Anisotropic wetting on simple, flat surfaces

Anisotropic wetting in nature and in the laboratory is typically associated with complicated chemical or topographical patterning. We show that, surprisingly, even soft, flat surfaces exhibit anisotropic wetting - both in statics and dynamics. Droplets sliding on a soft substrate with 23% stretch move approximately 70% faster parallel to the stretch direction than perpendicular. A stretched substrate with static droplets reveal increasing ellipticity with larger stretches. We connect both of these phenomena to the deformations of the substrate by the droplet along its contact line, the shape of which is strongly dependent on the applied stretch. Here we examine and analyze this deformation with a combination of confocal microscopy and optical profilometry in order to understand the factors that determine the anisotropic wetting behavior and to discuss the cause of the strong stretch-dependence of the anisotropic deformations.