arXiv subjects

Mykola Isaiev

Publications and source records attributed to Mykola Isaiev.

At least 19 recordsLinked to original sources

arXiv2026

Fluctuation-Dissipation Framework for Size-Dependent Surface Tension

The size-dependent liquid-vapor surface tension controls phase change, wetting, and transport at nanoscales, yet its first curvature correction, the Tolman length, remains difficult to determine. We develop a thermodynamic and statistical-mechanical framework that relates this correction to bulk response properties of a one-component liquid near liquid-vapor coexistence. For curved interfaces, the analysis considers two local formulations of the same capillary-chemical balance, in excess pressures and in relative density deviations. For weakly compressible liquids in the regime emphasized here, the adopted asymmetric density-based formulation is the practically relevant one, with finite-curvature effects entering through vapor supersaturation under capillary equilibrium. At coexistence, the planar-limit value of the same Tolman length reduces to a combination of the liquid isothermal compressibility and its pressure derivative and can be recast as a bulk fluctuation-response observable of the homogeneous liquid in the isothermal-isobaric ensemble. In this representation, the planar-limit coefficient is determined by second and third central moments of the volume distribution, equivalently by the pressure response of the relative fluctuation width. For water, homogeneous (N,P,T) simulations of SPC/E and TIP4P/2005 sample the bulk liquid, not an explicit liquid-vapor interface, and yield estimates near -0.7 Angstrom at 300 K. An independent evaluation based on the IAPWS-IF97 industrial formulation gives -0.713 +/- 0.004 Angstrom at the same coexistence state and predicts a weakly nonmonotonic temperature dependence along coexistence. Beyond water, the framework applies to other one-component liquids in regimes where an accurate thermal equation of state or sufficiently converged bulk volume statistics are available.

cond-mat.soft↗

arXiv2026

Interfacial Behavior from the Atomic Blueprint: Machine Learning-Guided Design of Spatially Functionalized a-SiO2 Surfaces

a-Quartz surfaces functionalized with hydroxyl and methyl groups provide a versatile platform for controlling interfacial properties critical to applications such as catalysis, protective coatings, and energy conversion. The arrangement of these functional groups strongly influences interfacial interactions at solid-liquid interfaces, highlighting their relevance to colloid and interface science. However, conventional models often treat surface functionalization as spatially homogeneous, overlooking the atomic-scale organization of surface groups. We hypothesize that this spatial distribution, beyond overall composition, plays a decisive role in governing surface stability and interfacial behavior. To test this hypothesis, we employ a multi-scale simulation workflow combining density functional theory, ab initio molecular dynamics (AIMD), and machine-learned force fields (MLFFs). This approach allows us to explore a range of spatial patterns of OH/CH3 functionalization on the a-quartz (0001) surface. We evaluate the impact of spatial arrangements on mixing energy, hydrogen bonding networks, and vibrational properties with high accuracy and robustness. Our results reveal that spatial patterning strongly influences surface stability and interfacial structure. A thermodynamically favored unpaired configuration emerges near 67 % CH3 substitution, where isolated OH groups form secondary hydrogen bonds through reorientation toward subsurface oxygen atoms. This rearrangement induces a characteristic blue shift in OH stretching frequencies, indicating weaker H-bonding. These effects are absent in clustered arrangements. By establishing a clear link between functional group patterning and interfacial behavior, our work uncovers the underlying mechanisms to guide and accelerate the rational design of silica-based materials and coatings, directly relevant to colloid and interface science.

cond-mat.mtrl-sci↗

arXiv2026

Tailored Thermal Transport in Phase Change Materials-Based Nanocomposites through Interfacial Structuring

