Droplet lift force enhancement due to internal 3D flow bifurcation

We study numerically the lift force experienced by a spherical droplet with droplet Reynolds numbers of order 100 experiencing a linear shear flow over a wide range of drop-to-fluid viscosity ratio. We reveal that the lift force exhibits, from zero (bubble) to infinite (solid sphere) viscosity ratio, a non-monotonous transition peaking at a viscosity ratio of order unity. This behavior is related to an internal 3D flow bifurcation also observed under uniform-flow conditions. In the presence of an ambiant shear, this internal 3D bifurcation results in the amplification of the inviscid vortex tilting-stretching mechanism responsible for the lift force, making the trailing vortices generated in the droplet wake stronger, and as a consequence an enhanced lift effect.