Prediction of droplet dynamics on an incline

Droplet motion is important in discrete microfluidic operations as well as in emerging site-specific cooling techniques based on electrowetting. The present work reports on carefully characterized experiments that map the shape, advancing and receding contact angles, and contact area of droplets sliding under gravitational actuation as a function of droplet size and velocity. A pseudo-Lagrangian methodology based on the Volume Of Fluid - Continuous Surface Force (VOF-CSF) model is developed to simulate droplet motion down an incline. The model is benchmarked against 2D stationary reference-frame simulations. The terminal velocity of droplets on an incline is predicted and is in good agreement with in-house experimental measurements. The effect of using different contact angle models on the terminal velocity predictions is investigated. The numerical solution is highly sensitive to the contact angle boundary condition. The internal fluid motion in moving droplets is also explained.