in the pre-exponential factor, which is linked to the number of molecule–surface collisions. Conversely, the accelerated decomposition of TNBP on iron oxide surfaces can be attributed to a reduction in the activation energy with increasing electric field strength. Single-molecule nudged-elastic band (NEB) calculations also show a linear reduction in the energy barrier for carbon–oxygen bond breaking with electric field strength, due to stabilization of the charged transition state. The simulation results are consistent with experimental observations of enhanced and asymmetric tribofilm growth under electrostatic fields.},
note = {Publisher: American Chemical Society},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Phosphate esters decompose on metal surfaces and form protective polyphosphate films. For many applications, such as in lubricants for electric vehicles and wind turbines, an understanding of the effect of electric fields on molecular decomposition is urgently required. Experimental investigations have yielded contradictory results, with some suggesting that electric fields improve tribological performance, while others have reported the opposite effect. Here, we use nonequilibrium molecular dynamics (NEMD) simulations to study the decomposition of tri-n-butyl phosphate (TNBP) molecules nanoconfined between ferrous surfaces (iron and iron oxide) under electrostatic fields. The reactive force field (ReaxFF) method is used to model the effects of chemical bonding and molecular dissociation. We show that the charge transfer with the polarization current equalization (QTPIE) method gives more realistic behavior compared to the standard charge equilibration (QEq) method under applied electrostatic fields. The rate of TNBP decomposition via carbon–oxygen bond dissociation is faster in the nanoconfined systems than that in the bulk due to the catalytic action of the surfaces. In all cases, the application of an electric field accelerates TNBP decomposition. When electric fields are applied to the confined systems, the phosphate anions are pulled toward the surface with high electric potential, while the alkyl cations are pulled to the surface with lower potential, leading to asymmetric film growth. Analysis of the temperature- and electric field strength-dependent dissociation rate constants using the Arrhenius equation suggests that, on reactive iron surfaces, the increased reactivity under an applied electric field is driven mostly by an increase in the pre-exponential factor, which is linked to the number of molecule–surface collisions. Conversely, the accelerated decomposition of TNBP on iron oxide surfaces can be attributed to a reduction in the activation energy with increasing electric field strength. Single-molecule nudged-elastic band (NEB) calculations also show a linear reduction in the energy barrier for carbon–oxygen bond breaking with electric field strength, due to stabilization of the charged transition state. The simulation results are consistent with experimental observations of enhanced and asymmetric tribofilm growth under electrostatic fields.
Efstratios M. Kritikos, Andrea Giusti
Investigation of Iron Nanoparticle Oxidation under External Electrostatic Fields Using Reactive Molecular Dynamics Journal Article
In: The Journal of Physical Chemistry C, vol. 128, no. 30, pp. 12364–12385, 2024, ISSN: 1932-7447, 1932-7455, (Publisher: American Chemical Society).
@article{kritikos_investigation_2024,
title = {Investigation of Iron Nanoparticle Oxidation under External Electrostatic Fields Using Reactive Molecular Dynamics},
author = {Efstratios M. Kritikos and Andrea Giusti},
url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.4c00722},
doi = {10.1021/acs.jpcc.4c00722},
issn = {1932-7447, 1932-7455},
year = {2024},
date = {2024-08-01},
urldate = {2025-08-21},
journal = {The Journal of Physical Chemistry C},
volume = {128},
number = {30},
pages = {12364–12385},
abstract = {A reactive molecular dynamics (MD) study of iron nanoparticle oxidation under externally applied electrostatic fields is performed, with a focus on the role of charge transfers and electrically induced polarization. Two charge equilibration methods that either enable or shield long-range charge transfers and two ReaxFF parameter sets that predict different bonded and nonbonded interactions are used in the evaluation. The field-induced polarization modeled in MD simulations is in good agreement with analytical solutions and density functional theory (DFT) computations. Results show that oriented external electric fields affect the transition states of reactions involved in oxygen adsorption on the nanoparticle surface and oxygen diffusion inside the iron lattice. The oxygen diffusion inside the nanoparticle can be either aided or hindered by the external electric field depending on the direction of motion of oxygen compared to the direction of the external electric field. Simulations also demonstrate that long-range charge transfers increase the reactivity of the system compared to shielded interactions. Regarding ambient conditions, a density increase can accelerate the kinetics of the system, while temperature variations have a smaller effect on the oxidation rate for the systems investigated here. The external electrostatic field affects the kinetic energy, collision frequency, and reactivity of the system. When charge transfers are limited to close interactions, the system’s kinetics is accelerated only under strong external electric fields. On the contrary, if long-range charge transfers are enabled, an increase in the oxidation rate is observed for weak electric fields, whereas for stronger electric fields the oxidation rate decreases due to a more coherent movement of the charged particles under the Lorentz forces. In addition, external electric fields affect the chemical composition of iron and iron oxide nanoparticles. The diffusion of the negatively charged absorbed oxygen atoms toward one side of the nanoparticle causes an anisotropic thickness of the oxidation layer. The influence of the external electric field on the system’s kinetics depends on the choice of the ReaxFF parameter set, highlighting the importance of accurate training of the force field with a focus on charge transfers and their distribution under external fields. These findings provide insight into the fundamental effects of charge transfers and field-induced polarization on the oxidation process of a nanoparticle in the presence of an external electrostatic field and offer a comprehensive evaluation of the capability of the MD-ReaxFF framework to predict the underlying physical mechanisms at the atomistic scale.},
note = {Publisher: American Chemical Society},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A reactive molecular dynamics (MD) study of iron nanoparticle oxidation under externally applied electrostatic fields is performed, with a focus on the role of charge transfers and electrically induced polarization. Two charge equilibration methods that either enable or shield long-range charge transfers and two ReaxFF parameter sets that predict different bonded and nonbonded interactions are used in the evaluation. The field-induced polarization modeled in MD simulations is in good agreement with analytical solutions and density functional theory (DFT) computations. Results show that oriented external electric fields affect the transition states of reactions involved in oxygen adsorption on the nanoparticle surface and oxygen diffusion inside the iron lattice. The oxygen diffusion inside the nanoparticle can be either aided or hindered by the external electric field depending on the direction of motion of oxygen compared to the direction of the external electric field. Simulations also demonstrate that long-range charge transfers increase the reactivity of the system compared to shielded interactions. Regarding ambient conditions, a density increase can accelerate the kinetics of the system, while temperature variations have a smaller effect on the oxidation rate for the systems investigated here. The external electrostatic field affects the kinetic energy, collision frequency, and reactivity of the system. When charge transfers are limited to close interactions, the system’s kinetics is accelerated only under strong external electric fields. On the contrary, if long-range charge transfers are enabled, an increase in the oxidation rate is observed for weak electric fields, whereas for stronger electric fields the oxidation rate decreases due to a more coherent movement of the charged particles under the Lorentz forces. In addition, external electric fields affect the chemical composition of iron and iron oxide nanoparticles. The diffusion of the negatively charged absorbed oxygen atoms toward one side of the nanoparticle causes an anisotropic thickness of the oxidation layer. The influence of the external electric field on the system’s kinetics depends on the choice of the ReaxFF parameter set, highlighting the importance of accurate training of the force field with a focus on charge transfers and their distribution under external fields. These findings provide insight into the fundamental effects of charge transfers and field-induced polarization on the oxidation process of a nanoparticle in the presence of an external electrostatic field and offer a comprehensive evaluation of the capability of the MD-ReaxFF framework to predict the underlying physical mechanisms at the atomistic scale.
