Figure 4: Origin of capillary torque on the gear.

A uniformly heated gear produces a temperature gradient resulting in smaller surface tension forces over the inner portions of the gear’s contour. The temperature profile, at the air–liquid interface, is computed by three-dimensional finite-element simulations and is represented by the colour level (see colour bar). Due to the asymmetric shape of the gear a non-zero total torque is generated (Equation (5)).