Spatters and spills: Spreading dynamics for partially wetting droplets

We present a solvable model inspired by dimensional analysis for the time-dependent spreading of droplets that partially wet a substrate, where the spreading eventually stops and the contact angle reaches a nonzero equilibrium value. We separately consider small droplets driven by capillarity and large droplets driven by gravity. To explore both regimes, we first measure the equilibrium radius vs a comprehensive range of droplet volumes for four household fluids, and we compare the results with predictions based on minimizing the sum of gravitational and interfacial energies. The agreement is good and gives a reliable measurement of an equilibrium contact angle that is consistent in both small and large droplet regimes. Next, we use energy considerations to develop equations of motion for the time dependence of the spreading, in both regimes, where the driving forces are balanced against viscous drag in the bulk of the droplet and by friction at the moving contact line. Our approach leads to explicit prediction of the functional form of the spreading dynamics. It successfully describes prior data for a small capillary-driven droplet, and it fits well to new data we obtain for large gravity-driven droplets with a wide range of volumes. While our prediction for the dynamics of small capillary-driven droplets assumes the case of thin nearly wetting droplets, with a small contact angle, this restriction is not otherwise invoked.

Automated summary

We present a solvable model to study the time-dependent spreading of partially wetting droplets on a substrate. The model considers both small droplets driven by capillarity and large droplets driven by gravity. We measure the equilibrium radius vs droplet volume for various household fluids and compare the results with predictions based on energy minimization, finding good agreement. We then develop equations of motion for the spreading dynamics, taking into account the forces balance inside the droplet and at the moving contact line. Our approach successfully describes prior data for capillary-driven droplets and fits well to new data for gravity-driven droplets.