Impact and spreading of a microdroplet on a solid wall

Abstract

Impact and spreading of a viscous micro droplet on dry solid surfaces are studied computationally using a finite-difference/front-tracking method. The problem is motivated by single cell epitaxy developed for printing biological cells on a solid substrate using ink-jet printer technology. In this study, we consider impact and spreading of a simple droplet on a partially wetting substrate as a first step in developing a complete compound droplet model for the single cell epitaxi. The numerical method is general and can treat non-wetting, partially wetting and fully wetting cases but the focus here is placed on partially wetting substrates. The contact angle is specified dynamically using the empirical correlation given by Kistler (1993). In addition, a precursive film model is also used especially for the highly wettable cases, i.e., the static contact angle is smaller than 30° due to numerical difficulty of resolving thin liquid later penetrating into surrounding gas near the solid surface. The numerical method is first applied to simple droplet spreading and the results are compared with experimental data of Sikalo et al. (2005). Then the effects of governing non-dimensional numbers on the spreading rate, apparent contact angle and deformation of the droplet are investigated. Finally a few preliminary results are presented for the impact and spreading of a compound microdroplet on a partially wetting surface.