Temperature-controlled directional spreading of water on a surface with high hysteresis

The actuation of microscale liquid droplets is a key point in the lab-on-chip field. Marangoni force actuation resulting from a temperature gradient has remarkable advantages. However, high hysteresis between the droplet and the surface is an obstacle to this motion. Here, we take advantage of the temperature-responsive wettability of a surface made of a block copolymer (BCP) to show the temperature-controlled directional spreading of water droplets. By applying a temperature gradient on the BCP surface, both the topologies and chemical components in the nanodomains could be changed gradually. As a result, a wettability gradient force would form with the same direction as the Marangoni force, which could also be formed due to the same temperature gradient, and the collaborative effect of these forces could help overcome the high hysteresis. This was confirmed theoretically by calculating the total force acting on the droplet. The liquid droplet was observed to move by forces of non-mechanical origin in experiments conducted using a thermal gradient on the BCP films. Furthermore, two water droplets were observed to merge into one when they were placed in a V-shaped temperature field. These results help us understand the motion of droplets on a surface with high hysteresis and provide potential applications in microfluidic devices.