Emin Saridede, Efstratios M. Kritikos, Andrea Giusti
Investigation of the effect of electrostatic fields and iron nanoparticles on hydrogen-oxygen combustion Journal Article
In: Proceedings of the Combustion Institute, vol. 40, no. 1-4, pp. 105769, 2024, ISSN: 15407489.
@article{saridede_investigation_2024,
title = {Investigation of the effect of electrostatic fields and iron nanoparticles on hydrogen-oxygen combustion},
author = {Emin Saridede and Efstratios M. Kritikos and Andrea Giusti},
url = {https://linkinghub.elsevier.com/retrieve/pii/S1540748924005777},
doi = {10.1016/j.proci.2024.105769},
issn = {15407489},
year = {2024},
date = {2024-01-01},
urldate = {2025-08-21},
journal = {Proceedings of the Combustion Institute},
volume = {40},
number = {1-4},
pages = {105769},
abstract = {The combustion of hydrogen–oxygen systems with iron nanoparticles and external electrostatic fields is investigated using reactive molecular dynamics simulations with the ReaxFF force field. The aim is to provide insight into the effect of iron nano-powder and electrostatic fields on the reaction dynamics of hydrogen for different temperatures of the system and different sizes of the nanoparticle. Results show that the presence of iron accelerates water formation and changes the related reaction pathway through an adsorption mechanism on the nanoparticle surface, an effect that is more evident with decreasing diameter of the nanoparticle. For small nanoparticles and relatively low system temperatures, strong electrostatic fields could affect this mechanism, resulting in increased hydrogen adsorption and accumulation of oxygen compounds on one side of the nanoparticle. An increase in size of the nanoparticle, for the same composition of the gas phase, leads to a decrease in the effects of the external electrostatic field. In addition, it was found that an increase in temperature of the system reduces the effect of external electrostatic fields on the reactivity of the hydrogen–oxygen system with iron nanoparticles. No significant effects on the reaction dynamics were observed for low-energy electric fields at all conditions investigated in this work. The present results provide new insights for the development of clean combustion technologies with enhanced control over the reactive process enabled by external electrostatic fields and iron nanoparticles.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The combustion of hydrogen–oxygen systems with iron nanoparticles and external electrostatic fields is investigated using reactive molecular dynamics simulations with the ReaxFF force field. The aim is to provide insight into the effect of iron nano-powder and electrostatic fields on the reaction dynamics of hydrogen for different temperatures of the system and different sizes of the nanoparticle. Results show that the presence of iron accelerates water formation and changes the related reaction pathway through an adsorption mechanism on the nanoparticle surface, an effect that is more evident with decreasing diameter of the nanoparticle. For small nanoparticles and relatively low system temperatures, strong electrostatic fields could affect this mechanism, resulting in increased hydrogen adsorption and accumulation of oxygen compounds on one side of the nanoparticle. An increase in size of the nanoparticle, for the same composition of the gas phase, leads to a decrease in the effects of the external electrostatic field. In addition, it was found that an increase in temperature of the system reduces the effect of external electrostatic fields on the reactivity of the hydrogen–oxygen system with iron nanoparticles. No significant effects on the reaction dynamics were observed for low-energy electric fields at all conditions investigated in this work. The present results provide new insights for the development of clean combustion technologies with enhanced control over the reactive process enabled by external electrostatic fields and iron nanoparticles.
Efstratios M Kritikos, William A Goddard III, Adri CT van Duin, Paul M Bellan
Atomistic insight into the plasma-catalyzed nucleation of ice grains Conference
American Physical Society, 2024.
@conference{kritikos2024atomistic,
title = {Atomistic insight into the plasma-catalyzed nucleation of ice grains},
author = {Efstratios M Kritikos and William A Goddard III and Adri CT van Duin and Paul M Bellan},
year = {2024},
date = {2024-01-01},
urldate = {2024-01-01},
journal = {Bulletin of the American Physical Society},
publisher = {American Physical Society},
keywords = {},
pubstate = {published},
tppubtype = {conference}
}
2023
Leon C. Thijs, Efstratios M. Kritikos, Andrea Giusti, Marie-Aline Ende, Adri C. T. Duin, XiaoCheng Mi
Effect of Fe–O ReaxFF on liquid iron oxide properties derived from reactive molecular dynamics Journal Article
In: Journal of Physical Chemistry A, vol. 127, no. 48, pp. 10339–10355, 2023, ISSN: 1089-5639, (Publisher: American Chemical Society).
@article{thijs_effect_2023,
title = {Effect of Fe–O ReaxFF on liquid iron oxide properties derived from reactive molecular dynamics},
author = {Leon C. Thijs and Efstratios M. Kritikos and Andrea Giusti and Marie-Aline Ende and Adri C. T. Duin and XiaoCheng Mi},
url = {https://doi.org/10.1021/acs.jpca.3c06646},
doi = {10.1021/acs.jpca.3c06646},
issn = {1089-5639},
year = {2023},
date = {2023-12-01},
urldate = {2025-08-21},
journal = {Journal of Physical Chemistry A},
volume = {127},
number = {48},
pages = {10339–10355},
abstract = {As iron powder nowadays attracts research attention as a carbon-free, circular energy carrier, molecular dynamics (MD) simulations can be used to better understand the mechanisms of liquid iron oxidation at elevated temperatures. However, prudence must be practiced in the selection of a reactive force field. This work investigates the influence of currently available reactive force fields (ReaxFFs) on a number of properties of the liquid iron–oxygen (Fe–O) system derived (or resulting) from MD simulations. Liquid Fe–O systems are considered over a range of oxidation degrees ZO, which represents the molar ratio of O/(O + Fe), with 0 < ZO < 0.6 and at a constant temperature of 2000 K, which is representative of the combustion temperature of micrometric iron particles burning in air. The investigated properties include the minimum energy path, system structure, (im)miscibility, transport properties, and the mass and thermal accommodation coefficients. The properties are compared to experimental values and thermodynamic calculation results if available. Results show that there are significant differences in the properties obtained with MD using the various ReaxFF parameter sets. Based on the available experimental data and equilibrium calculation results, an improved ReaxFF is required to better capture the properties of a liquid Fe–O system.},
note = {Publisher: American Chemical Society},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
As iron powder nowadays attracts research attention as a carbon-free, circular energy carrier, molecular dynamics (MD) simulations can be used to better understand the mechanisms of liquid iron oxidation at elevated temperatures. However, prudence must be practiced in the selection of a reactive force field. This work investigates the influence of currently available reactive force fields (ReaxFFs) on a number of properties of the liquid iron–oxygen (Fe–O) system derived (or resulting) from MD simulations. Liquid Fe–O systems are considered over a range of oxidation degrees ZO, which represents the molar ratio of O/(O + Fe), with 0 < ZO < 0.6 and at a constant temperature of 2000 K, which is representative of the combustion temperature of micrometric iron particles burning in air. The investigated properties include the minimum energy path, system structure, (im)miscibility, transport properties, and the mass and thermal accommodation coefficients. The properties are compared to experimental values and thermodynamic calculation results if available. Results show that there are significant differences in the properties obtained with MD using the various ReaxFF parameter sets. Based on the available experimental data and equilibrium calculation results, an improved ReaxFF is required to better capture the properties of a liquid Fe–O system.
