Perturbation and electrical field effects on the liquid dispensing system

Abstract

Droplet-based microfluidics, which refers to generation and manipulation of droplets at micron scale, has been widely used as vehicles for drug delivery, as micro-reactors for chemical and biological analysis and as inks for printing. It has the ability to precisely manipulate droplet sizes and structures. The monodispersity of the droplet sizes enables the standardization of droplet performances in their applications. The precise control over droplet structures realizes the integration of multiple functions in a single droplet. The conventional microfluidic method is used to manipulate fluid flows using hydrodynamic forces. It is achieved by adjusting the fluid flow rates and fabricating micro-channels with different geometries. However, this method is not applicable to control the size of droplets with ultra-low interfacial tension and is not efficient for generating monodisperse droplets with low interfacial tension. Besides, this conventional method is also not applicable to manipulate viscous liquids by increasing flow rates, due to the resultant increase in hydraulic resistances. Incorporating external forces with hydrodynamic forces is a potentially powerful way to manipulate the flow of fluids. It has been used to promote the generation of all aqueous droplets with ultra-low interfacial tension by imposing mechanical vibrations on fluids. Besides, it has been used to reduce droplet sizes to a few hundred nanometers by applying external electric forces to fluids. Applying these methods, the droplet sizes are precisely controlled by the external forces. To enhance the manipulation of oil/water fluids with low interfacial tension, we perturb the liquid-in-liquid jet by mechanically vibrating the micro-tubing which is used to deliver the dispersed fluid into microfluidic devices. The imposed perturbation is able to control not only droplet sizes but also their structures and configurations. The effectiveness of manipulation of droplet generation decreases with increasing fluid interfacial tension. Thus, the threshold of vibration amplitude increases to trigger controllable droplet generation. Moreover, our approach allows the generation of individual droplet rather than a train of droplets. To control the flow of viscous liquids, we inject viscous liquids into a charged nozzle. In the presence of the electric force, the liquid jet can be triggered to coil. In addition, the applied electric force enables tuning of the coiling frequency, coiling velocity and jet diameters. We have applied the electro-induced-coiling technique to realize a rapid mixing of viscous liquids and a fast printing of curved fibers. When an electric force is used to manipulate fluid flow, it is crucial to optimize the distribution of electric field to avoid undesired interruption in the operation of microfluidics. In chapter six, we introduce a method to shield the undesired electric field by inserting a metallic coil into the micro-channels. In summary, the incorporation of external forces, like mechanical vibrations and electric forces, and microfluidics provides a means to manipulate fluid flow at micron scale. It widens the range of fluid materials operated in microfluidic devices and realizes the precise manipulation in droplet sizes and structures of every individual droplet.