An experimental investigation of evaporation of sessile droplets is presented on nano-porous alumina surfaces with different pore distribution morphologies and pore sizes. Evaporation can be considered as a quasi-steady-state process, such that the vapor concentration distribution above the droplet satisfies the Laplace equation, but with a time-varying droplet surface. For benchmarking, the evaporation of sessile water and ethanol droplets is also investigated on standard borosilicate glass and Teflon surfaces respectively, and results are compared with the previous work of Picknett and Bexon (1977). Contact angle variation with time is also recorded and high speed videos showing the spreading process of droplets on nano-porous surfaces are taken. The results clearly show that nano-structuring is an effective tool to control wettability as well as the diffusive evaporation process.

Dynamics of micro droplets, coalescence underneath an inclined hydrophobic surface is explored, experimentally as well as computationally. Such a situation often arises during dropwise condensation, atmospheric dew formation, condensation in green houses, and metal vapor condensation during enrichment process, etc. Coalescence induced instability in the pendant mode is an effective means of passively enhancing heat transfer coefficient during dropwise condensation. Inclined substrates have natural advantages in terms of rendering effective passive sweeping of drops from the substrate, thereby exposing fresh preferred sites for renucleation. As compared to coalescences of sessile droplets, pendant mode induces flow instabilities at a much faster rate, thereby enhancing the associated heat/ mass transport characteristics. Against this background, the present study reports an experiment involving the coalescence of water drops in pendant mode on an inclined hydrophobic copper substrate of size 20 mm × 20 mm × 2 mm. The substrate has been prepared by developing a monolayer of n-octadecanethiol on it by chemical action. This work focuses on fundamental understanding of the mechanism of drop coalescence underneath an inclined superhydrophobic substrate to provide aid in controlling and promoting dropwise mode of condensation in order to increase efficiency of condenser or to aid in enrichment of heavy liquid metals in closed vacuum condition. The coalescence process gets initiated by the extra available surface energy which gets released in the process. Immediately afterwards, the process of oscillating free surface formed during the coalescence is limited by viscous and inertia forces. The free surface oscillations can last ~ 10 to 100 milliseconds, depending on the size of droplets and their thermophysical properties. Depending on the local wettability, size of drops which are coalescing, substrate inclination and pinning/de-pinning behavior, droplet instability conditions are generated, which result in its slide-off motion on the substrate.