Fig. 3.

Droplet motion on stiffness gradients. (A) Speed of droplets moving from thin to thick regions of the gel as a function of gel thickness at droplet center, h. Trajectories extracted from image sequences (see Movies S1–S3). Drop speed increased as drop radius increased and substrate thickness decreased. Droplets came to rest when . Droplets starting at locations much thicker than their radii did not move. (B) Solid curves show the theoretical contact angle as a function of substrate thickness for droplets of different sizes. The x symbols show droplet center and o symbols show droplet front and back at thickness where . The o’s are filled when the droplet straddles the trough in the substrate, and otherwise empty.