Enhanced Heat Flow between Charged Nanoparticles and an Aqueous Electrolyte.

Heat transfer through the interface between a metallic nanoparticle and an electrolyte solution has great importance in a number of applications, ranging from nanoparticle-based cancer treatments to nanofluids and solar energy conversion devices. However, the impact of the surface charge and dissolved ions on heat transfer has been scarcely explored so far. In this study, we compute the interface thermal conductance between hydrophilic and hydrophobic charged gold nanoparticles immersed in an electrolyte using equilibrium molecular dynamics simulations. Compared with an uncharged nanoparticle, we report a 3-fold increase of the Kapitza conductance for a nanoparticle surface charge of +320 mC/m2. This enhancement is shown to be approximately independent of the surface wettability, charge spatial distribution, and salt concentration. This allows us to express the Kapitza conductance enhancement in terms of the surface charge density on a master curve. Finally, we interpret the increase of the Kapitza conductance as a combined result of the shift of the water density distribution toward the charged nanoparticle and an accumulation of the counterions around the nanoparticle surface which increase the Coulombic interaction between the liquid and the charged nanoparticle. These considerations help us to apprehend the role of ions in heat transfer close to electrified surfaces.

Enhanced local viscosity around colloidal nanoparticles probed by equilibrium molecular dynamics simulations.

Nanofluids-dispersions of nanometer-sized particles in a liquid medium-have been proposed for a wide variety of thermal management applications. It is known that a solid-like nanolayer of liquid of typical thicknesses of 0.5-1 nm surrounding the colloidal nanoparticles can act as a thermal bridge between the nanoparticle and the bulk liquid. Yet, its effect on the nanofluid viscosity has not been elucidated so far. In this article, we compute the local viscosity of the nanolayer using equilibrium molecular dynamics based on the Green-Kubo formula. We first assess the validity of the method to predict the viscosity locally. We apply this methodology to the calculation of the local viscosity in the immediate vicinity of a metallic nanoparticle for a wide range of solid-liquid interaction strength, where a nanolayer of thickness 1 nm is observed as a result of the interaction with the nanoparticle. The viscosity of the nanolayer, which is found to be higher than its corresponding bulk value, is directly dependent on the solid-liquid interaction strength. We discuss the origin of this viscosity enhancement and show that the liquid density increment alone cannot explain the values of the viscosity observed. Rather, we suggest that the solid-like structure of the distribution of the liquid atoms in the vicinity of the nanoparticle contributes to the nanolayer viscosity enhancement. Finally, we observe a failure of the Stokes-Einstein relation between viscosity and diffusion close to the wall, depending on the liquid-solid interaction strength, which we rationalize in terms of the hydrodynamic slip.

Thermally induced stress in a nanoconfined gas medium.

Molecular dynamics simulations of static argon gas at three different levels of rarefaction are conducted for a channel of 5.4 nm height to investigate the simultaneous effect of the wall force field and the gas temperature on the stress distribution along the channel height. Using the interactive thermal wall model, different temperatures are applied on the channel walls to be able to investigate the effect of the wall temperature and the induced heat flux through the gas medium on the stress distribution. Considering the monoatomic neutral argon gas, the kinetic, particle-particle virial, and surface-particle virial are considered for computing the stress distribution along the channel height. The normal stress components in the bulk gas region are distributed isotropically regardless of the gas density, temperature, and induced heat flux through the domain, while an anisotropy is observed due to the presence of the surface-particle virial. As the gas becomes hotter, the velocity of the gas atoms increases, and thus the kinetic stress component also increases. Besides, the gas density in the wall force field region reduces which eventually attenuates the surface-particle and particle-particle virial stress within 1 nm from each wall. This effect was also observed as the gas becomes cooler. It is shown that the combination of gas density, wall temperature, and induced heat flux are the main parameters which determine the distribution of stress within the gas medium especially in the wall force field region where repulsive and attractive interactions exist.