Microfluidic devices functionalized with PNIPAm: Switchable wettability for reversible emulsion generation and capillary flow control

In this thesis, we developed several PDMS-based microfluidic devices for emulsion generation and capillary-driven flow control. By grafting the PDMS surface with PNIPAm using UV-induced surface polymerization, we obtain a PNIPAm-g-PDMS surface with it's surface wettability that is sensitive to temperature. By locally integrate a microheater underneath the channel, reversible hydrophilic/hydrophobic surface can be obtained by switching the temperature at 20/38 °C. The switchable wettability was used for reversible emulsion generation (chapter 2) and stop valves for capillary flow control (chapter 3). By integrate hierarchical structures on the surface, the switchable wettability range was extended, thus we obtained a stop valve with a more stable functionality. By further integrated the stop valves with thermal actuated air-gates, we developed a device for programmable capillary flow control (chapter 5). In addition, in chapter 3, a constant capillary filling rate was observed in our devices instead of Lucas-Washburn behavior. We attributed it to the diffusive hydration of water on PNIPAm thin film which is induced by the slow wetting kinetic of PNIPAm in water. Moreover, since the wetting kinetic of PNIPAm is sensitive to the liquid component such as ethanol due to the co-nonsolvency of PNIPAm in water-ethanol mixture. In chapter 4, we further studied the wetting behavior of water/ethanol mixture on PNIPAm-g-PDMS surface and the capillary filling behavior of water/ethanol mixture in PNIPAm-g-PDMS microchannel. Constant capillary filling flow rate was obtained among water/ethanol mixture of a molar fraction between 0-4%, with the filling flow rate dependent on molar fraction. The dependence of the filling rate on the ethanol content makes the device a promising tool for the detection of low alcohol concentrations in alcoholic beverages by simply comparing the filling times. By using this method, Grolsch beers with 0%, 2%, and 5% ABV were distinguished from each other within 10 s.