Marangoni contracted droplets: the role of the solid substrate

Marangoni contracted droplets are stabilized by Marangoni flow rather than an energy equilibrium. Recent work has revealed how droplets in the Marangoni contracted state have negligible contact angle hysteresis, allowing them to sense and respond to subtle forces with high sensitivity. This unique ability enables the creation of droplet-based fluidic machines with advanced capabilities. It is known that Marangoni contracted droplets can form on high energy surfaces where liquids are ordinarily expected to spread into a thin film. Here we systematically investigate the role of the solid substrate, showing how texture and surface energy impact whether or not a droplet is in the Marangoni contracted state. We then show how roughness and surface energy can be patterned for tunable control of droplet behavior. We also show how stable surfaces can be created that support the formation of Marangoni contracted droplets for unprecedented amounts of time. Finally, we discuss how these concepts may have application to the cleaning of high energy surfaces with limited equipment and minimal liquid volumes.