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. Directing the manner in which a liquid spreads across a surface — through patterns, for example — is important in a variety of fields ranging from printing to microfluidic device fabrication. Longcheng Gao and colleagues from Beihang University, China, have now achieved such control over a liquid through temperature variations. The researchers prepared a material with a temperature-dependent surface wettability by combining the polymers poly(methyl methylacrylate) (PMMA) and poly(N-isopropylacrylamide) (PNIPPAAm). The resulting PMMA-b-PNIPPAAm block copolymer is hydrophobic at high temperatures yet hydrophilic at low temperatures. Under a temperature gradient, the consequent difference in wettability causes a liquid drop to flow along the surface. Furthermore, as the surface tension of a liquid varies according to its temperature, an additional ‘Marangoni’ flow is induced. Since the directions of the Marangoni effect and the wettability gradient are the same, they work in concert to promote the movement of the droplet along the block copolymer. By applying temperature gradient on the temperature-responsive wettablitity surface of block copolymer, water droplet movement by the nonmechanical origin forces is realized on the surface with high hysteresis, due to the binary collaboration effect of wettability gradient force and Marangoni force. The results help to understand the motion of droplets on the surface with high hysteresis and also provide potential application in microfluidic devices.