When drops of liquid are trapped in a thin gap between two solids, a strong negative pressure develops inside the drops. If the solids are flexible, this pressure deforms the solids to close the gap.
In our experiment the solids are transparent, which allows us to image the drops from above. Alternating dark and light lines represent lines of constant gap height, much like the lines on a topological map. These lines are caused by light interference, which is the phenomenon responsible for the beautiful rainbow pattern in an oil slick. The blue areas denote the extent of the drops. Since the drops pull the gap closed, the areas of minimum gap height (i.e. maximum deformation) are inside the drops, at the center of the concentric rings.
– Jason Wexler and Howard A. Stone
Jason Wexler (graduate student) and Howard A. Stone (faculty), Department of Mechanical and Aerospace Engineering