Microfluidics: Fabrication, Droplets, Bubbles and Nanofluids Synthesis

Present studies include a series of investigations on microfluidics from fabrication to application. In the application this study focuses on some fundamental problems regarding the manipulation of droplets and bubbles inside microfluidics, including the size control during generation, the critical condition for breaking droplets, chaotic mixing inside moving droplets and nanofluids synthesis.

A low-cost fabrication method for manufacturing glass-based microfluidic devices is developed in a routine laboratory without the requirement of a clean room. Direct bonding of glass material is realized without using any additives. The fabrication method is very reliable and the yield is larger than 90%. Channel surface modification from hydrophilic to hydrophobic is realized inside the microfluidic devices after fabrication. With the help of the surface modification, both of the two types of droplets, oil-in-water and water-in-oil, can be generated inside glass-based microfluidic devices.

Droplet formation under controlled flow rates and bubble formation under controlled pressures in confined T-shaped junctions are experimentally investigated by changing thermophysical properties of continuous phases and by changing the controlled dynamic parameters. A pressure-driven mechanism of droplet/bubble formation is experimentally discovered. The influence of the continuous phase thermophysical properties and the controlled dynamic parameters on droplet/bubble volume and formation time is systematically investigated. Empirical correlations are obtained for predicting the droplet volume and formation time.

Droplet breakup in either symmetrically or asymmetrically confined T-shaped junctions is experimentally studied. The critical condition with which microfluidic droplets will break equally is theoretically analyzed based on the pressure-driven mechanism. A semi-empirical correlation is obtained for predicting the equal breakup in symmetric T-shaped junctions. Besides the equal breakup, a new droplet breakup pattern, unequal breakup, is observed in the symmetric T-shaped junction. In asymmetric T-shaped junctions the droplet breakup is found to be very difficult.

Scaling analysis of the chaotic mixing inside moving droplets is conducted based on the idealized recirculating flow and the Baker’s transformation. Experimental investigations on the mixing efficiency inside the droplets moving in curved microchannels are performed with the help of the micro visualization system. It is found that the mixing efficiency is significantly enhanced by the chaotic advection inside the moving droplets and the full mixing time can be reasonably estimated by the scaling analysis. A significant mixing enhancement during the droplet formation process is observed, which is not so frequently reported by others. An effective microstructure is designed for mixing two or more individual droplets after their formation.

Synthesis of copper nanofluids is realized in microfluidic reactors. In contrast to the traditional method, the copper nanofluids synthesized in the microfluidic reactors have a narrower size distribution. The synthesis time is also reduced by one order of magnitude. It is also found that the particle size and size distribution are insensitive to the flow rate of reactants, the reactants concentration and the surfactant concentration.