Abstract
Understanding thermo-osmosis in nanoscale channels and pores is essential for both theoretical advances of thermally induced mass flow and a wide range of emerging industrial applications. We present a new mechanistic understanding and quantification of thermo-osmosis at nanometric/sub-nanometric length scales and link the outcomes with the non-equilibrium thermodynamics of the phenomenon. The work is focused on thermo-osmosis of water in quartz slit nanochannels, which is analysed by molecular dynamics (MD) simulations of mechano-caloric and thermo-osmotic systems. We investigate the applicability of Onsager reciprocal relation, irreversible thermodynamics, and continuum fluid mechanics at the nanoscale. Further, we analyse the effects of channel size on the thermo-osmosis coefficient, and show, for the first time, that these arise from specific liquid structures dictated by the channel size. The mechanical conditions of the interfacial water under different temperatures are quantified using a continuum approach (pressure tensor distribution) and a discrete approach (body force per molecule) to elucidate the underlying mechanism of thermo-osmosis. The results show that the fluid molecules located in the boundary layers adjacent to the solid surfaces experience a driving force which generates the thermo-osmotic flow. While the findings provide a fundamental understanding of thermo-osmosis, the methods developed provide a route for analysis of the entire class of coupled heat and mass transport phenomena in nanoscale structures.
Highlights
Thermo-osmosis is a non-equilibrium heat and mass transfer cross-phenomenon
While the other observed mechanisms, generically referred to as the thermophoresis, involve thermally induced migration of particle mixtures, e.g., thermally induced flux of aqueous species relative to the fluid, thermo-osmosis describes the migration of a single-component fluid due to thermal gradients
There is a debate between two approaches for describing the underlying mechanism for thermo-osmosis, namely the interfacial approach and the energetic approach.[18]
Summary
Compared to the theoretical studies of thermophoresis, in particular thermal diffusion,[9,10,11,12,13] the microscopic mechanistic understanding of thermo-osmosis is rather incomplete, its macroscopic thermodynamical description has been widely discussed and used both in modelling multiscale– multiphase couplings problems[14,15] and in experimental works.[16,17] there is a debate between two approaches for describing the underlying mechanism for thermo-osmosis, namely the interfacial approach and the energetic approach.[18]. The existing formulas for the thermo-osmotic coefficient are derived on the basis of a local continuum theory,[21] e.g. Navier–Stokes equation, where assumptions are made on the fluid properties near the solid surface including the excess enthalpy distribution and fluid viscosity Such continuum approximations may be sufficient for solid–fluid interfaces at larger length scales. Understanding water interaction in quartz or other silica phases is of interest to many fields such as geology,[28] biology,[29] physics[30,31] and chemistry.[32,33] This specific system is relevant to membrane science as there are membranes made up quartz and other silica phases such as hydrophilic quartz fibre membranes[34] and silica membranes.[35] While a number of studies have been dedicated to the interactions of water with quartz and other silica phases, research on temperature effects, such as thermo-osmosis, in quartz–water systems is very limited. The systems analysed, the analysis methods, and the insights of this work provide a solid foundation for future research in the field of thermophoretic phenomena
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