Sarith P Sathian

A better understanding of submicron-scale heat transfer is rapidly gaining interest due to the complex phenomena involved in nanometer scales. We discuss the role of interfacial resistance, in particular that of curvature effects, and the possibility of achieving high temperatures inside the particles without creating a phase transition in the surrounding fluid. The heat transfer from a heated nanoparticle into surrounding fluid is studied using molecular dynamics (MD) simulations. The results show that the particle size and wetting strength between the nanoparticle-liquid influence the heat transfer characteristics. The interfacial conductance and Kapitza length for a model solid-liquid interface were calculated. Both quantities are found to be strongly dependent on particle size and temperature. Smaller nanoparticles are observed to have a stronger bonding with the interfacial fluid when the temperature of the particle is higher, while larger nanoparticles have better affinity with the liquid at lower temperatures.

In this paper, we study the structural, electronic, vibrational, thermoelectric and elastic properties of tin disulfide (SnS 2 ) using first principles density functional theory calculations in the pressure range 0 ≤ p ≤ 5 GPa. The variation of lattice constant along c-axis is found to be higher than that along the a-axis which significantly affects the properties. The electronic band gap is observed to decrease with the applied pressure. The Raman shift of E g and A 1g modes increases with applied pressure. Furthermore, SnS 2 remains dynamically stable up to 5 GPa. Thermoelectric properties such as thermopower (S), electrical conductivity (σ), power factor (S 2 σ) show anisotropy. While the in-plane direction is more dominant at ambient pressure, the out-of-plane is more dominant with the increase in pressure. The calculated power factor is higher in the hole concentration than the electron concentration in the defined pressure range at room temperature. This suggests that SnS 2 could be an excellent candidate material of p-type thermoelectric under high pressure conditions.

A new method is proposed to model the thermal boundary resistance (TBR) at the nanoscale, solid-liquid interface from macroscopic observables that characterize a nanoscale interface. We correlated the TBR with thermodynamic state variables, material properties, and geometric parameters to derive a generalized relationship with the help of data-driven heuristic algorithms. The results show that TBR can be expressed in terms of physical observables of the systems and material-specific parameters. We investigated the mutual independence of descriptor variables and quantified the weightage for each observable parameter in the TBR models. The interfacial liquid layering has a robust correlation with TBR. However, for systems with phonon size effects and under extreme thermodynamic conditions, the work of adhesion and system geometry also affects the variation in TBR. The data-driven approach followed in this study helps us gain better insight into the mechanism of TBR at nanoscale solid-liquid interfaces and shows significant improvement in our knowledge about interfacial thermal transport.

The controlled transport of water through nanoscale devices is an important requirement in the design and development of various nanofluidic systems. Molecular dynamics simulations are performed to investigate the phonon coupling induced thermophoretic transport of water through a carbon nanotube (CNT). Phonon coupling is believed to have a significant role in the transport of heat at the liquid-solid interface. The thermally induced vibrational modes of water-filled and empty CNTs are examined at various thermal gradients. The spatial asymmetry along the length of a CNT due to the imposed thermal gradient contributes to the diffusion enhancement of water confined in the CNT, but does not have a strong correlation with the applied thermal gradient. Analysis shows that the vibrational modes present in the center-of-mass oscillations of CNTs do not play any significant role in the development of the thermophoretic force on water. The low-frequency phonon vibrational modes of CNTs are suppressed due to the phonon coupling between water and the CNT. Also, we observed that the spectral heat current across the water-CNT interface dominates at frequencies below 5 THz, which is the same frequency range as radial breathing modes observed in the vibrational spectrum of CNT. This observation leads us to the conclusion that the coupling of radial breathing phonon modes contributes significantly to thermophoresis. This study substantiates the existence of phonon coupling at the water-CNT interface and quantifies the accumulated heat transfer across the interface.

The details of laser reflection method (LRM) for the determination of shear stress in low density transitional flows are presented. The method is employed to determine the shear stress due to impingement of a low density supersonic free jet issuing out from a convergent divergent nozzle on a flat plate. The plate is smeared with a thin oil film and kept parallel to the nozzle axis. For a thin oil film moving under the action of aerodynamic boundary layer, the shear stress at the air-oil interface is equal to the shear stress between the surface and air. A direct and dynamic measurement of the oil film slope generated by the shear force is done using a position sensing detector (PSD). The thinning rate of the oil film is directly measured which is the major advantage of the LRM. From the oil film slope history, calculation of the shear stress is done using a three-point formula. The range of Knudsen numbers investigated is from 0.028 to 0.516. Pressure ratio across the nozzle varied from 3,500 to 8,500 giving highly under expanded free jets. The measured values of shear, in the overlapping region of experimental parameters, show fair agreement with those obtained by force balance method and laser interferometric method.

