Time evolution of coiled fibers inside evaporating sessile drops*

When a hydrophilic elastic fiber with a length of around 10 cm is introduced into a millimeter-sized water drop, it can form a multiply coiled structure completely immersed in liquid. We have studied the time evolution of fiber-drop systems that evaporate on a superhydrophobic surface. After introduction, the fiber is either in an ordered or a disordered state. Ordered initial states are preferably found for short fibers, while long fibers exhibit disorder. Upon evaporation, drop shapes largely deviating from a spherical cap are observed. The time evolution of the fibers is mostly smooth, interrupted by a few sudden reconfiguration events. Generally, as the drop evaporates, the fibers transition from a less ordered to an ordered state. Specifically, while the initial state is often a three-dimensional morphology, the final configuration deposited on the surface has a two-dimensional character. We present a morphology diagram of the fiber deposits on the surface, with the non-dimensional fiber length and elastocapillary length as independent variables. Only three different morphologies of the fiber deposits are found: circles, ellipses, and 8-shaped morphologies. The deposit morphology is mainly determined by the fiber length, while the elastocapillary length only plays a subordinate role.