Coalescence of Drops Near A Hydrophilic Boundary Leads to Long Range Directed Motion

TL;DR

This paper introduces a passive method for removing drops from surfaces by using alternating hydrophilic and hydrophobic stripes, enabling directed motion without pre-fabricated energy gradients, with potential applications in thermal management.

Contribution

It demonstrates a novel passive drop removal technique using surface patterning, eliminating the need for pre-existing wettability gradients.

Findings

Drops coalesce and move towards hydrophilic zones

Directed motion occurs without thermal gradient necessity

Method enables efficient drop removal from surfaces

Abstract

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.