Effects of solid/liquid interface on size-dependent specific heat capacity of nanoscale water films: Insight from molecular simulations

Highlights

• Effects of thickness, interface and temperature on thin film specific heat capacity are investigated.

• Substantial differences are found between freestanding, sessile, and confined films.

• Interface effect is demonstrated by comparative analysis such as density and interface energy.

• Temperature effect is negligible compared to the size and interface effects in the temperature range.

• Form of a thin liquid film greatly affects its thermophysical properties.

Abstract

The thermophysical properties of an evaporative thin water film that is near the triple line may change because of the effects of size, geometry, interface, temperature, etc. However, the factor-dependence of the properties was seldom considered for the corresponding nanoscale transport phenomena or processes. In this work, the effects of the interface and temperature on the thickness-dependent specific heat capacities of nanoscale water films are investigated by comparing the values of the freestanding film between vacuum, the sessile film on a copper plate, and the confined film between two copper plates. It is found that the specific heat capacities of the thin water films decrease exponentially with reducing thickness, with the confined film having the highest value, followed by the sessile film and the freestanding film, at the same film thickness. The solid/liquid interface presents a substantial impact on the increase in the specific heat capacity, demonstrated by the analysis of density profile, vibration density of state of hydrogen atoms of water molecules, radial distribution function, and interface energy, compared between the films of different types. The analysis of the temperature effect by comparing the vibration densities of state in different regions of the films at different temperatures shows that the temperature has a negligible influence as compared to the size and interface effects. Therefore, the form of a thin water film will greatly affect its thermophysical properties, including specific heat capacity, which should be considered in applications such as evaporation of thin water film, nanoconfinement transfer in membrane, and interface separation of porous media.

Introduction

The heat transfer of evaporative thin water film near the three-phase contact line plays a key role in many applications, such as phase-change cooling, inkjet printing, controlled deposition of self-assembled surfaces, and microfluidic chips. The three-phase contact line can be divided into the intrinsic meniscus region, the thin water film region (or the transition region), and the adsorption layer area, according to the relationship between the force and spacing between the liquid/gas interface and the solid/liquid interface [1]. The thickness in the thin water film region is usually between 10 ∼ 1000 nm. It is reported that the vast majority of evaporative heat transfer occurs in this small transition region [2]. However, changes in the thermophysical properties of thin water films due to reduced size have not attracted enough attention, even though we know thermophysical properties are the basis of heat transfer research of fluid. In fact, because of the interface effect caused by the limitations of size and geometry, there are significant differences between the thermophysical properties of fluids in nanoconfined spaces and the bulk value, including the transport properties such as thermal conductivity and viscosity [3], [4], [5], [6], [7], [8]. Even if there is no effect of size or interface, the temperature effect on liquid molecules structure [9] and hence the thermophysical properties in confined space should be considered in phase-change heat transfer studies. Due to the limitations of size and geometry, it is also difficult to measure the thermophysical properties of the thin water films at the nanoscale. In addition, freestanding thin water films can exist in the Leidenfrost phenomenon on a hot surface [10], floating water bridge in an electric field [11], amorphous polymer to a vacuum, etc. [12] However, the study of the thermophysical properties of the thin water films in such phenomena is often ignored, although the thermophysical properties of freestanding thin water films should significantly affect the corresponding phenomena or processes. For these different thin water films, namely the confined, sessile, and freestanding films, the theoretical and computational corrections of the macroscopic models can be revised by a microscopic analysis, which needs systematically study of the effects of size, interface, and temperature on the thermophysical properties of nanoscale thin water films.

Heat capacity is one of the fundamental thermophysical properties in the heat transfer study of fluid. Tombari et al. [13,14] investigated the thermal characteristics of water confined in silica nanopores through static and dynamic calorimetry during heating and cooling and found that the specific heat capacity has a broad peak and it gets higher while the amount of water increases during heating. However, the features found on heating were not observed on cooling due to a temperature hysteresis of the peaks at a certain temperature. Nagoe et al. [15] used adiabatic calorimetry to study the thermal characteristics of liquid benzene confined in silica nanopores at low temperatures. The isobaric heat capacity increases initially and decreases afterward, while the pore diameter increases since there is the ordering and excitation in the agglomeration of molecules. Gautam et al. [16] utilized inelastic neutron scattering measurements and MD simulations to measure the isochoric specific heat capacities of nanoconfined propane. At a temperature lower than the melting point, the change of the isochoric specific heat capacity of confined propane is consistent with that of bulk propane. While the temperature is higher, it is also consistent with the isochoric specific heat capacity of saturated liquid propane.

Molecular dynamics (MD) is an important method used in chemical physics, materials science, and biophysics to solve the multi-body problem at the atomic and molecular levels by describing and analyzing a group of molecules (ensemble). It can study the thermophysical properties of liquid water, including thermal conductivity, shear viscosity, and specific heat capacity, using rigid water models [17]. Faraone et al. [18] performed the neutron scattering experiments for confined water and observed a fragile-to-strong liquid transition, while Xu et al. [19] correlated the measured isobaric specific heat maxima with the fragility transition by the MD simulation method. Kumar et al. [20] calculated the thermodynamic properties of water confined between two plates with a gap of 1.10 nm and observed an anomalous shift to lower temperatures relative to the bulk values. Recently, Morshed et al. [9] performed an MD simulation to study the mobility of liquid molecules and found it reduces significantly because of the confinement in a solid nano-gap. Later, Mahmud et al. [21] simulated the confined liquid argon between nano-gap and observed that the molar heat capacity of nanoconfined liquid argon is higher than that of the bulk liquid for a specific range of gap thickness through the MD simulation method. However, it is observed that the molar heat capacity of nanoconfined liquid argon is close to that of the bulk liquid as the gap thickness is greater than 6.14 nm for the confined liquid at 100 K. Mahmud et al. [22] also calculated the molar heat capacity of liquid argon confined in a nano-gap at different temperatures. The decrease rate of molar heat capacity of nanoconfined liquid argon is obviously high than that of the bulk when the liquid argon is confined between a certain range of gap thickness. However, the value approached that of bulk liquid beyond this range, because of the change of the heat carrier number in liquid, variation of the thermal resistance, the relaxation of density oscillation, and increase in molecular mobility.

It can be seen from the above literature survey that the experimental researches on the specific heat capacity are very limited and mostly for thin water films in confined space, and the molecular simulation researches are also limited to the nanoscale thin water film confined between two plates. At present, there is still a lack of comparative and systematic studying on the specific heat capacities of confined film, sessile film, and freestanding film. In this work, we study using the MD method the thickness-dependent specific heat capacities of nanoscale water films, including freestanding film between vacuum, sessile film on copper plate, and confined film between two copper plates, with consideration of the interface and temperature effects on the specific heat capacity. The interface effect on the specific heat capacities is discussed by comparison of the thin water films. Influencing factors including the density distribution, the velocity auto-correlation function (VACF), and the vibration density of state (VDOS) of hydrogen (H) atoms of water molecules in the thin water films are analyzed to explore the thickness-dependent specific heat capacities. The radial distribution function (RDF) is also used to study the interface effect. The influence of temperature on the specific heat capacity between 280 ∼ 330 K is further investigated, combined with the VDOS curve of hydrogen atoms in the thin water films at these typical temperatures.