Thin film evaporation in microchannels with interfacial slip

Abstract

We investigate the role of interfacial slip on evaporation of a thin liquid film in a microfluidic channel. The effective slip mechanism is attributed to the formation of a depleted layer adhering to the substrate-fluid interface, either in a continuum or in a rarefied gas regime, as a consequence of intricate hydrophobic interactions in the narrow confinement. We appeal to the fundamental principles of conservation in relating the evaporation mechanisms with fluid flow and heat transfer over interfacial scales. We obtain semi-analytical solutions of the pertinent governing equations, with coupled heat and mass transfer boundary conditions at the liquid-vapor interface. We observe that a general consequence of interfacial slip is to elongate the liquid film, thereby leading to a film thickening effect. Thicker liquid films, in turn, result in lower heat transfer rates from the wall to liquid film, and consequently lower mass transfer rates from the liquid film to the vapor phase. Nevertheless, the total mass of evaporation (or equivalently, the net heat transfer) turns out to be higher in case of interfacial slip due to the longer film length. We also develop significant physical insights on the implications of the relative thickness of the depleted layer with reference to characteristic length scales of the microfluidic channel on the evaporation process, under combined influences of the capillary pressure, disjoining pressure, and the driving temperature differential for the interfacial transport.

Near-Wall Liquid Layering, Velocity Slip, and Solid–Liquid Interfacial Thermal Resistance for Thin-Film Evaporation in Microchannels

“…The liquid velocity slip arises from the less effective momentum transfer between the wall and liquid molecules when the liquid does not completely wet the wall. The momentum deficit is due to the imbalance between the wall adhesion force and the shear force, especially when the liquid is depleted near the wall [4,14,20]. Similarly, the temperature slip and the Kapitza resistance arise because the solid-liquid heat transfer is less efficient than the solid-solid heat transfer when the solid-liquid interactions are weak.…”

“…The bulk liquid and the liquid in the interface region have different physical properties. The liquid in the nanolayer as shown in Figures 1-3 is (1) more ordered and stratified than the bulk liquid because it is adsorbed on the wall with a crystalline or semi-solid structure for the wetting case [5] or is (2) sparse and disordered like a rarefied gas for the extremely nonwetting case [14] and is (3) mobile and flowing with an intrinsic slip velocity, u w , at the wall surface for both cases. The density, viscosity, and thermal conductivity of the nanolayer continuously decrease with increased surface hydrophobicity.…”

“…Liquid velocity slip also has a pronounced effect in the evaporating thin-film region [12][13][14]. The velocity slip effect without the temperature slip tends to elongate the thinfilm region and increase the evaporative mass flux.…”

“…Most studies of thin liquid film evaporation [9][10][11][12][13][14] have assumed that there is no temperature slip at the wall and the effect of the Kapitza resistance is negligible, so T w = T w . Atomistic simulations by Freund [15] showed that the Kapitza resistance at the solidliquid boundary is negligible at macroscopic scales but important for evaporating thin meniscus films.…”

Evaporation of an isolated liquid plug moving inside a capillary tube

“…This work gave the relationship of heat flow in terms of the contact angle, superheat and material properties and can also be applied to different tube geometries. Recently, Biswal et al [12] studied the impact of including interfacial slip in the evaporation of thin-film. Based on their semi-analytical solution, it was concluded that the interfacial slip thickens the liquid film, which in turn lowers the mass transfer from the thin-film to the vapour phase.…”