Water droplets prefer the soft touch

Researchers have found a way to drive water droplets along a flat surface without applying heat, chemicals, electricity, or other forces: All that's required is varying the stiffness of the surface in the desired direction. The droplets, it turns out, prefer the soft spots. "Our findings show that simple physical parameters, like the surface tension of a liquid droplet, can drive spontaneous motion from one stiffness to another," said Eric Dufresne, associate professor of engineering at Yale University and principal investigator of a study published the week of June 24 in the journal Proceedings of the National Academy of Sciences. The researchers' approach was inspired by the behavior of living cells. Certain biological cells are known to detect and move along stiffness gradients in a process called durotaxis. While eukaryotic cells migrate toward harder surfaces, researchers found that liquid droplets move toward softer ones. "Droplet durotaxis is much simpler than cell durotaxis," said Dufresne.