Majd Sayed Ahmad, Efstratios M. Kritikos, Andrea Giusti
A Reactive Molecular Dynamics Investigation of Nanoparticle Interactions in Hydrocarbon Combustion Journal Article
In: Combustion Science and Technology, vol. 195, no. 14, pp. 3281–3295, 2023, ISSN: 0010-2202, 1563-521X, (Publisher: Taylor & Francis _eprint: https://doi.org/10.1080/00102202.2023.2240451).
@article{sayed_ahmad_reactive_2023,
title = {A Reactive Molecular Dynamics Investigation of Nanoparticle Interactions in Hydrocarbon Combustion},
author = {Majd Sayed Ahmad and Efstratios M. Kritikos and Andrea Giusti},
url = {https://www.tandfonline.com/doi/full/10.1080/00102202.2023.2240451},
doi = {10.1080/00102202.2023.2240451},
issn = {0010-2202, 1563-521X},
year = {2023},
date = {2023-10-01},
urldate = {2025-08-21},
journal = {Combustion Science and Technology},
volume = {195},
number = {14},
pages = {3281–3295},
abstract = {The use of energetic nanoparticles to tailor the properties of a base liquid fuel has attracted attention due to the possibility of decreasing fuel consumption and increasing control over the combustion process. In this study, the role of nanomaterials in the consumption of hydrocarbon fuel vapor is investigated using reactive molecular dynamics. Simulations are performed with aluminum and iron nanoparticles inside an n-heptane and oxygen gas mixture. The role of atomic charges on the dynamics of nanoparticle-hydrocarbon interactions is also investigated using different charge equilibration methods. Results show that both nanomaterials act as catalysts and enhance fuel decomposition. The decomposition of fuel molecules is initiated by dehydrogenation at the particle’s surface. This reaction path occurs significantly faster than the oxidation and pyrolysis paths observed for n-heptane in absence of nanoparticles. The oxidation in the presence of aluminum is characterized by more rapid particle heating and fragmentation compared to iron. Metal fragments further enhance the reactivity of the system due to a higher surface area available for reactions. The atomic charge distribution was found to affect the kinetics and reactivity of the system, showing that the non-bonded interactions influence the oxidation process. This study confirms that the use of nanomaterials is beneficial to accelerate the decomposition of fuel and that the combustion behavior of the selected hydrocarbon is strongly dependent on the type of nanomaterial used in combination with the base fuel.},
note = {Publisher: Taylor & Francis
_eprint: https://doi.org/10.1080/00102202.2023.2240451},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The use of energetic nanoparticles to tailor the properties of a base liquid fuel has attracted attention due to the possibility of decreasing fuel consumption and increasing control over the combustion process. In this study, the role of nanomaterials in the consumption of hydrocarbon fuel vapor is investigated using reactive molecular dynamics. Simulations are performed with aluminum and iron nanoparticles inside an n-heptane and oxygen gas mixture. The role of atomic charges on the dynamics of nanoparticle-hydrocarbon interactions is also investigated using different charge equilibration methods. Results show that both nanomaterials act as catalysts and enhance fuel decomposition. The decomposition of fuel molecules is initiated by dehydrogenation at the particle’s surface. This reaction path occurs significantly faster than the oxidation and pyrolysis paths observed for n-heptane in absence of nanoparticles. The oxidation in the presence of aluminum is characterized by more rapid particle heating and fragmentation compared to iron. Metal fragments further enhance the reactivity of the system due to a higher surface area available for reactions. The atomic charge distribution was found to affect the kinetics and reactivity of the system, showing that the non-bonded interactions influence the oxidation process. This study confirms that the use of nanomaterials is beneficial to accelerate the decomposition of fuel and that the combustion behavior of the selected hydrocarbon is strongly dependent on the type of nanomaterial used in combination with the base fuel.
Leon C. Thijs, Efstratios M. Kritikos, Andrea Giusti, Giel Ramaekers, Jeroen A. Van Oijen, Philip De Goey, XiaoCheng Mi
On the surface chemisorption of oxidizing fine iron particles: Insights gained from molecular dynamics simulations Journal Article
In: Combustion and Flame, vol. 254, pp. 112871, 2023, ISSN: 00102180.
