Contribution of droplet dynamics on contact line depinning in shearing gas flow

The depinning of sessile drop contact line with a solid substrate is a complex phenomenon that involves a complex balance of forces at the three-phase contact line. When a droplet is exposed to shearing gas flow, the drag force from the core gas flow tries to deform the droplet and depin its contact line. This external force, however, is competed against by contact angle hysteresis which describes the maximum deformation of the droplet without being dislodged from the surface. The adhesion force that pins the droplet to the solid surface is a function of the droplet surface tension, size, and its upstream and downstream contact angles. In this study, dynamics of droplets exposed to shearing gas flow are investigated by high speed imaging. We observed that droplets undergo an oscillatory pattern before contact line depinning. This oscillatory pattern induces inertia force within the droplet. When the droplet accelerates in streamwise direction, the inertia force adds to the drag force and the combined effect compete against the adhesion force. Our results indicate that droplet inertia force can reach up to half of the adhesion force.