Interfacial thermal transport is a critical bottleneck in nanoscale systems, where heat dissipation and energy efficiency are strongly modulated by molecular ordering at solid-liquid boundaries. Here, using atomistic simulations of hexadecane confined by structured silica substrates, we reveal how interfacial geometry, specifically curvature, governs the density distribution and thermal transport across the interface. On flat and mildly curved surfaces, the liquid exhibits surface-templated layering, promoting efficient heat transfer, which is enhanced as the contact surface area increases. As curvature increases, this ordering breaks down, giving rise to interference-like density patterns, reduced molecular packing, and localized depletion zones. This structural reorganization leads to a systematic increase of up to 10 % in interfacial thermal resistance (ITR), even when the contact area is kept constant. By decomposing the interface into convex ("hills" of the solid) and concave ("valleys" of the solid) regions, we find that valleys consistently exhibit lower ITR. In contrast, hills act as bottlenecks to heat flow, leading to interfacial thermal resistance values up to 70% higher than those of valleys, depending on the surface configuration. Remarkably, we show that the work of adhesion and entropy-related energy gain upon liquid detachment scale non-trivially with curvature: while adhesion increases with contact area from 30 mJ*m^(-2) for a flat surface to 40 mJ*m^(-2) for a maximum curvaceous surface, the entropic penalty dominates the total energy change, reflecting curvature-induced frustration of molecular alignment.

cond-mat.mes-hall↗

arXiv2025

Tailoring Heat Transfer at Silica-Water Interfaces via Hydroxyl and Methyl Surface Groups

Efficient thermal transport across solid-liquid interfaces is essential for optimizing heat dissipation in modern technological applications. This study employs molecular dynamics (MD) simulations to investigate the impact of surface functionalization on heat transfer at the silica/water interface. It has been shown that the surface functionalization changes significantly the wetting characteristics of silica surface: from one hand hydroxyl groups render such surfaces more hydrophilic, while methyl groups more hydrophobic. Here, we reveal that modifying the surface functionalization from methylated to hydroxylated groups results in: (i) up to an approximately eightfold increase in adhesion energy, (ii) a reorientation of interfacial water molecules to align perpendicular to the surface normal, (iii) a reduction in the liquid depletion length near the interface, and (iv) an overall enhancement of interfacial heat conduction. We quantify interfacial thermal resistance through the calculation of the contribution of each functional group to the total heat flux, providing insights into the physical mechanisms governing heat transfer at functionalized interfaces. We demonstrated that manipulation of the concentrations of the functional groups can be used to tailor interfacial thermal transport.

physics.atm-clus↗

arXiv2024

Features of the Contact Angle Hysteresis at the Nanoscale: A Molecular Dynamics Insight

Understanding the physics of a three-phase contact line between gas, liquid, and solid is important for numerous applications. At the macroscale, the three-phase contact line response to an external force action is often characterized by a contact angle hysteresis, and several models are presented in the literature for its description. Yet, there is still a need for more information about such model applications at the nanoscale. In this study, a molecular dynamics approach was used to investigate the shape of a liquid droplet under an external force for different wetting regimes. In addition, an analytic model for describing the droplet shape was developed. It gives us the possibility to evaluate the receding and advancing wetting angle accurately. With our modeling, we found that the interplay between capillary forces and viscous forces is crucial to characterize the droplet shape at the nanoscale. In this frame, the importance of the rolling movement of the interface between liquid and vapor was pointed out. We also demonstrate that in the range of the external forces when capillary forces are most significant compared to others, hysteresis is well described by the macroscale Cox-Voinov model.

cond-mat.soft↗

arXiv2024

Spectral Mechanisms of Solid/Liquid Interfacial Heat Transfer in the Presence of a Meniscus

In this study, we employ molecular simulations to investigate the enhancement in thermal conductance at the solid/liquid interface in the presence of a meniscus reported previously (Klochko et al., Phys. Chem. Chem. Phys. 25(4):3298-3308, 2023). We vary the solid/liquid interaction strength at Lennard-Jones interfaces for both confined liquid and meniscus systems, finding that the presence of a meniscus yields an enhancement in the interfacial thermal conductance across all wettabilities. However, the magnitude of the enhancement is found to depend on the surface wettability, initially rising monotonously for low to moderate wettabilities, followed by a sharp rise between moderate and high wettabilities. The spectral decomposition of heat flux formalism was applied to understand the nature of this phenomenon further. By computing the in-plane and out-of-plane components of the heat fluxes within both the interfacial solid and liquid, we show that the initial monotonous rise in conductance enhancement predominantly stems from a rise in the coupling of out-of-plane vibrations within both the solid and the liquid. In contrast, the subsequent sharp rise at more wetting interfaces is linked to sharp increases in the utilization of the in-plane modes of the solid and liquid. These observations result from the interplay between the solid/liquid adhesive forces and the liquid/vapor interfacial tension. Our results can aid engineers in optimizing thermal transport at realistic interfaces, which is critical to designing effective cooling solutions for electronics, among other applications.