@article{thijs_surface_2023,
title = {On the surface chemisorption of oxidizing fine iron particles: Insights gained from molecular dynamics simulations},
author = {Leon C. Thijs and Efstratios M. Kritikos and Andrea Giusti and Giel Ramaekers and Jeroen A. Van Oijen and Philip De Goey and XiaoCheng Mi},
url = {https://linkinghub.elsevier.com/retrieve/pii/S0010218023002523},
doi = {10.1016/j.combustflame.2023.112871},
issn = {00102180},
year = {2023},
date = {2023-08-01},
urldate = {2025-08-21},
journal = {Combustion and Flame},
volume = {254},
pages = {112871},
abstract = {Molecular dynamics (MD) simulations are performed to investigate the thermal and mass accommodation coefficients (TAC and MAC, respectively) for the combination of iron(-oxide) and air. The obtained values of TAC and MAC are then used in a point-particle Knudsen model to investigate the effect of chemisorption and the Knudsen transition regime on the combustion behavior of (fine) iron particles. The thermal accommodation for the interactions of Fe with N2 and FexOy with O2 is investigated for different surface temperatures, while the mass accommodation coefficient for iron(-oxide) with oxygen is investigated for different initial oxidation stages ZO, which represents the molar ratio of O/(O+Fe), and different surface temperatures. The MAC decreases fast from unity to 0.03 as ZO increases from 0 to 0.5 and then diminishes as ZO further increases to 0.57. By incorporating the MD-informed accommodation coefficients into the single iron particle combustion model, the oxidation beyond ZO=0.5 (from stoichiometric FeO to Fe3O4) is modeled. A new temperature evolution for single iron particles is observed compared to results obtained with previously developed continuum models. Specifically, results of the present simulations show that the oxidation process continues after the particle reaching the peak temperature, while previous models predicting that the maximum temperature was attained when the particle is oxidized to ZO=0.5. Since the rate of oxidation slows down as the MAC decreases with an increasing oxidation stage, the rate of heat loss exceeds the rate of heat release upon reaching the maximum temperature, while the particle is not yet oxidized to ZO=0.5. Finally, the effect of transition-regime heat and mass transfer on the combustion behavior of fine iron particles is investigated and discussed.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Molecular dynamics (MD) simulations are performed to investigate the thermal and mass accommodation coefficients (TAC and MAC, respectively) for the combination of iron(-oxide) and air. The obtained values of TAC and MAC are then used in a point-particle Knudsen model to investigate the effect of chemisorption and the Knudsen transition regime on the combustion behavior of (fine) iron particles. The thermal accommodation for the interactions of Fe with N2 and FexOy with O2 is investigated for different surface temperatures, while the mass accommodation coefficient for iron(-oxide) with oxygen is investigated for different initial oxidation stages ZO, which represents the molar ratio of O/(O+Fe), and different surface temperatures. The MAC decreases fast from unity to 0.03 as ZO increases from 0 to 0.5 and then diminishes as ZO further increases to 0.57. By incorporating the MD-informed accommodation coefficients into the single iron particle combustion model, the oxidation beyond ZO=0.5 (from stoichiometric FeO to Fe3O4) is modeled. A new temperature evolution for single iron particles is observed compared to results obtained with previously developed continuum models. Specifically, results of the present simulations show that the oxidation process continues after the particle reaching the peak temperature, while previous models predicting that the maximum temperature was attained when the particle is oxidized to ZO=0.5. Since the rate of oxidation slows down as the MAC decreases with an increasing oxidation stage, the rate of heat loss exceeds the rate of heat release upon reaching the maximum temperature, while the particle is not yet oxidized to ZO=0.5. Finally, the effect of transition-regime heat and mass transfer on the combustion behavior of fine iron particles is investigated and discussed.
Efstratios Kritikos
Electromagnetic fields and nanoenergetic particles in reacting flows PhD Thesis
Imperial College London, 2023.
@phdthesis{kritikos_electromagnetic_2023,
title = {Electromagnetic fields and nanoenergetic particles in reacting flows},
author = {Efstratios Kritikos},
url = {https://doi.org/10.25560/108708},
year = {2023},
date = {2023-08-01},
school = {Imperial College London},
abstract = {To increase the environmental sustainability of the transportation and energy generation sectors, new technologies must be developed. This research provides a theoretical and numerical investigation of multiphysics phenomena in reacting flows as a possible path for innovation towards more effective control of reacting flows. The main focus is on the interaction of reacting flows with electromagnetic fields and nanoenergetic particles across a range of scales. First, quantum mechanics computations of hydrocarbon oxidation reactions are performed. Results show that electric fields affect the electronic structure of the transition states, leading to catalysis or inhibition. The electron distribution is also affected by strong magnetic fields, as a consequence of the induced currents. Second, reactive molecular dynamics simulations of hydrocarbon oxidation are performed. Findings indicate that electrostatic fields affect the collision frequency and translational, rotational, and vibrational degrees of freedom of reactants and products. The Lorentz force could introduce stabilization and alignment effects. Results also highlight the impact of the used charge equilibration method on the prediction of the system's electrodynamics. Conversely, the magnetic response of hydrocarbon kinetics was negligible. Subsequently, the behaviour of metal nanoparticles, either as fuel substitutes or fuel additives, under external electric fields is analyzed. Nanoparticles act as catalysts in the dissociation process of heavy hydrocarbon and oxygen molecules. External electric fields can further increase the reactivity of the system. Moreover, the negatively charged absorbed species diffuse along the surface under the Lorentz forces, causing anisotropic chemical compositions and shell thicknesses in the nanoparticle. Lastly, to study the electromagnetic field effects in reacting flows at macroscales, a computational fluid dynamics code is developed. The numerical framework describes electromagnetic wave interactions with neutrals, ions, and electrons, considering classical electrodynamics and quantum mechanical effects. Results show that electrostatic and inhomogeneous magnetostatic fields can significantly influence the reacting flow. Additionally, for sufficiently high ion and electron concentration and mobility, time-dependent electromagnetic waves are induced. This research offers a comprehensive evaluation of multiphysics phenomena at the quantum, atomistic, and macroscopic levels, with the potential of inspiring novel technologies for the propulsion and power generation sectors.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
To increase the environmental sustainability of the transportation and energy generation sectors, new technologies must be developed. This research provides a theoretical and numerical investigation of multiphysics phenomena in reacting flows as a possible path for innovation towards more effective control of reacting flows. The main focus is on the interaction of reacting flows with electromagnetic fields and nanoenergetic particles across a range of scales. First, quantum mechanics computations of hydrocarbon oxidation reactions are performed. Results show that electric fields affect the electronic structure of the transition states, leading to catalysis or inhibition. The electron distribution is also affected by strong magnetic fields, as a consequence of the induced currents. Second, reactive molecular dynamics simulations of hydrocarbon oxidation are performed. Findings indicate that electrostatic fields affect the collision frequency and translational, rotational, and vibrational degrees of freedom of reactants and products. The Lorentz force could introduce stabilization and alignment effects. Results also highlight the impact of the used charge equilibration method on the prediction of the system’s electrodynamics. Conversely, the magnetic response of hydrocarbon kinetics was negligible. Subsequently, the behaviour of metal nanoparticles, either as fuel substitutes or fuel additives, under external electric fields is analyzed. Nanoparticles act as catalysts in the dissociation process of heavy hydrocarbon and oxygen molecules. External electric fields can further increase the reactivity of the system. Moreover, the negatively charged absorbed species diffuse along the surface under the Lorentz forces, causing anisotropic chemical compositions and shell thicknesses in the nanoparticle. Lastly, to study the electromagnetic field effects in reacting flows at macroscales, a computational fluid dynamics code is developed. The numerical framework describes electromagnetic wave interactions with neutrals, ions, and electrons, considering classical electrodynamics and quantum mechanical effects. Results show that electrostatic and inhomogeneous magnetostatic fields can significantly influence the reacting flow. Additionally, for sufficiently high ion and electron concentration and mobility, time-dependent electromagnetic waves are induced. This research offers a comprehensive evaluation of multiphysics phenomena at the quantum, atomistic, and macroscopic levels, with the potential of inspiring novel technologies for the propulsion and power generation sectors.
