Spontaneous thermocapillary motion of condensation droplets

The spontaneous movements of condensate droplets on either superhydrophobic surfaces or homogenous slippery surfaces are generally driven by capillary forces. It is difficult to shift a millidroplet without using, e.g., a wettability gradient or asymmetric bump. Its motion direction is not related to the surface temperature of the substrate, although the condensation strongly depends on the surface temperature. This Letter reports a self-excited thermocapillary motion during condensation on a heterogeneous slippery liquid-infused porous surface without an externally imposed tangential temperature gradient, where the droplet moves directionally toward cold areas on the surface. The spontaneous thermocapillary motion is driven by the thermocapillary force originating from the local nonuniform temperature distribution on the surface, which is several orders of magnitude larger than the capillary force for a millidroplet. Even a millidroplet could move on such a heterogeneous surface and move upward against the gravity on an inclined surface. In addition to the spontaneous motion directly related to the temperature of the cooling substrate, the dropwise condensation rate may be significantly increased up to two times compared to that of a homogenous slippery surface.