cond-mat.soft↗

arXiv2024

Features of phonon scattering by a spherical pore: molecular dynamics insight

There is still a gap in understanding phonons scattering by geometrical defects at the nanoscale, and it remains a significant challenge for heat transfer management in nanoscale devices and systems. In this study, we aim to explore the characteristics of phonon scattering by a single pore to gain insights into thermal transport in nanostructures. The paper outlines a methodology for assessing the spatial distribution of the magnitude of the radial, azimuthal, and polar components of the velocity of scattered phonons by a spherical pore. We demonstrated that the size parameter, commonly employed in electromagnetic wave scattering theory, is vital in determining the scattering regime. Specifically, we show that calculated scattering efficiency has the same pattern as one commonly obtained in classical wave scattering theory. However, we found that crystallographic directions are pivotal in shaping the scattering patterns, especially in the regions where scattering patterns are defined by the Mie resonances. This observation holds significance in understanding the influence of phonon coherence on thermal transport in nanostructured materials.

cond-mat.mes-hall↗

arXiv2023

Thermal Transport Properties of Nanoporous Silicon with Significant Specific Surface Area

This paper studies thermal transport in nanoporous silicon with a significant specific surface area. First, the equilibrium molecular dynamics approach was used to obtain the dependence of thermal conductivity on a specific surface area. Then, a modified phonon transport kinetic theory-based approach was developed to analyze thermal conductivity. Two models were used to evaluate the phonon mean free path in the porous materials. The first model approximates the dependence of the mean free path only with the specific surface area, and the second one investigates the dependence of the mean free path variation with the porosity in the peculiar case of a highly porous matrix. Both models approximate molecular dynamics data well for the smaller porosity values, while the first model fails for large porosities. The second model matches well with molecular dynamics simulations for all considered ranges of the porosities. This work illustrates that the phonon mean free path dependence with the porosity/volume fraction of composite materials is essential for describing thermal transport in systems with significant surface-to-volume fractions.

cond-mat.mes-hall↗

arXiv2022

Thermal transport properties of the nanocomposite system "porous silicon/ionic liquid"

This paper investigates thermal transport in a nanocomposite system "porous silicon matrix filled with ionic liquid". First, the thermal conductivity and heat capacity of two imidazolium and one ammonium ionic liquids were evaluated using the photoacoustic approach in piezoelectric configuration and differential scanning calorimetry, respectively. Then, the thermal transport properties of the composite system "ionic liquid confined inside porous silicon matrix" were investigated with the photoacoustic approach in gas-microphone configuration. It was found that a significant enhancement of the thermal conductivity of the composite when compared to the individual components, i.e. (i) more than two times for pristine porous silicon and (ii) more than eight times for ionic liquids. These results provide new paths for innovative solutions in the field of thermal management in highly efficient energy storage devices.

cond-mat.mes-hall↗

arXiv2021

Bi-modal photothermal/optical microscopy for complementary bio-imaging with high resolution and contrast

In the paper, combined bimodal microscopic visualization of biological objects based on reflection-mode optical and photoacoustic measurements was presented. Gasmicrophone configuration was chosen for the registration of photothermal response. Precise positioning of the scanning laser beam with modulated intensity was performed employing acousto-optic deflectors. Photoacoustic images are shown to give complementary details to the optical images obtained in the reflected light. Specifically, the photoacoustic imaging mode allows much better visualization of the features with enhanced heat localization due to the reduced heat outflow. For example, the application of the photoacoustic imaging mode was especially successful to visualize Drosophila fly's micro-hairs. Furthermore, the photoacoustic image quality was shown to be adjusted through modulation frequency.

