In this work, a novel thiol-ene based photopolymerizable resin formulation was shown to exhibit highly desirable characteristics, such as low cure time and the ability to overcome oxygen inhibition, for the photolithographic fabrication of microfluidic devices. The feature fidelity, as well as various aspects of the feature shape and quality, were assessed as functions of various resin attributes, particularly the exposure conditions, initiator concentration and inhibitor to initiator ratio. An optical technique was utilized to evaluate the feature fidelity as well as the feature shape and quality. These results were used to optimize the thiol-ene resin formulation to produce high fidelity, high aspect ratio features without significant reductions in feature quality. For structures with aspect ratios below 2, little difference (<3%) in feature quality was observed between thiol-ene and acrylate based formulations. However, at higher aspect ratios, the thiol-ene resin exhibited significantly improved feature quality. At an aspect ratio of 8, raised feature quality for the thiol-ene resin was dramatically better than that achieved by using the acrylate resin. The use of the thiol-ene based resin enabled fabrication of a pinched-flow microfluidic device that has complex channel geometry, small (50 μm) channel dimensions, and high aspect ratio (14) features.

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Rigid particles transported through a pinched-flow fractionation (PFF) device are simulated using boundary-integral methods (BIM). The PFF device separates particles by size using a bifurcated microfluidic channel. The critical flow ratio of the two input channels required to achieve complete separation of large and small particles decreases with increasing diameter of the larger particles relative to the pinch height, and is nearly independent of the smaller particle size. A narrow pinch with a square exit was shown to have the lowest critical flow ratio and was selected as the model device to be fabricated. Experiments conducted using this device confirm that the larger particles exit further from the top wall than do the smaller particles, due to steric exclusion, and the final exit positions are within a few percent of the simulation results. It is shown that BIM is a valuable tool in the design of microfluidic devices.

A new, three-dimensional algorithm is developed to accurately simulate low-Reynolds number, flow-driven motion of a neutrally buoyant spherical particle in plane-parallel microchannels of complex shape. The channel profile may consist of an arbitrary number of straight line segments with sharp corners in an arbitrary configuration. This geometry provides a suitable model for particle transport in many microfluidic devices with multiple branch bifurcations. The particle may be comparable with the narrowest channel dimensions, but is typically much smaller than the overall channel domain, which creates difficulties with a standard boundary-integral approach. To make simulations feasible, the 3D problem is solved locally in a computational cell that is smaller than the full domain and is dynamically constructed around the particle as it moves through the channel; the outer boundary conditions are provided by the 2D flow that would exist in the channel in the absence of the particle. Difficulties with particle-corner close interactions are alleviated using special iterative techniques, (near-) singularity subtractions and corner-fitted, gap-adaptive discretizations of the cell boundary. The algorithm is applied to simulate “pinched-flow fractionation” and predict how particle interactions with a narrow pinch region and sharp corners result in particle focusing and separation in the outlet according to their size. As another application, the particle motion through a T-bifurcation with sharp corners is simulated, with calculation of the particle flux partition ratio for a broad range of parameters. It is demonstrated how the particle-corner interactions can make the side branch inaccessible to particles, even for relatively strong fluid suction through this branch.