Evaporation Of A Nanodroplet On Rough Substrate

When the surrounding vapor pressure is not saturated, a droplet will evaporate, which is commonly seen in our daily life. This phenomenon is widely used in industries, such as cooling, printing, washing and coating. Recently, due to its new applications(e.g., inkjet printing, DNA stretching and DNA mapping) in the fields of technology and medicine, this topic becomes even more popular. Although studies on evaporation of macroscopic and mesoscopic droplets have achieved significant progress, Knowledge on the evaporation of nanodroplet is still in lack. In order to make the best use of the evaporation process, it is necessary to extend the droplet size to nanoscal.In the beginning of this thesis, we introduce some concepts and basic knowledge on substrate wetting, and wetting functions for different kind of substrates were given. Then, some hot topics were listed out, for example, studies on the evaporation models, the “coffee ring effect” and its influence factors. Chapter four is the main work of this paper. In order to investigate the effect of substrate roughness on nanodroplet evaporation, molecular dynamics simulations were carried out in both hydrophilic and hydrophobic systems. Finally, the article?s summary and prospects.In this work substrates were classified into two kinds: hydrophobic and hydrophilic substrates. Some roughness was added to the substrates. Comparing with the evaporation process of nanodroplets on a smooth substrate, we found that substrate roughness could make the hydrophilic substrate more hydrophilic, while make the hydrophobic substrate more hydrophobic. This leads to the increase of evaporation rate of nanodroplet on a hydrophilic substrate, and the reduction of evaporation rate of nanodroplet on a hydrophobic substrate. As the sphericity, which is closely related to the evaporation rate, is the indicator of droplet deformation, it can visually show that substrate roughness can make nanodroplet on hydrophilic substrate deform stronger, while make nanodroplet on hydrophobic substrate deform weaker. This agrees with what we saw in the simulation. In addition, substrate roughness also has an effect on the evaporation model. On smooth substrate constant contact angle model is always found, but when the substrate is rough the evaporation model turned out to be a mixed model. Both contact line “pinning” and “depinning” phenomenon were not seen in the rough system.