Parametric Studies of the Thermal and Momentum Accommodation of Monoatomic and Diatomic Gases on Solid Surfaces

The Thermal Accommodation Coefficient (TAC) and the Momentum Accommodation Coefficient (MAC) Are the Two Fundamental Parameters Quantifying the Solid-Gas Energy and Momentum Exchange Efficiencies. We Use Molecular Dynamics (MD) Simulations to Study the Effect of Individual Interfacial Parameters Including, (I) Solid-Gas Interaction Strength, (Ii) Gas-Solid Atomic Mass Ratio, (Iii) Solid Elastic Stiffness, and (Iv) Temperature, on TAC and MAC at Solid Surfaces in Contact with Monoatomic and Diatomic Gases. in Addition to Offering a Fundamental Understanding on How These Individual Parameters Affect the Nature of Gas-Solid Collisions, We Provide an Extensive Database for the TAC and MAC. We Also Study the Effect of Surface Functionalization with Molecular Monolayers on the Energy and Momentum Transfer at the Interface. These Results Are Useful in Developing Interfaces with Enhanced Heat Transfer under Various Operation Conditions. © 2014 Elsevier Ltd. All Rights Reserved

Finite-Size Effects on Molecular Dynamics Interfacial Thermal-Resistance Predictions

Using Molecular Dynamics Simulations, We Study the Role of Finite Size Effects on the Determination of Interfacial Thermal Resistance between Two Solids Characterized by High Phonon Mean Free Paths. in Particular, We Will Show that a Direct, Heat Source-Sink Method Leads to Strong Size Effect, Associated with Ballistic Phonon Transport to and From, and Specular Reflections at the Simulation Domain Boundary. Lack of Proper Account for These Effects Can Lead to Incorrect Predictions About the Role of Interfacial Bonding and Structure on Interfacial Thermal Resistance. We Also Show that the Finite Size Effect Can Be Dramatically Reduced by Introduction of Rough External Boundaries Leading to Diffuse Phonon Scattering, as Explicitly Demonstrated by Phonon Wave-Packet Simulations. Finally, We Demonstrate that When Careful Considerations Are Given to the Effects Associated with the Finite Heat Capacity of the Simulation Domains and Phonon Scattering from the External Surfaces, a Size-Independent Interfacial Resistance Can Be Properly Extracted from the Time Integral of the Correlation Function of Heat Power Across the Interface. Our Work Demonstrates that Reliable and Consistent Values of the Interfacial Thermal Resistance Can Be Obtained by Equilibrium and Nonequilibrium Methods with a Relatively Small Computational Cost. © 2014 American Physical Society

Sound Attenuation in Amorphous Silica at Frequencies Near the Boson Peak

We Use Molecular Dynamics Phonon Wave Packet (WP) Simulations to Study Acoustic Propagation and Attenuation in Amorphous Silica (A-SiO2) at Frequencies Near the Boson Peak (BP) Position and Compare Them with the Results of Equilibrium Molecular Dynamics (EMD) Simulations. the Sound Attenuation Coefficients Obtained from WP Simulations Are Generally Consistent with Those from EMD Predictions and Have Reasonable Agreement with the Existing Experimental Data. Near the BP Position, We Found the Frequency-Dependent Sound Attenuation Coefficients for Longitudinal and Transverse Modes Both Follow the Rayleigh-Scattering Fourth Power Law. above the BP Frequency, However, the Propagating Phonon is Essentially Attenuated in A-SiO2 within a Few Nanometers, and the Accurate Determination of the Sound Attenuation Coefficients by the WP Simulation Becomes Challenging. the Modeling Results Provide a Reference for Future Experimental Investigations of Sound Attenuation in A-SiO2 Thin Film using Narrow-Band Coherent Phonons

Slip Length Crossover on a Graphene Surface

Using Equilibrium and Non-Equilibrium Molecular Dynamics Simulations, We Study the Flow of Argon Fluid above the Critical Temperature in a Planar Nanochannel Delimited by Graphene Walls. We Observe that, as a Function of Pressure, the Slip Length First Decreases Due to the Decreasing Mean Free Path of Gas Molecules, Reaches the Minimum Value When the Pressure is Close to the Critical Pressure, and Then Increases with Further Increase in Pressure. We Demonstrate that the Slip Length Increase at High Pressures is Due to the Fact that the Viscosity of Fluid Increases Much Faster with Pressure Than the Friction Coefficient between the Fluid and the Graphene. This Behavior is Clearly Exhibited in the Case of Graphene Due to a Very Smooth Potential Landscape Originating from a Very High Atomic Density of Graphene Planes. by Contrast, on Surfaces with Lower Atomic Density, Such as an (100) Au Surface, the Slip Length for High Fluid Pressures is Essentially Zero, Regardless of the Nature of Interaction between Fluid and the Solid Wall

