Reversible self-propelled droplet movement: a new driving mechanism.

Reversible self-propelled movements of droplets of long chain alkanes are presented and analyzed. Slightly above the bulk melting temperature, solid alkane multilayers melt into droplets which move in a self-avoiding, random path. While moving, the droplets consume the solid alkane and leave behind a widening groove. At temperatures slightly below bulk melting this process can be reversed. Now the droplets move backwards leaving behind a narrowing solid trail which is nourished by the shrinking droplet. The speeds of the droplets are analyzed quantitatively and the melting enthalpy is identified as energetic source for their movement.