Macromolecular Migration in Microfluidics

Abstract

One of the great fascinations chemical engineers have with polymeric liquids is their curious behaviour when confined. In this thesis, we use macromolecular theory that relies entirely on flow orientation to explain the rheology of polymeric liquids in confinement. Specifically, we use rigid dumbbell theory to explain the curious effects of polymer confinement: (a) diminishing complex viscosity, and (b) emigration from high-shear. Whereas much is known about the complex viscosity of polymeric liquids, far less is understood about the behaviour of this material function when macromolecules are confined. By confined, we mean that the gap along the velocity gradient is small enough to reorient the polymers. In the first part of this thesis, we examine classical analytical solutions for a confined rigid dumbbell suspension in small-amplitude oscillatory shear flow. We test these analytical solutions against the measured effects of confinement on both parts of the complex viscosity of a Carbopol suspension and three polystyrene solutions. Microfluidic design, fabrication and experiments have developed rapidly, leading to lab-on-chip separation or fractionation. In the second part of this thesis, we design a continuous concentrator for macromolecular solutions. Our design relies on the analytical solutions for orientational diffusion under laminar pressure-driven slot flow through a microchannel. Using rigid dumbbell theory, we provide analytical solutions for the design of our microfluidic macromolecular hydrodynamic chromatography. We arrive at our design through the use of well-known confinement-driven composition profiles. Using a pair of razor-sharp blades, our design separates the slot flow into a symmetric core inner slot (concentrated) between two outer slots (diluted). We discover a minimum dimensionless blade leading edge separation for complete fractionation, and that this decreases with confinement, and also decreases with dimensionless shear rate. In the last part of this thesis, using our earlier analytical rigid dumbbell theory for orientational diffusion under laminar pressure-driven slot flow through a microchannel, we design and fabricate a passive, continuous microfluidic separator. By passive, we mean channel geometry or particle inertia control the migration inside the microchannels. We designed and fabricated (soft lithography) slot-flow and square duct straight microfluidic channels with one inlet and three outlets.