Liquid drops on soft solids generate strong deformations below the contact line, resulting from a balance of capillary and elastic forces. The movement of these drops may cause strong, potentially singular dissipation in the soft solid. Here we show that a drop on a soft substrate moves by surfing a ridge: the initially flat solid surface is deformed into a sharp ridge whose orientation angle depends on the contact line velocity. We measure this angle for water on a silicone gel and develop a theory based on the substrate rheology. We quantitatively recover the dynamic contact angle and provide a mechanism for stick–slip motion when a drop is forced strongly: the contact line depins and slides down the wetting ridge, forming a new one after a transient. We anticipate that our theory will have implications in problems such as self-organization of cell tissues or the design of capillarity-based microrheometers.

Soft solids differ from stiff solids in an important way: their surface stresses can drive large deformations. Based on a topical workshop held in the Lorentz Center in Leiden, this Opinion highlights some recent advances in the growing field of solid capillarity and poses key questions for its advancement. Capillarity of Soft InterfacesW o r k s h o p : 2 -6 N o v e m b e r 2 0 1 5 , L e i d e n , t h e N e t h e r l a n d s T h e Lo re nt z Ce nte r is a n i nte r n at i o n a l center in the sciences. Its aim is to organize workshops for scientists in an atm osp h ere that fos ter s co llab o r ati ve work, discussions and interactions. F o r r e g i s t r a t i o n s e e : w w w . l o r e n t z c e n t e r . n lThe forces that hold a water droplet together also pull on delicate objects such as the leaves and petals of plants. Photo CC BY-NC-SA 2.0 by Cé dric Fayemendy. Pos ter design: Sup erNova Studios . NL