Xiaocheng Mi, Leon C Thijs, Efstratios M Kritikos, Giel Ramaekers, Shyam Sundar Hemamalini, Swagnik Guhathakurta, Toos Gool, Aravind Ravi, Andrea Giusti, Cuenot Benedicte, Jeroen van Oijen, Philip Goey
Unique Problems in Iron-powder Combustion Tackled by Multiphysics Modeling Conference
2023.
@conference{miunique,
title = {Unique Problems in Iron-powder Combustion Tackled by Multiphysics Modeling},
author = {Xiaocheng Mi and Leon C Thijs and Efstratios M Kritikos and Giel Ramaekers and Shyam Sundar Hemamalini and Swagnik Guhathakurta and Toos Gool and Aravind Ravi and Andrea Giusti and Cuenot Benedicte and Jeroen van Oijen and Philip Goey},
year = {2023},
date = {2023-06-01},
urldate = {2023-04-19},
keywords = {},
pubstate = {published},
tppubtype = {conference}
}
Leon C Thijs, Efstratios M Kritikos, Andrea Giusti, Giel Ramaekers, Jeroen van Oijen, Philip Goey, XiaoCheng Mi
Theoretical Modeling of Iron-droplet Combustion Informed by Molecular Dynamics Simulations Conference
2023.
@conference{thijstheoretical,
title = {Theoretical Modeling of Iron-droplet Combustion Informed by Molecular Dynamics Simulations},
author = {Leon C Thijs and Efstratios M Kritikos and Andrea Giusti and Giel Ramaekers and Jeroen van Oijen and Philip Goey and XiaoCheng Mi},
year = {2023},
date = {2023-04-19},
urldate = {2023-04-19},
keywords = {},
pubstate = {published},
tppubtype = {conference}
}
Efstratios M. Kritikos, Aditya Lele, Adri C. T. Van Duin, Andrea Giusti
Atomistic insight into the effects of electrostatic fields on hydrocarbon reaction kinetics Journal Article
In: The Journal of Chemical Physics, vol. 158, no. 5, pp. 054109, 2023, ISSN: 0021-9606, 1089-7690.
@article{kritikos_atomistic_2023,
title = {Atomistic insight into the effects of electrostatic fields on hydrocarbon reaction kinetics},
author = {Efstratios M. Kritikos and Aditya Lele and Adri C. T. Van Duin and Andrea Giusti},
url = {https://pubs.aip.org/jcp/article/158/5/054109/2871534/Atomistic-insight-into-the-effects-of},
doi = {10.1063/5.0134785},
issn = {0021-9606, 1089-7690},
year = {2023},
date = {2023-02-01},
urldate = {2025-08-21},
journal = {The Journal of Chemical Physics},
volume = {158},
number = {5},
pages = {054109},
abstract = {Reactive Molecular Dynamics (MD) and Density Functional Theory (DFT) computations are performed to provide insight into the effects of external electrostatic fields on hydrocarbon reaction kinetics. By comparing the results from MD and DFT, the suitability of the MD method in modeling electrodynamics is first assessed. Results show that the electric field-induced polarization predicted by the MD charge equilibration method is in good agreement with various DFT charge partitioning schemes. Then, the effects of oriented external electric fields on the transition pathways of non-redox reactions are investigated. Results on the minimum energy path suggest that electric fields can cause catalysis or inhibition of oxidation reactions, whereas pyrolysis reactions are not affected due to the weaker electronegativity of the hydrogen and carbon atoms. MD simulations of isolated reactions show that the reaction kinetics is also affected by applied external Lorentz forces and interatomic Coulomb forces since they can increase or decrease the energy of collision depending on the molecular conformation. In addition, electric fields can affect the kinetics of polar species and force them to align in the direction of field lines. These effects are attributed to energy transfer via intermolecular collisions and stabilization under the external Lorentz force. The kinetics of apolar species is not significantly affected mainly due to the weak induced dipole moment even under strong electric fields. The dynamics and reaction rates of species are studied by means of large-scale combustion simulations of n-dodecane and oxygen mixtures. Results show that under strong electric fields, the fuel, oxidizer, and most product molecules experience translational and rotational acceleration mainly due to close charge transfer along with a reduction in their vibrational energy due to stabilization. This study will serve as a basis to improve the current methods used in MD and to develop novel methodologies for the modeling of macroscale reacting flows under external electrostatic fields.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Reactive Molecular Dynamics (MD) and Density Functional Theory (DFT) computations are performed to provide insight into the effects of external electrostatic fields on hydrocarbon reaction kinetics. By comparing the results from MD and DFT, the suitability of the MD method in modeling electrodynamics is first assessed. Results show that the electric field-induced polarization predicted by the MD charge equilibration method is in good agreement with various DFT charge partitioning schemes. Then, the effects of oriented external electric fields on the transition pathways of non-redox reactions are investigated. Results on the minimum energy path suggest that electric fields can cause catalysis or inhibition of oxidation reactions, whereas pyrolysis reactions are not affected due to the weaker electronegativity of the hydrogen and carbon atoms. MD simulations of isolated reactions show that the reaction kinetics is also affected by applied external Lorentz forces and interatomic Coulomb forces since they can increase or decrease the energy of collision depending on the molecular conformation. In addition, electric fields can affect the kinetics of polar species and force them to align in the direction of field lines. These effects are attributed to energy transfer via intermolecular collisions and stabilization under the external Lorentz force. The kinetics of apolar species is not significantly affected mainly due to the weak induced dipole moment even under strong electric fields. The dynamics and reaction rates of species are studied by means of large-scale combustion simulations of n-dodecane and oxygen mixtures. Results show that under strong electric fields, the fuel, oxidizer, and most product molecules experience translational and rotational acceleration mainly due to close charge transfer along with a reduction in their vibrational energy due to stabilization. This study will serve as a basis to improve the current methods used in MD and to develop novel methodologies for the modeling of macroscale reacting flows under external electrostatic fields.
Efstratios M. Kritikos, Andrea Giusti
Investigation of the effect of iron nanoparticles on n-dodecane combustion under external electrostatic fields Journal Article
In: Proceedings of the Combustion Institute, vol. 39, no. 4, pp. 5667–5676, 2023, ISSN: 15407489.
