Droplet microfluidics : droplet generation and synthesis of superwetting materials

Abstract

Droplet microfluidics is a technology for processing and manipulating small amounts of fluids (nanoliter to attoliter) in channels with dimensions at the micrometer scale (sub-micrometer to hundreds of micrometres). It has developed into a versatile tool in diverse fields, ranging from foods, to cosmetics, pharmaceuticals, and diagnostics, due to its ability to produce monodisperse droplets and control each droplet independently. Applications of microfluidic droplets arise from two distinct but complementary aspects: lab-on-a-chip systems and materials synthesis. To facilitate diverse applications, microfluidic droplets should possess the following features: (i) a wide range of tunable size, (ii) controllable size distribution, and (iii) on-demand droplet generation where droplets with prescribed volume are produced at a predetermined frequency. Therefore, control of droplet generation is at the heart of droplet microfluidics and its applications. The present work first focuses on uncovering the mechanism of droplet generation in microfluidic devices. Droplet generation originates from the competition among various forces; while viscous and inertial forces act to deform the liquid interface, interfacial tension effects resist the deformation. The magnitude of these forces is manipulated by parameters of flow rates, material properties, and channel dimensions. In capillary microfluidic devices with an expansion-contraction structure, droplets are produced either in the expansion or in the contraction region when viscous shear forces dominate over the interfacial tension. Five modes of droplet generation are observed in the contraction region: squeezing, dripping, jetting, tip-streaming, and tip-multi-breaking. The first four modes produce continuous streams of droplets, and the last one produces intermittent droplet sequences containing non-uniform droplets with predictable size distribution. Besides passive droplet generation, active elements are incorporated into microfluidic devices to provide additional handle for droplet generation. A mechanical vibrator is connected to the microtubing to perturb the dispersed phase flow rate. Pinch-off of droplet is accelerated by mechanical vibration due to the enhancement in fluid inertia that counteracts viscous shear. As such, a robust control over droplet generation is achieved by mechanical vibration, where the size and generation frequency of droplets can be independently manipulated in dripping mode. The jetting flow is transferred into dripping for the production of uniform droplet in a wider range of flow conditions. The well-controlled droplets are used as templates for the fabrication of porous membranes via droplet assembly. The fabrication process works with various materials including nanoparticles and polymers. The structure of porous membrane is delicately tailored by the deformation of droplet templates and precisely predicted by a theoretical model. The porous membrane is designed to fabricate diverse superwetting materials: omniphobic, superhydrophobic, under-liquid superlyophobic, and liquid-infused slippery surfaces. Moreover, the porous membrane displays high mechanical robustness attributed to the interconnected microstructures, which is of vital importance for improving the longevity of superwetting materials in pragmatic applications. Delicate control over droplet generation is enabled by comprehensive understandings of the underlying mechanism and development of novel droplet maker systems. The produced droplets would open new avenues for various applications such as fabrication of next-generation functional materials with tunable and programmable properties.