Lithographically Defined Micropost Arrays for Programmable Actuation and Interfacial Hydrodynamics

Magnetically actuating surface-attached post (ASAP) arrays have great potential in microfluidic flow control, including mixing and pumping. Both passive (nonactuating) and active (actuating) micropillar arrays can also be used to control pressure-driven flow and the motion of microscopic particles carried by the fluid through microfluidic channels. Molding techniques are popular for generating these microstructures. However, fabricating high aspect ratio elastomeric microstructures over large surface areas suffers from practical problems such as damage incurred in the demolding process. Here, we report on a fabrication protocol that generates ASAP with an aspect ratio as high as 23:1 and a cross-sectional area less than 1 μm2 using straightforward photolithography processes. We generated 50 unique ASAP arrays, each occupying an area of 1 mm2 on a silicon mold; these arrays have varied cross-sectional shape and size, aspect ratio, and spacings between neighboring posts. Our protocol also controls the level of magnetic material in the ASAP tips with a centrifugation step. Using a herringbone pattern ASAP array, we have demonstrated control over the relative phase of actuation between neighboring posts. Such ASAP serve as an experimental platform to test current models predicting that reciprocal actuators in close proximity can successfully drive flow in a low Reynolds (Re) number environment.