Spreading of large drops on patterned surfaces

Abstract

We report an experimental study of the shape of large liquid drops spreading on surfaces with patterns of wettability. The patterns are sets of disks of a material less wettable than the rest of the plane substrate. When few defects are present, or when the average distance between defects is large compared to the capillary length, the drop edge is a set of circular arcs connecting the pinning points. When the density of defects is smaller than the capillary length, the contact line shape is more complex and we analyze its morphology through a kind of box counting method. The roughness of the contact line is maximum when the average distance between defects is comparable to two times the defects size. Setting the non-wetting defects on a periodic or almost periodic array produces drops with faceted edges. The transition between circular and faceted drops can be understood as a competition between the tension of the drop edge and the local force exerted on the contact line by the defects. Although the drops are grown at a fixed flow rate, the motion of the contact line is irregular, with jumps between pinning sites. In some cases, the jumps of the contact line are correlated in space, leading to "avalanches" and large scale motions of the drop.