Coalescence of drops near a hydrophilic boundary leads to long range directed motion

A new mechanism for the passive removal of drop on a horizontal surface is described that does not require pre-fabrication of a surface energy gradient. The method relies upon the preparation of alternate hydrophilic/hydrophobic stripes on a surface. When one side of this surface is exposed to steam, with its other surface convectively cooled with cold water, steam condenses as a continuous film on the hydrophilic stripes but as droplets on the hydrophobic stripes. Coalescence leads to a self-generated noise that in turn leads to a random motion of the center of mass of the fused drops on the surface, which are readily removed as they reach near the boundary of the hydrophobic and hydrophilic zones thus resulting in a net diffusive flux of the coalesced drops moving from the hydrophobic to the hydrophilic stripes on the surface. This phenomenon is, indeed, similar to that of the random walk of particles with an absorbing wall. This method of creating directed motion of drops does not require a pre-existing wettability gradient and may have useful applications in thermal management devices.