@article{kritikos_investigation_2023,
title = {Investigation of the effect of iron nanoparticles on n-dodecane combustion under external electrostatic fields},
author = {Efstratios M. Kritikos and Andrea Giusti},
url = {https://linkinghub.elsevier.com/retrieve/pii/S154074892200030X},
doi = {10.1016/j.proci.2022.07.003},
issn = {15407489},
year = {2023},
date = {2023-01-01},
urldate = {2025-08-21},
journal = {Proceedings of the Combustion Institute},
volume = {39},
number = {4},
pages = {5667–5676},
abstract = {Reactive molecular dynamics simulations are performed to investigate the combined effects of iron nanoparticles and external electrostatic fields on the combustion of n-dodecane. Results suggest that iron nanoparticle additives significantly accelerate fuel and oxidizer consumption. In particular, the decomposition of n-dodecane is initiated at the nanoparticle’s surface by hydrogen abstraction and subsequent absorption of the hydrogen and carbon atoms. Products, such as H2 and H2O, are formed in the nanoparticle’s shell and released back into the gas phase, demonstrating a catalytic behaviour of the nanoparticle. Additionally, the application of an external electrostatic field further increases the n-dodecane consumption rate. A rise in the variety of product species is also observed when an external electrostatic field is applied due to the overall accelerated kinetics of the system. Analysis of the system’s kinetic energy suggests that the presence of an external electrostatic field leads to an increase in the translational energy of the molecules. The chemical composition of the nanoparticle is also affected. The absorbed species diffuse along the surface of the nanoparticle to counteract the externally applied electric field. This species rearrangement leads to the formation of an anisotropic shell with varying chemical composition. This study suggests that the use of electrostatic fields with nanomaterial-based catalysis can offer new possibilities for the control of the reaction process as well as for the synthesis of tailored nanoparticles.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Reactive molecular dynamics simulations are performed to investigate the combined effects of iron nanoparticles and external electrostatic fields on the combustion of n-dodecane. Results suggest that iron nanoparticle additives significantly accelerate fuel and oxidizer consumption. In particular, the decomposition of n-dodecane is initiated at the nanoparticle’s surface by hydrogen abstraction and subsequent absorption of the hydrogen and carbon atoms. Products, such as H2 and H2O, are formed in the nanoparticle’s shell and released back into the gas phase, demonstrating a catalytic behaviour of the nanoparticle. Additionally, the application of an external electrostatic field further increases the n-dodecane consumption rate. A rise in the variety of product species is also observed when an external electrostatic field is applied due to the overall accelerated kinetics of the system. Analysis of the system’s kinetic energy suggests that the presence of an external electrostatic field leads to an increase in the translational energy of the molecules. The chemical composition of the nanoparticle is also affected. The absorbed species diffuse along the surface of the nanoparticle to counteract the externally applied electric field. This species rearrangement leads to the formation of an anisotropic shell with varying chemical composition. This study suggests that the use of electrostatic fields with nanomaterial-based catalysis can offer new possibilities for the control of the reaction process as well as for the synthesis of tailored nanoparticles.
Christos Tantos, Efstratios Kritikos, Stylianos Varoutis, Christian Day
Kinetic modeling of polyatomic heat and mass transfer in rectangular microchannels Journal Article
In: Heat and Mass Transfer, vol. 59, no. 1, pp. 167–184, 2023, ISSN: 0947-7411, 1432-1181.
@article{tantos_kinetic_2023,
title = {Kinetic modeling of polyatomic heat and mass transfer in rectangular microchannels},
author = {Christos Tantos and Efstratios Kritikos and Stylianos Varoutis and Christian Day},
url = {https://link.springer.com/10.1007/s00231-022-03224-z},
doi = {10.1007/s00231-022-03224-z},
issn = {0947-7411, 1432-1181},
year = {2023},
date = {2023-01-01},
urldate = {2025-08-21},
journal = {Heat and Mass Transfer},
volume = {59},
number = {1},
pages = {167–184},
abstract = {Abstract
The present study aims at estimating the heat and the mass transfer coefficients in the case of the polyatomic gas flows through long rectangular microchannels driven by small and large pressure (Poiseuille flow) and temperature (Thermal creep flow) drops. The heat and mass transfer coefficients are presented for all gas flow regimes, from free molecular up to hydrodynamic ones, and for channels with different aspect ratios as well as for various values of translational and rotational Eucken factors. The applied values of the Eucken factors were extracted based on the Rayleigh-Brillouin experiments and the kinetic theory of gases. The numerical study has been performed on the basis of a kinetic model for linear and non-linear gas molecules considering the translational and rotational degrees of freedom. The solution of the obtained system of the kinetic equations is implemented on the Graphics Processing Units (GPUs), allowing the reduction of the computational time by two orders of magnitude. The results show that the Poiseuille mass transfer coefficient is not affected by the internal degrees of freedom and the non-dependence of the previous observed deviations with the experimental data on the molecular nature of the gas molecules is confirmed. However, the study shows that the deviation between monatomic and polyatomic values of the mass transfer coefficient in the thermal creep flow is increased as the gas rarefaction is decreased, and for several polyatomic gases met in practical applications in the temperature range from 300 to 900 K might reach 15%. In addition, the effect of the internal degrees of freedom on the heat transfer coefficient is found to be rather significant. The polyatomic heat transfer coefficients are obtained essentially higher than the monatomic ones, with the maximum difference reaching about 44% and 67% for linear and non-linear gas molecules. In view of the large differences between monatomic and polyatomic gases, the present results may be useful in the design of technological devices in which the thermal creep phenomenon plays a dominant role.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Abstract
The present study aims at estimating the heat and the mass transfer coefficients in the case of the polyatomic gas flows through long rectangular microchannels driven by small and large pressure (Poiseuille flow) and temperature (Thermal creep flow) drops. The heat and mass transfer coefficients are presented for all gas flow regimes, from free molecular up to hydrodynamic ones, and for channels with different aspect ratios as well as for various values of translational and rotational Eucken factors. The applied values of the Eucken factors were extracted based on the Rayleigh-Brillouin experiments and the kinetic theory of gases. The numerical study has been performed on the basis of a kinetic model for linear and non-linear gas molecules considering the translational and rotational degrees of freedom. The solution of the obtained system of the kinetic equations is implemented on the Graphics Processing Units (GPUs), allowing the reduction of the computational time by two orders of magnitude. The results show that the Poiseuille mass transfer coefficient is not affected by the internal degrees of freedom and the non-dependence of the previous observed deviations with the experimental data on the molecular nature of the gas molecules is confirmed. However, the study shows that the deviation between monatomic and polyatomic values of the mass transfer coefficient in the thermal creep flow is increased as the gas rarefaction is decreased, and for several polyatomic gases met in practical applications in the temperature range from 300 to 900 K might reach 15%. In addition, the effect of the internal degrees of freedom on the heat transfer coefficient is found to be rather significant. The polyatomic heat transfer coefficients are obtained essentially higher than the monatomic ones, with the maximum difference reaching about 44% and 67% for linear and non-linear gas molecules. In view of the large differences between monatomic and polyatomic gases, the present results may be useful in the design of technological devices in which the thermal creep phenomenon plays a dominant role.