Coalescence-Induced Jumping of Nanoscale Droplets on Super-Hydrophobic Surfaces

The Coalescence-Induced Jumping of Tens of Microns Size Droplets on Super-Hydrophobic Surfaces Has Been Observed in Both Experiments and Simulations. However, Whether the Coalescence-Induced Jumping Would Occur for Smaller, Particularly Nanoscale Droplets, is an Open Question. using Molecular Dynamics Simulations, We Demonstrate that in Spite of the Large Internal Viscous Dissipation, Coalescence of Two Nanoscale Droplets on a Super-Hydrophobic Surface Can Result in a Jumping of the Coalesced Droplet from the Surface with a Speed of a Few M/s. Similar to the Coalescence-Induced Jumping of Microscale Droplets, We Observe that the Bridge between the Coalescing Nano-Droplets Expands and Impacts the Solid Surface, Which Leads to an Acceleration of the Coalesced Droplet by the Pressure Force from the Solid Surface. We Observe that the Jumping Velocity Decreases with the Droplet Size and its Ratio to the Inertial-Capillary Velocity is a Constant of About 0.126, Which is Close to the Minimum Value of 0.111 Predicted by Continuum-Level Modeling of Enright Et Al. [ACS Nano 8, 10352 (2014)]

Molecular Simulation of Steady-State Evaporation and Condensation in the Presence of a Non-Condensable Gas

Using Molecular Dynamics Simulations, We Study Evaporation and Condensation of Fluid Ar in the Presence of a Non-Condensable Ne Gas in a Nanochannel. the Evaporation and Condensation Are Driven by the Temperature Difference, ΔTL, between the Evaporating and Condensing Liquid Surfaces. the Steady-State Evaporation and Condensation Fluxes (JMD) Are Also Affected by the Ne Concentration, ΡNe, and the Nanochannel Length. We Find that Across a Wide Range of ΔTL and ΡNe, JMD is in Good Agreement with the Prediction from Stefan\u27s Law and from Schrage Relationships. Furthermore, for ΔTL Less Than ∼20% of the Absolute Average Temperature, We Find that Both Steady-State Heat and Mass Fluxes Are Proportional to ΔTL. This Allows Us to Determine the Interfacial Resistance to the Heat and Mass Transfer and Compare It with the Corresponding Resistances in the Gas Phase. in This Context, We Derive an Analytical Expression for the Effective Thermal Conductivity of the Gas Region in the Nanochannel and the Mass Transport Interfacial Resistance Equivalent Length, I.e., the Length of the Nanochannel for Which the Resistance to the Mass Flow is the Same as the Interfacial Resistance to the Mass Flow

Fundamentals of Energy Transport in Nanofluids

We performed computational simulations and theoretical analysis to investigate the underlying origins of large thermal conductivity enhancements observed in nanofluids (colloidal suspensions of solid nanoparticles and/or nanofibers in thermal fluids) and to identify strategies towards tailoring nanofluids for better thermal performance

Thermal Transport Across a Substrate-Thin-Film Interface: Effects of Film Thickness and Surface Roughness

Using Molecular Dynamics Simulations and a Model AlN-GaN Interface, We Demonstrate that the Interfacial Thermal Resistance RK (Kapitza Resistance) between a Substrate and Thin Film Depends on the Thickness of the Film and the Film Surface Roughness When the Phonon Mean Free Path is Larger Than Film Thickness. in Particular, When the Film (External) Surface is Atomistically Smooth, Phonons Transmitted from the Substrate Can Travel Ballistically in the Thin Film, Be Scattered Specularly at the Surface, and Return to the Substrate Without Energy Transfer. If the External Surface Scatters Phonons Diffusely, Which is Characteristic of Rough Surfaces, RK is Independent of Film Thickness and is the Same as RK that Characterizes Smooth Surfaces in the Limit of Large Film Thickness. at Interfaces Where Phonon Transmission Coefficients Are Low, the Thickness Dependence is Greatly Diminished Regardless of the Nature of Surface Scattering. the Film Thickness Dependence of RK is Analogous to the Well-Known Fact of Lateral Thermal Conductivity Thickness Dependence in Thin Films. the Difference is that Phonon-Boundary Scattering Lowers the In-Plane Thermal Transport in Thin Films, But It Facilitates Thermal Transport from the Substrate to the Thin Film. © 2014 American Physical Society

Liquid Phase Stability under an Extreme Temperature Gradient

Using Nonequilibrium Molecular Dynamics Simulations, We Subject Bulk Liquid to a Very High-Temperature Gradient and Observe a Stable Liquid Phase with a Local Temperature Well above the Boiling Point. Also, under This High-Temperature Gradient, the Vapor Phase Exhibits Condensation into a Liquid at a Temperature Higher Than the Saturation Temperature, Indicating that the Observed Liquid Stability is Not Caused by Nucleation Barrier Kinetics. We Show that, Assuming Local Thermal Equilibrium, the Phase Change Can Be Understood from the Thermodynamic Analysis. the Observed Elevation of the Boiling Point is Associated with the Interplay between the Bulk Driving Force for the Phase Change and Surface Tension of the Liquid-Vapor Interface that Suppresses the Transformation. This Phenomenon is Analogous to that Observed for Liquids in Confined Geometries. in Our Study, However, a Low-Temperature Liquid, Rather Than a Solid, Confines the High-Temperature Liquid. © 2013 American Physical Society