Investigation of Sessile Droplet Wetting, Dynamics and Evaporation on Micro-Structured Substrates

Abstract

The wetting, spreading and drying of pure liquid and nanofluid sessile droplets on a patterned solid surface were investigated systematically in terms of liquid and surface property. The patterned nickel surface was characterized with diamond, circular, hexagon and rectangular pillars. The size ratio between interval and pillars varies from 1.0 to 5.0. The study was firstly carried out for the effect of pure water droplet size on liquid spreading and droplet evaporation process on diamond-shape micro structured substrate with LInterval/LPillar=1.0. Larger amount of liquid leads to a larger wetting area. With fixed substrate (diamond, LInterval/LPillar=1.0) and droplet size (1 μm), mixture of DI water and Ethanol (volume ratio varies from 0.5 to 2.0) was used for generating droplets with different surface tension and evaporation coefficient. Fingering shape would generate on the contact line. With higher concentration of ethanol, the fingering effect is stronger and appeared in a shorter time. The contact area shrinks when increase the size ratio of interval and pillar. This would reduce the length of the contact line, and thus slow down the liquid evaporation. The role of pillar shape was examined based on time for complete evaporation. The effect of surface material on evaporation process was conducted on nickel and PMMA substrate fabricated with the same design. Additionally, investigations were conducted with solutions consisted with nanoparticles and DI water. The mixture were made at different weight ration to achieve concentration of nanoparticles varies from 0.02% to 0.18% with an interval at 0.04%.