A thermoresponsive film applicable to diverse substrates for controllable sessile droplets motion

Thermoresponsive films used to actuate microscale liquid droplets play an important role in lab-on-chip field. However, the limitation of applicable substrates and the rigid requirements for building block copolymers are obstacles. In this study, a thermosensitive poly(N-isopropylacrylamide) (PNIPAm) film was prepared by photo-grafting on modified polydimethylsiloxane (PDMS) layer. Polytetrafluoroethylene microparticles were sprayed and embedded in PDMS to obtain the modified layer with low surface energy and micro-roughness, which enhanced the wettability of PNIPAm switching between superhydrophilic and hydrophobic. As PDMS layer can be easily attained by either spin-coating or dip-coating, the thermoresponsive film (TRF) can be applied on diverse substrates without limitation. With applied temperature gradient on the TRF, the contact lines of sessile droplets moved spontaneously along on-demand directions by temperature control. Forces acting on droplets were calculated to analyze the factors that affect the contact line motion as well as the available temperature range. Such novel film will provide potential applications in microfluidic devices especially in portable paper-like lab-on-chip accompanied by the advantages of simple preparation, high adaptability to nonplanar surfaces, easy access to temperature control and low energy consumption.