Disorder-to-order transition of long fibers contained in evaporating sessile drops.

A liquid drop containing a long fiber is a complex system whose configuration is determined by an interplay of elastic stresses in the fiber and capillary forces due to the liquid. We study the morphological evolution of fibers that are much longer than the drop diameter in evaporating sessile drops. After insertion, the fibers are either found in an ordered or disordered state, with increasing disorder for increasing fiber length. Upon evaporation, the order increases, in such a way that the final configuration deposited on the solid surface is either a circle, an ellipse, or 8-shaped. The morphology of the deposit depends on the fiber length and the elastocapillary length, both non-dimensionalized with the characteristic drop size, which we classify in a morphology regime map. The disorder-to-order transition allows depositing ordered fiber structures on solid surfaces even in cases of a strongly disordered state after fiber insertion. Combined with technologies such as inkjet printing, this process could open new avenues to decorate surfaces with filamental structures whose morphology can be controlled by varying the fiber length.