Investigation of multiphase liquid systems in microchannels.

Abstract

Droplet-based microfluidics plays an important role in biological and chemical sciences. Different operations such as transport, reagent reaction, particles sorting, and merging can be achieved within a confined droplet. Droplet manipulation can be achieved in both passive and active ways. The passive way focuses on the ingenious device design and arrangement, while the active way incorporates external forces including but not limiting to thermocapillary force, electroweting force, and magnetic force.

Numerical procedures were implemented to study multiphase systems in microchannels. The combined governing equations were employed to calculate the physical fields, which are solved using a finite volume method on the uniform Cartesian grid. The interfacial tension force and the magnetic force are coupled in the Navier-Stokes equation. The interface between two immiscible phases was captured using a particle level-set method. The accuracy of the present numerical codes was validated before calculating droplet-based microfluidic problems.

In this thesis, two study cases were investigated. The first one employed a series of diffuser/nozzle structures to understand the behavior of microdroplets flowing in microchannels as a passive control. At first, the pressure drop between two ends of the diffuser/nozzle microchannel was measured with pressure sensor in both directions. The experimental and numerical results show that the pressure drop is linearly proportional to the flow rate. Furthermore, the rectification effect was observed in all tested devices. Secondly, at the same flow rates of the continuous and the dispersed phases, the velocity of the droplet is determined by the viscosity of the continuous phase and the interfacial tension between the two phases. Both numerical and experimental results show that the velocity of the droplet increases with increasing capillary number. The droplet velocity is higher than the mean velocity of the fluid system and increases with increasing viscosity of the continuous phase or decreasing interfacial tension.

In the second case study, the magnetic force is employed for an active control mechanism. The effect of magnetic force on the formation of ferrofluid droplets in a flow focusing channel was investigated numerically and experimentally. A three-dimensional model was built for this purpose. Two phases of ferrofluid and silicone oil were employed in the simulation. The interaction between hydrodynamics and capillarity force acting on the ferrofluid tip was analyzed numerically in the conditions of without and with magnetic field. The evolution of droplet formation and the time dependent velocity field are discussed. Increasing magnetic susceptibility or increasing magnetic field lead to the formation of larger droplets.