Dynamic Contact Angle on a Surface with Gradient in Wettability

The retention and drainage of water on heat exchangers is extremely important in air-conditioning, refrigeration, and heat-pumping systems. In this work, droplets of varying sizes sliding on an inclined heat exchanger materials with and without a wettability gradient are observed using a high-speed camera. The dynamic contact angles, the shape evolution and the velocity of the droplet are obtained by image processing. Aluminum and copper surfaces are examined and the gradient is created by partially treating the base surface. The hypothesis is that the momentum of a sliding droplet on a treated surface will push the droplet onto the part without any treatment, so that water retention and drainage can be improved with limited surface treatment. It is found that the dynamic contact angle, the shape evolution and the velocity can be very different for droplets sliding in a wettability-increasing direction when compared to those sliding in a wettability-decreasing direction. The results are very important for the design of specialized heat transfer surfaces operating under dehumidification or defrosting conditions.