  Numerous cell-types have shown a remarkable ability to detect and move along
gradients in stiffness of an underlying substrate -- a process known as
durotaxis. The mechanisms underlying durotaxis are still unresolved, but
generally believed to involve active sensing and locomotion. Here, we show that
simple liquid droplets also undergo durotaxis. By modulating substrate
stiffness, we obtain fine control of droplet position on soft, flat substrates.
Unlike other control mechanisms, droplet durotaxis works without imposing
chemical, thermal, electrical or topographical gradients. This enables a new
approach to large-scale droplet patterning and is potentially useful for many
applications, such as microfluidics, thermal control and microfabrication.
