ABSTRACT
Digital microfluidics employs water-repellant surfaces to exquisitely manipulate droplets of water for chemical analysis. However, the actuation and manipulation of organic droplets is still relatively unexplored as it is significantly more difficult to synthesize organic-repellent surfaces compared to water-repellent surfaces. Here, we present the fabrication of slippery liquid-infused porous surfaces (SLIPS) based on a porous polymer monolithic approach. The synthesized SLIPS were able to repel organic liquids such as hexane and methanol with a contact angle of 42.1 ± 0.4° and 69.0 ± 1.8°, respectively, as well as water with a contact angle of 115.8 ± 0.8°. More importantly for digital microfluidic applications, the sliding angle of liquids tested was between 4° and 6°. As a result, droplets containing magnetically susceptible material could be facilely manipulated on the SLIPS surface. A systematic actuation study was carried out to explore how actuation parameters including speed, paramagnetic particle (PMP) concentrations, and droplet volume impacted the outcomes (droplet actuation, disengagement, and PMP extraction). Two different applications were used to demonstrate the utility of actuating organic droplets on SLIPS surfaces including on-chip liquid-liquid extractions of natural products (NPs) from marine bacteria and droplet-based polymer synthesis with different polymerization conditions. Both applications employ an aqueous droplet and organic droplet interface at which either phase transfer or a chemical reaction is carried out. Two NPs (prodigiosin from Pseudoalteromonas rubra and violacein from Pseudoalteromonas luteoviolacea) were extracted, from aqueous droplets containing the bacteria, into butanol droplets and characterized with matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS). Nylon 6,6 was synthesized on-chip via magnetic actuation of organic droplets containing adipoyl chloride and hexamethylamine. Relative intensities of the characteristic polymer masses suggest that droplet-based microfluidic synthesis on slips can be used to probe reaction conditions. The compatibility of SLIPS with both aqueous and organic solutions opens up a wider number of droplet-based sample preparation protocols and chemical transformations.
