  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 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 from the hydrophobic to the hydrophilic
stripes of the surface. Although an in-situ produced thermal gradient due to
differential heat transfer coefficients of the hydrophilic and hydrophobic
stripes could provide additional driving force for such a motion, it is,
however, not a necessary condition for motion to occur. 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.
