Effects of fluid rheology and wetting properties on the droplet motion by a shear flow

Abstract

AbstractThe paper presents an experimental investigation of the detaching mechanisms of adhering droplets exposed to a turbulent air flow, focusing on the specific case of water, glycerine and ethanol droplets on a PMMA and coated silicon wafer surface. A rectangular Plexiglas channel (22 x 22 mm) with a replaceable substrate is used. A controlled air flow is established and increased during the experiment, until the droplet deforms and starts to move from its initial position along the surface. Shadowgraph technique is applied to measure the droplet behavior. The images are automatically analyzed by an edge‐detection algorithm to gain information on the droplet contour, the contact angles and the contact line movement. It was found that the critical velocity required for the onset of the droplet movement decreases with the droplet volume. As the viscosity of the droplet increases, the critical velocity increases. For a particular liquid it is easier to detach the droplets on coated silicon wafers compared to PMMA due to the increase of the hydrophobicity of the substrate. Glycerine droplets move at a slow rate over the surface due to the high viscous damping and reach a certain position only at higher air velocities. The results show that contact angle hysteresis seems not to be the unique factor influencing the critical velocity, since several parameters superpose and affect the droplet detachment process.