Manipulations of viscoelastic instability and interfacial surface forces in microfluidics devices for biomedical and material science applications

As a highly viscoelastic liquid, flowing blood exerts a shearing force that has a significant effect on the functioning of vascular endothelial cells (ECs), which regulates the human circulatory system. The first part of the thesis describes a microfluidic device along with a specially formulated media to provide an in-vitro testing microenvironment where cultured endothelial cell layers can be subjected to shearing forces from both stable and unstable flows. Complex and unstable flow patterns are generated within this microchannel device by engineering the viscoelastic properties of the EC culture media without the need of an extensive flow agitation apparatus. In-vitro shearing tests showed significant differences in the responses of Human Umbilical Vein Endothelial Cell (HUVEC) layers to laminar stable and complex unstable flows. The second part of the thesis describes a microfluidic method to generate uniform-sized polydimethylsiloxane (PDMS) microspheres over a size range of 85--200 microns by manipulation of the microchannel two-phase flow. Viscous PDMS prepolymer is pushed out of the middle channel of a 3-inlet-1-outlet converging microchannel flanked on each side by flow of an aqueous surfactant solution. Unique surface crack patterns are generated on the surfaces of PDMS microspheres, and they are decorated by coating them with fluorescent protein and gold nanoparticles, which could be further enhanced into gold or silver nanowires. The unique ability to generate controllable selective 3D deposition patterns on PDMS microspheres introduces a new class of microscale functional materials, and provides opportunities for a multitude of material science and biomedical applications. Finally, the effect of channel surface properties on air-liquid two-phase flows and plug flows in a microchannel are investigated. Manipulation of the surface properties creates a several distinct flow regimes in a Y-shaped microchannel; and affects different plug propagation conditions in a K-shaped design, with important clinical implications for pulmonary air cell injury.