Do droplets always move following the wettability gradient?

Impacting droplets on rough surfaces with a wettability gradient have been reported to rebound obliquely or migrate following the wettability gradient due to the unbalanced interfacial forces created by such heterogeneous architectures. Here we demonstrate under certain conditions, droplets can be self-propelled against the wettability gradient. We show that the local Cassie-to-Wenzel transition in the droplet is critical for such a counter-intuitive phenomenon. We believe that our letter extends our conventional understanding of wettability dynamics to heterogeneous surfaces and provides important insight for the design of micro/nanotextured surfaces for controlled droplet manipulation. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3592997]