Role of Droplet Dynamics on Contact Line Depinning in Shearing Gas Flow

Sessile droplets exposed to shearinggas flows resist depinningowing to surface tension and contact angle hysteresis. It is knownthat contact line depinning occurs when the shearing gas flow is largeenough to deform the droplet beyond its contact angle hysteresis.This work explores the contact line depinning process by visualizinggrowing droplets on a porous layer in laminar shear gas flows. High-speedimaging of droplets revealed an oscillatory motion in droplets, whichis speculated to originate from an interaction between the drag forceand surface tension effects. This oscillatory motion creates an inertialforce within the droplet which combines with the drag force when dropletacceleration is in the stream-wise direction. The combined effectcompetes against the droplet adhesion force, setting the depinningcriteria. Analyzing droplet images revealed that droplet local velocityand acceleration (i.e., sessile droplet dynamics prior to detachmentfrom the substrate) increase with the superficial gas velocity. Atthe same time, the contact line depinning occurs at a smaller dropletsize for higher superficial gas velocities. This results in a hill-likevariation of the inertial force as a function of the convective Webernumber, We(conv), causing a local maximum in the inertialforce data (We(conv) scales the inertia effects of the shearflow to surface tension effects). For the experimental condition testedin the current study, the inertial force created in the droplet couldreach up to half of the adhesion force, making the drag force onlyresponsible for the other half to depin the droplet contact line.Even at low superficial gas velocities, which featured lower dropletoscillations, the inertial force created in the droplet was considerablewith respect to the adhesion force, reaching around one-third of theadhesion force.

Automated summary

This work investigates the depinning process of sessile droplets exposed to shearing gas flows. High-speed imaging reveals an oscillatory motion in droplets, speculated to be caused by the interaction between drag force and surface tension. The analysis shows that droplet dynamics prior to detachment increase with superficial gas velocity, and contact line depinning occurs at smaller droplet sizes for higher gas velocities.