The present study aims at estimating the heat and the mass transfer coefficients in the case of the polyatomic gas flows through long rectangular microchannels driven by small and large pressure (Poiseuille flow) and temperature (Thermal creep flow) drops. The heat and mass transfer coefficients are presented for all gas flow regimes, from free molecular up to hydrodynamic ones, and for channels with different aspect ratios as well as for various values of translational and rotational Eucken factors. The applied values of the Eucken factors were extracted based on the Rayleigh-Brillouin experiments and the kinetic theory of gases. The numerical study has been performed on the basis of a kinetic model for linear and non-linear gas molecules considering the translational and rotational degrees of freedom. The solution of the obtained system of the kinetic equations is implemented on the Graphics Processing Units (GPUs), allowing the reduction of the computational time by two orders of magnitude. The results show that the Poiseuille mass transfer coefficient is not affected by the internal degrees of freedom and the non-dependence of the previous observed deviations with the experimental data on the molecular nature of the gas molecules is confirmed. However, the study shows that the deviation between monatomic and polyatomic values of the mass transfer coefficient in the thermal creep flow is increased as the gas rarefaction is decreased, and for several polyatomic gases met in practical applications in the temperature range from 300 to 900 K might reach 15%. In addition, the effect of the internal degrees of freedom on the heat transfer coefficient is found to be rather significant. The polyatomic heat transfer coefficients are obtained essentially higher than the monatomic ones, with the maximum difference reaching about 44% and 67% for linear and non-linear gas molecules. In view of the large differences between monatomic and polyatomic gases, the present results may be useful in the design of technological devices in which the thermal creep phenomenon plays a dominant role.
2022
Efstratios M. Kritikos, Aditya Lele, Adri C. T. Van Duin, Andrea Giusti
A reactive molecular dynamics study of the effects of an electric field on n-dodecane combustion Journal Article
In: Combustion and Flame, vol. 244, pp. 112238, 2022, ISSN: 00102180.
@article{kritikos_reactive_2022,
title = {A reactive molecular dynamics study of the effects of an electric field on n-dodecane combustion},
author = {Efstratios M. Kritikos and Aditya Lele and Adri C. T. Van Duin and Andrea Giusti},
url = {https://linkinghub.elsevier.com/retrieve/pii/S001021802200253X},
doi = {10.1016/j.combustflame.2022.112238},
issn = {00102180},
year = {2022},
date = {2022-10-01},
urldate = {2025-08-21},
journal = {Combustion and Flame},
volume = {244},
pages = {112238},
abstract = {A reactive Molecular Dynamics (MD) study of n-dodecane combustion at high temperatures under externally applied electrostatic fields is performed to investigate their effect on chemical kinetics. A local charge equilibration method is used to enable charge transfers up to the overlap of the atomic orbitals and introduce molecular polarization induced by an electric field. The atomic charges of an isolated n-dodecane molecule with and without external electrostatic fields are first compared with Density Functional Theory (DFT) computations, to assess the accuracy of the charge equilibration method and its ability to capture polarization. Then, the impact of external electrostatic fields on the reaction kinetics of fuel, oxidizer and products is studied for a range of ambient temperatures and densities. The activation energy and pre-exponential factor of Arrhenius-type reactions under various electrostatic fields are also investigated by performing Nudged Elastic Band (NEB) computations on selected reactions’ Minimum Energy Path (MEP) and by analysing the collision frequency, respectively. Results show that the atomic charge transfers due to close interactions and molecular polarisation are relatively weak in all investigated conditions, leading to the necessity of strong external electric fields to induce changes to chemical kinetics. The consumption rate of n-dodecane decreases for strong electrostatic fields, whereas for low values of the electrostatic field strength no clear trend is observed. In addition, at high temperature and density conditions, oxygen consumption increases under strong electrostatic fields, whereas the opposite trend is observed as the temperature and density decrease. NEB analysis shows alterations of the activation energy up to 2.3 kcal/mol for oxygen compound reactions with varying strength of the external electrostatic field. Furthermore, analysis of the translational, rotational and vibrational kinetic energy modes and collision frequency reveals the influence of translational motion and molecular stabilization on the reaction rates. The kinetics of oxygen molecules was found to be of primary importance to determine the reaction behaviour under external electrostatic fields, as oxygen molecules have a direct effect on the oxidation reactions and also indirectly affect n-dodecane pyrolysis when an electrostatic field is present. This study provides fundamental understanding of the interactions between heavy hydrocarbons and electrostatic fields for the development of future hybrid thermal-electrical technologies.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A reactive Molecular Dynamics (MD) study of n-dodecane combustion at high temperatures under externally applied electrostatic fields is performed to investigate their effect on chemical kinetics. A local charge equilibration method is used to enable charge transfers up to the overlap of the atomic orbitals and introduce molecular polarization induced by an electric field. The atomic charges of an isolated n-dodecane molecule with and without external electrostatic fields are first compared with Density Functional Theory (DFT) computations, to assess the accuracy of the charge equilibration method and its ability to capture polarization. Then, the impact of external electrostatic fields on the reaction kinetics of fuel, oxidizer and products is studied for a range of ambient temperatures and densities. The activation energy and pre-exponential factor of Arrhenius-type reactions under various electrostatic fields are also investigated by performing Nudged Elastic Band (NEB) computations on selected reactions’ Minimum Energy Path (MEP) and by analysing the collision frequency, respectively. Results show that the atomic charge transfers due to close interactions and molecular polarisation are relatively weak in all investigated conditions, leading to the necessity of strong external electric fields to induce changes to chemical kinetics. The consumption rate of n-dodecane decreases for strong electrostatic fields, whereas for low values of the electrostatic field strength no clear trend is observed. In addition, at high temperature and density conditions, oxygen consumption increases under strong electrostatic fields, whereas the opposite trend is observed as the temperature and density decrease. NEB analysis shows alterations of the activation energy up to 2.3 kcal/mol for oxygen compound reactions with varying strength of the external electrostatic field. Furthermore, analysis of the translational, rotational and vibrational kinetic energy modes and collision frequency reveals the influence of translational motion and molecular stabilization on the reaction rates. The kinetics of oxygen molecules was found to be of primary importance to determine the reaction behaviour under external electrostatic fields, as oxygen molecules have a direct effect on the oxidation reactions and also indirectly affect n-dodecane pyrolysis when an electrostatic field is present. This study provides fundamental understanding of the interactions between heavy hydrocarbons and electrostatic fields for the development of future hybrid thermal-electrical technologies.
