In-Channel Responsive Surface Wettability for Reversible and Multiform Emulsion Droplet Preparation and Applications

Abstract

We report on a simple approach for in-channel functionalization of a polydimethylsiloxane (PDMS) surface to obtain a switchable and reversible wettability change between hydrophilic and hydrophobic states. The thermally responsive polymer, poly(N-Isopropylacrylamide) (PNIPAAm), was grafted on the surface of PDMS channels by UV-induced surface grafting. PNIPAAm-grafted PDMS (PNIPAAm-g-PDMS) surface wettability can be thermally tuned to obtain water contact angles varying in the range of 24.3 to 106.1°by varying temperature at 25−38°C. By selectively modifying the functionalized area in the microfluidic channels, multiform emulsion droplets of oil-in-water (O/W), water-in-oil (W/O), oil-in-water-in-oil (O/W/O), and water-in-oil-inwater (W/O/W) could be created on-demand. Combining solid surface wettability and liquid−liquid interfacial properties, tunable generation of O/W and W/O droplet and stratified flows were enabled in the same microfluidic device with either different or the same two-phase fluidic systems, by properly heating/cooling thermal-responsive microfluidic channels and choosing suitable surfactants. Controllable creation of O/W/O and W/O/W droplets was also achieved in the same microfluidic device, by locally heating or cooling the droplet generation areas with integrated electric heaters to achieve opposite surface wettability. Hollow microcapsules were prepared using double emulsion droplets as templates in the microfluidic device with sequential hydrophobic and hydrophilic channel segments, demonstrating the strength of the proposed approach in practical applications.

Nanoemulsions: Using emulsifiers from natural sources replacing synthetic ones—A review

“…An emulsion is a colloidal system that consists of two or more entirely or partially immiscible phases where one of them was dispersed as small droplets in another phase (Komaiko, Sastrosubroto, & McClements, 2016a), and can be of the types water-in-oil (W/O), oil-in-water (O/W), or multiple-emulsions in a complex system in the form of O/W/O or W/O/W (Li et al, 2019;Sheth, Seshadri, Prileszky, & Helgeson, 2020). The emulsions can be prepared not only by surface-active substances, that is, emulsifiers, although solid nanoparticles may be used to formulate the so-called Pickering emulsions (Pickering, 1907).…”

Autonomous capillary microfluidic devices with constant flow rate and temperature-controlled valving

“…3.5 UV-induced surface grafting of NIPAm on PDMS mediated by benzophenone UV-induced grafting of PNIPAm was conducted by UV-initiated surface polymerization 34,35 , a schematic drawing of the grafting process is shown in Figure S2. Benzophenone-acetone (20 wt%) solution was introduced into the PDMS channels for 5 min, followed by a deep wash of the channels with DI water and ethanol.…”

Autonomous Capillary Microfluidic Devices with Constant Flow Rate and Temperature-Controlled Valving

“…UV-induced grafting of PNIPAm was conducted by UV-initiated surface polymerization 34,35 , a schematic drawing of the grafting process is shown in Figure S2. Benzophenone-acetone (20 wt%) solution was introduced into the PDMS channels for 5 min, followed by a deep wash of the channels with DI water and ethanol.…”