physics.app-ph↗

arXiv2021

Molecular dynamics simulation of thermal transport across solid/liquid interface created by meniscus

Understandings heat transfer across a solid/liquid interface is important to develop new pathways to improve thermal management in various energy applications. One of the important questions that arises in this context is the impact of three-phase contact line between solid, liquid and gas on the perturbations of the heat fluxes at the nanoscale. Therefore, this paper is devoted to the investigations of features of thermal transport across nanosized meniscus constrained between two solid walls. Different wetting states of the meniscus were considered with molecular dynamics approach by the variation of the interactional potential between atoms of the substrate and the liquid. The effect of the size of the meniscus on the exchange of energy between two solid walls was also investigated. It was shown that the presence of a three phase contact line leads to a decrease of the interfacial boundary resistance between solid and liquid. Further, investigations with the finite element method were used to link atomistic simulations with the continuum mechanics. We demonstrate that the wetting angle and the interfacial boundary resistance are the required key-parameters to perform multiscale simulations of such engineering problems with an accurate microscale parametrization.

cond-mat.mes-hall↗

arXiv2020

Thermal properties study of silicon nanostructures by photoacoustic techniques

The photoacoustic method with piezoelectric detection for the simultaneous evaluation of the thermophysical properties is proposed. The approach is based on the settling of an additional heat sink for redistribution of heat fluxes deposited on the sample surface. Firstly, the approach was tested on the porous silicon with well-defined morphology and well-studied properties. Then, heat capacity and thermal conductivity of silicon nanowires arrays have been investigated by recovering the experimental data through numerical simulations. The decrease of heat capacity and effective thermal conductivity of the samples upon increasing thickness and porosity of the sample is observed. Such behavior could be caused by the increase of the structure heterogeneity. In particular, this can be related to larger disorder (increased density of broken nanowires and larger porosity) that appears during the etching process of the thick layers.

cond-mat.mes-hall↗

arXiv2020

Thermal transport enhancement of hybrid nanocomposites; impact of confined water inside nanoporous silicon

The thermal transport properties of porous silicon and nano-hybrid "porous silicon/water" systems are presented here. The thermal conductivity was evaluated with equilibrium molecular dynamics technique for porous systems made of spherical voids or water-filled cavities. We revealed large thermal conductivity enhancement in the nano-hybrid systems as compared to their dry porous counterparts, which cannot be captured by effective media theory. This rise of thermal conductivity is related to the increases of the specific surface of the liquid/solid interface. We demonstrated that significant difference for more than two folds of thermal conductivity of pristine porous silicon and "porous silicon liquid/composite" is due to the liquid density fluctuation close to "solid/liquid interface" (layering effect). This effect is getting more important for the high specific surface of the interfacial area. Specifically, the enhancement of the effective thermal conductivity is 50 % for specific surface area of 0.3 (1/nm), and it increases further upon the increase of the surface to volume ratio. Our study provides valuable insights into the thermal properties of hybrid liquid/solid nanocomposites and about the importance of confined liquids within nanoporous materials.

cond-mat.mes-hall↗

arXiv2020

Frequency domain measurements of thermal properties using 3omega-Scanning Thermal Microscope in a vacuum environment

Material thermal properties characterization at nanoscales remains a challenge even if progresses were done in developing specific characterization techniques like the Scanning Thermal Microscopy (SThM). In the present work, we propose a detailed procedure based on the combined use of a SThM probe characterization and its Finite Element Modeling (FEM) to recover in-operando 3omega measurements achieved under high vacuum. This approach is based on a two-step methodology: (i) a fine description of the probe s electrical and frequency behaviors in out of contact mode to determine intrinsic parameters of the SThM tip, (b) a minimization of the free parameter of our model, i.e. the contact thermal resistance, by comparing 3omega measurements to our simulations of the probe operating in contact mode. Such an approach allows us to accurately measure thermal interface resistances of the probe as a function of the strength applied between the tip and the surface for three different materials (silicon, silica and gold). In addition, FEM modeling provides insights about the 3omega-SThM technique sensitivity, as a function of probe / sample interface resistance to measure material thermal conductivity, paving the way to quantitative SThM measurements.

