Buoyancy-induced axis-asymmetric convection inside droplets

In addition to the well-known diffusion-only and toroidal convection, a seldom reported axis-asymmetric (single circulating) flow can also be induced by heat/mass transfer within a droplet (i.e., droplet rolling). To investigate the mechanism, this work compares their characteristic times and finds that (1) a large viscosity ratio between the droplet and its environment is required for the axis-asymmetric flow and (2) in contrast to precedent prediction, heat/mass transfer cannot incur the axis-asymmetric flow within a vanishingly tiny droplet, as verified by our experiments together with dimensionless dynamical equations. Moreover, our experiments reveal an unprecedented shift of the bound between the axis-asymmetric flow and toroidal convection zones when reversing the transition direction.