Programming droplet transport on ultrasoft solids via surface creases

Ultrasoft, polymeric solids present a unique opportunity to maneuver liquid droplets on a substrate without the need for chemical coatings, complex surface patterns, or electric fields. By inducing mechanical stretch or compression to the substrate, the motion of a sliding droplet can be sped up or slowed down. This mechano-adaptive wetting behavior arises from the elastocapillary deformation field around the droplet, which governs its spreading dynamics on ultrasoft materials. In this talk, we demonstrate how creases which are localized surface folds appearing due to compressive stresses, can halt a sliding droplet at a distance, but can be reversible to let a droplet resumes its motion. Unlike surface defects that stop droplets by pinning their contact lines, this mechanism involves a long-range repulsion between the deformation fields of the crease and the droplet, arresting the droplet without direct contact. We will demonstrate two key functionalities that this reconfigurable approach enables: (1) sorting and filtering droplets based on size and surface tension, and (2) programming droplet trajectories. Our design offers a scalable solution for droplet path planning with no additional flow or electrical controls and have potential applications in digital microfluidics systems.