Experimental and CFD Study of Internal Flow inside a Sessile Microdroplet undergoing Lateral Vibration

In this study, the internal flow pattern of a sessile microdroplet undergoing a lateral vibration was analyzed by using both the experimental and CFD simulation methods. By initially staining the droplet partially with fluorescent dye, the main flow inside the laterally vibrating microdrop was experimentally demonstrated to be that the main fluid flows downward along the central axis and ascends upward along the surface to form two counterflow circuits. Experimental evaluation of fluid mixing inside the droplets verified that the internal flowing velocity is dependent on the vibrating frequency, the main fluid flows faster at the resonant modes. CFD simulation using the VOF-CSF model showed that extra flow circuits exit inside the oscillating droplet besides the main flow. The diffusion of substrate momentum within the Stokes layer results in the two flow circuits near the bottom substrate, and the Laplace force due to the droplet deformation induces the two counter-current flow circuits near the surfaces of the microdroplet.