Thermoresponsive Film Enhances Fluid Migration within the Capillary under a Dynamic Wettability Gradient

The study developed a thermoresponsive film to achieve autonomous fluid driving in microfluidic channels, simplify microfluidic systems, and improve their operability. This film has a lower critical solution temperature (LCST), exhibiting different wettabilities on each side of the LCST, and showed improved hydrophilicity-to-hydrophobicity conversion with increased substrate roughness, maintaining stability after repeated cycles. The thermoresponsive film was applied to the inner wall of the glass capillary, which showed a hydrophilic and enhanced capillary effect below the LCST and a hydrophobic and weakened capillary effect above the LCST. Subsequently, the modified capillary was placed under a dynamic temperature gradient, and the force analysis of the fluid in the flow channel was carried out. It was found that only when the driving force exceeded the axial resistance could the fluid migrate. Experimental analysis showed that fluid length was directly proportional to axial resistance and inversely proportional to both driving force and migration velocity at a constant dynamic temperature gradient. Additionally, the temperature of the hot end of the capillary was varied to form different dynamic temperature gradients. A higher dynamic temperature gradient resulted in greater fluid displacement and velocity at a constant fluid length. The results presented in this study were expected to provide new insights into the design and optimization of thermally driven microfluidic systems.