The behaviour of nano-scale jets emanating from a reservoir under the action of an external force is studied using Langevin dynamics simulations. The advantage of employing a Langevin thermostat to maintain the temperature of the fluid reservoir is highlighted. The effect of hydrodynamic screening introduced by the Langevin thermostat is considered. It is seen that the nature of thermostat plays a crucial role in simulating the onset of instabilities in the liquid structures. A plunger action has been chosen to initiate jet generation. Langevin dynamics is seen to be able to model the physics of nano-scale jets quite accurately. The study also shows that the Langevin dynamics simulations are capable of capturing the dynamics of nano-scale liquid jets, as the nanojet simulated by this method is found to behave in close agreement with the theoretical predictions for nano-scale jets.

We introduce an analytical method to predict the slip length (L s) in cylindrical nanopores using equilibrium molecular dynamics (EMD) simulations, following the approach proposed by Sokhan and Quirke for planar channels [39]. Using this approach, we determined the slip length of water in carbon nanotubes (CNTs) of various diameters. The slip length predicted from our method shows excellent agreement with the results obtained from nonequilibrium molecular dynamics (NEMD) simulations. The data show a monotonically decreasing slip length with an increasing nanotube diameter. The proposed EMD method can be used to precisely estimate slip length in high slip cylindrical systems, whereas, L s calculated from NEMD is highly sensitive to the velocity profile and may cause large statistical errors due to large velocity slip at the channel surface. We also demonstrated the validity of the EMD method in a BNNT-water system, where the slip length is very small compared to that in a CNT pore of similar diameter. The developed method enables us to calculate the interfacial friction coefficient directly from EMD simulations, while friction can be estimated using NEMD by performing simulations at various external driving forces, thereby increasing the overall computational time. The EMD analysis revealed a curvature dependence in the friction coefficient, which induces the slip length dependency on the tube diameter. Conversely, in flat graphene nanopores, both L s and friction coefficient show no strong dependency on the channel width.

Combined kinetic theory-hydrodynamics treatment has been proven effective in the prediction of biomolecule dynamics, generally if a single biomolecule is present in the bulk solvent. But the validity of such a theory in many physiological conditions is controversial. In the present study, a sample protein surrounded by other large biomolecules is approximated as the protein in a cylindrical nanopore. The hydrodynamic radius of the protein is chosen as an indicator to check whether one of the widely used kinetic theoryhydrodynamics relation namely Stokes-Einstein-Debye relation, is genuine for confined conditions of the protein. It has been found that Stokes-Einstein-Debye relation cannot be satisfied by the protein if confinement dimensions are very close. The reason for the violation can be attributed to van der Waals interaction between pore and the protein.

The anisotropic heat transport in graphene-CNT based materials provoked the development of three-dimensional pillared-graphene (PG) systems. In this study, we performed non-equilibrium molecular dynamics simulations to analyze PG thermal conductivity and thermal boundary conductance. For the first time, we have considered the influence of pillar chirality and the temperature effect on PG heat transport. We analyzed the influence of pillar chirality and pillar length on the in- and out-of-plane transport properties. For the temperature-dependent analysis, the chosen temperatures were in the range of 100 K to 500 K. To elucidate the mechanism underlying the heat transport, we investigated the phonon density of states (DOS) in the different regions of PG systems. The overlap factor was calculated to quantify the mismatch in the phonon DOS profiles. Across the pillar region, the overlap factor correlates directly with the thermal boundary conductance. When heat is transported in an out-of-plane direction, the zig-zag PG system performs better than the armchair PG system. The atomic arrangement at the graphene-CNT interface plays an inevitable role in limiting heat transport in PG systems. The calculated phonon energy in the zig-zag PG interface is higher than that in the armchair PG interface.

We investigated the effects of the chirality of carbon nanotubes (CNTs) on water transport using molecular dynamics simulations. For the study, we considered CNTs with similar diameter and varying chiralities, obtained by altering the chiral indices (n,m) of the nanotubes. The tubes with an armchair (n = m) structure show the maximum streaming velocity, flux, flow rate enhancement and slip length, whereas the corresponding values are lower for chiral (n ≠ m) tubes, and are the lowest in zigzag (m = 0) CNTs. The difference in flow rates with varying tube structures can be primarily attributed to the alteration in potential energy landscape experienced by the water molecules, leading to changes in the friction coefficient at the fluid-solid interface. The water molecules experienced the least resistance to flow in an armchair tube, while the force exerted by the CNT surface on the water molecules increased monotonically with the change in the CNT type to chiral and then to zigzag. The chirality effects on water transport are, however, found to decrease with an increase in tube diameter. Furthermore, an analysis of the influence of the CNT type on ion (Na+ or Cl-) transport in water-filled CNTs showed the interaction energy of ions with water to be much higher than that with the CNT surface, demonstrating minimal dependence of ion transport on the chiral structure. Hence, the tube chirality should be considered an ineludible factor in controlling the water transport through CNTs and in the designing of novel devices in nanotechnology.