2020
Efstratios Kritikos, Andrea Giusti
Reactive Molecular Dynamics Investigation of Toluene Oxidation under Electrostatic Fields: Effect of the Modeling of Local Charge Distribution Journal Article
In: The Journal of Physical Chemistry A, vol. 124, no. 51, pp. 10705–10716, 2020, ISSN: 1089-5639, 1520-5215.
@article{kritikos_reactive_2020,
title = {Reactive Molecular Dynamics Investigation of Toluene Oxidation under Electrostatic Fields: Effect of the Modeling of Local Charge Distribution},
author = {Efstratios Kritikos and Andrea Giusti},
url = {https://pubs.acs.org/doi/10.1021/acs.jpca.0c08040},
doi = {10.1021/acs.jpca.0c08040},
issn = {1089-5639, 1520-5215},
year = {2020},
date = {2020-12-01},
urldate = {2025-08-21},
journal = {The Journal of Physical Chemistry A},
volume = {124},
number = {51},
pages = {10705–10716},
abstract = {A reactive Molecular Dynamics (MD) study of toluene oxidation at high temperatures under externally applied electrostatic fields has been performed. The impact of the modeling of local charge distribution has been investigated by comparing the widely used Charge Equilibration (QEq) method with the Charge Transfer with Polarization Current Equalization (QTPIE) method, which shields charge transfers up to atomic orbitals and introduces molecular polarization. Using the latter method, it is possible to improve the computation of the atomic charges, which are a critical aspect for the numerical study of electric fields, and to capture important effects of the electric field on rotational and vibrational energies of the toluene molecule. Results show that a more comprehensive treatment of inter-and intramolecular charge distribution achieved through the QTPIE method leads to substantially different applied forces and oxidation rates of toluene compared to the QEq method. Using the QTPIE method, no significant effects of the electrostatic field on the toluene oxidation rate were observed for the range of temperatures and pressures studied here, which is in disagreement with the results obtained with the QEq method where a clear impact of the electrostatic field on the average oxidation rate was found. Therefore, when studying electric field effects with MD simulations, the choice of the method used for the charge equilibration is a key modeling assumption whose impact should be carefully evaluated.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A reactive Molecular Dynamics (MD) study of toluene oxidation at high temperatures under externally applied electrostatic fields has been performed. The impact of the modeling of local charge distribution has been investigated by comparing the widely used Charge Equilibration (QEq) method with the Charge Transfer with Polarization Current Equalization (QTPIE) method, which shields charge transfers up to atomic orbitals and introduces molecular polarization. Using the latter method, it is possible to improve the computation of the atomic charges, which are a critical aspect for the numerical study of electric fields, and to capture important effects of the electric field on rotational and vibrational energies of the toluene molecule. Results show that a more comprehensive treatment of inter-and intramolecular charge distribution achieved through the QTPIE method leads to substantially different applied forces and oxidation rates of toluene compared to the QEq method. Using the QTPIE method, no significant effects of the electrostatic field on the toluene oxidation rate were observed for the range of temperatures and pressures studied here, which is in disagreement with the results obtained with the QEq method where a clear impact of the electrostatic field on the average oxidation rate was found. Therefore, when studying electric field effects with MD simulations, the choice of the method used for the charge equilibration is a key modeling assumption whose impact should be carefully evaluated.
C. P. Koutsou, E. M. Kritikos, A. J. Karabelas, M. Kostoglou
Analysis of temperature effects on the specific energy consumption in reverse osmosis desalination processes Journal Article
In: Desalination, vol. 476, pp. 114213, 2020, ISSN: 00119164.
@article{koutsou_analysis_2020,
title = {Analysis of temperature effects on the specific energy consumption in reverse osmosis desalination processes},
author = {C. P. Koutsou and E. M. Kritikos and A. J. Karabelas and M. Kostoglou},
url = {https://linkinghub.elsevier.com/retrieve/pii/S0011916419315607},
doi = {10.1016/j.desal.2019.114213},
issn = {00119164},
year = {2020},
date = {2020-02-01},
urldate = {2025-08-21},
journal = {Desalination},
volume = {476},
pages = {114213},
abstract = {Specific Energy Consumption (SEC) is key parameter involved in optimizing design and operation of RO-desa- lination plants, used for treating various feed-waters. This study deals with feed-water temperature Tf effects on the performance of a typical RO unit (operating in constant recovery mode), that includes an energy recovery device (ERD), focusing on SEC for brackish and sea-water desalination. Account is taken of temperature-effects on physico-chemical water-properties and membrane performance-parameters, that enable reliable determina- tion of itemized contributions to SEC; the latter include energy losses in mechanical devices (pumps and ERD) and in the RO process, mainly due to species separation and fluid-friction through membranes and narrow flow- channels. In general, within a Tf range textasciitilde15 to 40 °C, increased feed-water temperature results in SEC reduction in desalinating low salinity waters. However, for high salinities as in sea-water, the significantly increasing osmotic pressure (with Tf) tends to neutralize other positive effects, thus leading to a minimum SEC at textasciitilde30 °C for typical membrane-module parameter-values. Furthermore, increased Tf in desalination of both low- and high- salinity waters tends to negatively impact on membrane salt rejection and scaling, which should be considered in optimization studies. The methodology adopted in this work would be useful in such studies.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Specific Energy Consumption (SEC) is key parameter involved in optimizing design and operation of RO-desa- lination plants, used for treating various feed-waters. This study deals with feed-water temperature Tf effects on the performance of a typical RO unit (operating in constant recovery mode), that includes an energy recovery device (ERD), focusing on SEC for brackish and sea-water desalination. Account is taken of temperature-effects on physico-chemical water-properties and membrane performance-parameters, that enable reliable determina- tion of itemized contributions to SEC; the latter include energy losses in mechanical devices (pumps and ERD) and in the RO process, mainly due to species separation and fluid-friction through membranes and narrow flow- channels. In general, within a Tf range textasciitilde15 to 40 °C, increased feed-water temperature results in SEC reduction in desalinating low salinity waters. However, for high salinities as in sea-water, the significantly increasing osmotic pressure (with Tf) tends to neutralize other positive effects, thus leading to a minimum SEC at textasciitilde30 °C for typical membrane-module parameter-values. Furthermore, increased Tf in desalination of both low- and high- salinity waters tends to negatively impact on membrane salt rejection and scaling, which should be considered in optimization studies. The methodology adopted in this work would be useful in such studies.