Hierarchical thermal transport in nanoconfined water

Abstract

The structure of nanoconfined fluids is particularly non-uniform owing to the wall interaction, resulting in the distinctive characteristic of thermal transport compared to bulk fluids. We present the molecular simulations on the thermal transport of water confined in nanochannels with a major investigation of its spatial distribution under the effects of wall interaction. The results show that the thermal conductivity of nanoconfined water is inhomogeneous and its layered distribution is very similar to the density profile. The layered thermal conductivity is the coupling result of inhomogeneous density and energy distributions that are generally diametrical, and their contributions to the thermal conductivity compensate with each other. However, the accumulative effect of water molecules is really dominating, resulting in a high thermal conductivity in the high-density layers with the low-energy molecules, and vice versa. Moreover, it is found that the adsorptive and repulsive interactions from solid walls have different roles in the hierarchical thermal transport in nanoconfined water. The adsorptive interaction is only responsible for the layered distribution of thermal conductivity, while the repulsive interaction is responsible for the overall thermal conductivity; accordingly, the thermal conductivity is independent of the strength of water–solid interactions. The identified hierarchical thermal transport in nanoconfined water and its underlying mechanisms have a great significance for the understanding of nanoscale thermal transport and even the mass and energy transport of nanoconfined fluids.

Unexpected Behavior in Thermal Conductivity of Confined Monolayer Water

“…These vacancies gradually reduce with the increase of ρ A and the linear increase of thermal conductivity of confined water. The increase in thermal conductivity can be attributed to the increasing heat flux carrier, namely, the increasing number of water molecules in the channel, which has been fully discussed in previous literature …”

“…The increase in thermal conductivity can be attributed to the increasing heat flux carrier, namely, the increasing number of water molecules in the channel, which has been fully discussed in previous literature. 27 Afterward, if more water molecules are put in the channel, a large-scale ordered quadrilateral structure is formed (see Figure 2c) just like a fishing net, which is called structure SII. This near-square symmetry structure can be demonstrated by the structure factor of the confined water corresponding to the case of ρ A = 12.5 nm −2 shown in Figure 2d, which is comparable to the structure of confined water from previous studies.…”

“…Zhou et al reported similar commensurability effects in the specific heat capacity of nanoconfined water owing to the structure change of water adjacent to the walls. In recent years, we have systematically studied the thermal conductivity of nanoconfined water and revealed that its anisotropy, size dependence, and inhomogeneous distribution are utterly different from those of bulk water. , In addition to the thermophysical properties, water could present many fantastic kinetic and dynamical behaviors . Experimental studies − have reported the ultrafast transport of water through graphene oxide (GO) membranes or in the graphene nanocapillary, which is several orders of magnitude in enhancement compared to the prediction with non-slip conditions.…”

“…In recent years, we have systematically studied the thermal conductivity of nanoconfined water and revealed that its anisotropy, size dependence, and inhomogeneous distribution are utterly different from those of bulk water. 26,27 In addition to the thermophysical properties, water could present many fantastic kinetic and dynamical behaviors. 28 Experimental studies 29−31 or in the graphene nanocapillary, which is several orders of magnitude in enhancement compared to the prediction with non-slip conditions.…”

Local thermal conductivity of inhomogeneous nano-fluidic films: A density functional theory perspective

“…They also revealed that the thermal energy is less effectively transferred through the first adsorption layer near the wall than that in the central region. Recently, Zhao et al [19] presented the molecular simulations on the thermal transport of water confined in nanochannels. They also found the inhomogeneous profiles of thermal conductivity, which is nearly in phase with the density profile.…”