Water drops on nanofiber-coated substrates: Influence of wall temperature and coating thickness on hydrodynamics and wall heat flux distribution

Abstract

Heat and mass transport during drop impact or a gentle deposition onto a heated substrate depends on the surface chemistry, morphology, thermal and mechanical properties of the substrate, as well as the substrate temperature. In particular, if a substrate is coated with a porous layer of a wettable material, the drop spreading is accompanied by the imbibition of a liquid into the layer. The wetted area is significantly enhanced in comparison with the area which can be covered by a drop spreading over a bare substrate. As a result, the liquid evaporation rate increases and the evaporation time decreases. Understanding the interaction between the liquid spreading, imbibition and evaporation is important, both for enhancement of heat transfer in cooling applications and for functionalizing of porous media. In our experimental studies, a disk of infrared-transparent calcium fluoride is used as the base substrate. The disk is coated with a submicrometer layer of black chromium nitride and an electrically conductive chromium layer, used for heating of the substrate by electrical current. Nanofiber coating layers are applied on top of the chromium layer using electrospinning. The dynamics of drop spreading is captured by a high-speed camera in a side view, the imbibition is observed with a top view camera, and the temperature distribution at the substrate-coating interface is captured by an infrared camera. Based on the transient temperature field, corresponding heat flux distributions are determined. For the detailed analysis of the interplay between hydrodynamics and heat transfer, the nanofiber coating thickness and substrate temperature have been varied. It has been found that the drop drying time and maximal imbibed area depend non-monotonously on the coating thickness. Changing the initial wall temperature leads to qualitatively different distribution of the heat flux at the substrate surface.