Internal flow patterns of a droplet pinned to the hydrophobic surfaces of a confined microchannel using micro-PIV and VOF simulations

We present both experimental results and numerical simulations of the fluid dynamics of a droplet pinned to the hydrophobic surfaces of a confined microfluidic channel, as a result of contact angle hysteresis. Internal circulations in the droplet are observed and quantified using micro-particle image velocimetry (μPIV). As the channel inlet velocity increases, the difference between the contact angles at the front and the rear part of the contact line is also increased, while the equilibrium Young's contact angle remains essentially constant. Numerical simulations based on a Volume-Of-Fluid (VOF) method combined with a Laplacian filter for the phase function are also performed to consider contact angle hysteresis effects. Major quantities from the simulations, including the velocity distribution inside the droplet, the contact angles, and the vortex structures, show good agreement with experimental results. In addition, force balance models of the pinned droplet have been built for various inlet conditions, indicating that the adhesion force at the side walls and the blockage of the droplet have significant effects on the liquid motion within the droplet. The recirculation flow rate inside the droplet is found to vary linearly with the Capillary number.