physics.app-ph↗

arXiv2020

Size dependence of the surface tension of a free surface of an isotropic fluid

We report on the size dependence of the surface tension of a free surface of an isotropic fluid. The size dependence of the surface tension is evaluated based on the Gibbs-Tolman-Koenig-Buff equation for positive and negative values of curvatures and the Tolman lengths. For all combinations of positive and negative signs of curvature and the Tolman length, we succeed to have a continuous function, avoiding the existing discontinuity at zero curvature (flat interfaces). As an example, a water droplet in the thermodynamical equilibrium with the vapor is analyzed in detail. The size dependence of the surface tension and the Tolman length are evaluated with the use of experimental data of the International Association for the Properties of Water and Steam. The evaluated Tolman length of our approach is in good agreement with molecular dynamics and experimental data

cond-mat.soft↗

arXiv2019

Decorated dislocations against phonon propagation for thermal management

The impact of decorated dislocations on the effective thermal conductivity of GaN is investigated by means of equilibrium molecular dynamics simulations via the Green-Kubo approach. The formation of "nanowires" by a few atoms of In in the core of dislocations in wurtzite GaN is found to affect the thermal properties of the material, as it leads to a significant decrease of the thermal conductivity, along with an enhancement of its anisotropic character. The thermal conductivity of In-decorated dislocations is compared to the ones of pristine GaN, InN, and random and ordered InxGa1-xN alloy, to examine the impact of doping. Results are explained by the stress maps, the bonding properties and the phonon density of states of the aforementioned systems. The decorated dislocations engineering is a novel way to tune, among other transport properties, the effective thermal conductivity of materials at the nanoscale, which can lead to the manufacturing of interesting candidates for thermoelectric or anisotropic thermal dissipation devices.

cond-mat.mtrl-sci↗

arXiv2019

Thermal conductivity of strained silicon: molecular dynamics insight and kinetic theory approach

In this work, we investigated tensile and compression forces effect on the thermal conductivity of silicon. We used equilibrium molecular dynamics approach for the evaluation of thermal conductivity considering different interatomic potentials. More specifically, we tested Stillinger-Weber, Tersoff, Environment-Dependent Interatomic Potential and Modified Embedded Atom Method potentials for the description of silicon atom motion under different strain and temperature conditions. Additionally, we extracted phonon density of states and dispersion curves from molecular dynamics simulations. These data were used for direct calculations of thermal conductivity considering the kinetic theory approach. Comparison of molecular dynamics and kinetic theory simulations results as a function of strain and temperature allowed us to investigate the different factors affecting the thermal conductivity of strained silicon.

cond-mat.mtrl-sci↗

arXiv2018

Interfacial thermal resistance between porous layers: impact on thermal conductivity of a multilayered porous structure

Features of thermal transport in multilayered porous silicon nanostructures are considered. Such nanostructures were fabricated by electrochemical etching of monocrystalline Si substrates by applying periodically changed current density. Hereby, the multilayered structures with specific phononic properties were formed. Photoacoustic (PA) technique in gas-microphone configuration was applied for thermal conductivity evaluation. Experimental amplitude-frequency dependencies were adjusted by temperature distribution simulation with thermal conductivity of the multilayered porous structure as a fitting parameter. The experimentally determined values of thermal conductivity were found to be significantly lower than theoretically calculated ones. Such difference was associated with the presence of thermal resistance at the interfaces between porous layers with different porosities arising because of elastic parameters mismatch (acoustical mismatch). Accordingly, the magnitude of this interfacial thermal resistance was experimentally evaluated for the first time. Furthermore, crucial impact of the resistance on thermal transport perturbation in a multilayered porous silicon structure was revealed.

physics.app-ph↗