Parallel nano-manufacturing via electro-hydrodynamic jetting from externally-fed emitter arrays

Abstract

The accelerating growth of our ability to engineer at the nanoscale offers unprecedented opportunity to control the world around us in meaningful ways. One particularly exciting development is the production of nanofibers, whose unique morphological properties promise to improve the quality and efficiency of countless technologies. Unfortunately, their integration into almost all of these technologies is unfeasible due to the low throughput and high cost of current production methods. The most common production process, known as electrospinning, involves pumping a viscous, conducting liquid at very low flow rates through a syringe needle in a strong electric field. The emitted charged jet is stretched and whipped extensively creating fibers with diameters as small as tens of nanometers. Existing approaches to increase throughput via multiplexing of jets are either too complex to scale up effectively, or they sacrifice precision and control. In this thesis research, we report the design, fabrication, and experimental characterization of externally-fed emitter arrays for electro-hydrodynamic jetting. We microfabricate monolithic, emitter blades that consist of pointed structures etched out of silicon using DRIE and assemble these into a slotted base to form two-dimensional arrays. By patterning the emitter surface with appropriately dimensioned microstructures, we enable and control the wicking of liquid toward the emission site via passive capillary action. Our results confirm greater flow rate per unit area through wicking structures comprised of open microchannels as compared to those consisting of micropillars. We also demonstrate the existence and location of a flow maximum with respect to the width of the microchannels. We test arrays with as many as 225 emitters (25 emitters/cm²) and with emitter densities as high as 100 emitters/cm². The densest arrays (1 mm emitter spacing) fail to electrospin fibers but demonstrate electrospray of droplets. Sparser arrays (>/- 2 mm emitter spacing) are capable of both emission modes, sometimes simultaneously. This can degrade fibers via re-dissolution on the collector electrode and suggests the need for finer control over emission characteristics. Arrays capable of electrospinning exhibit a mass flux as high as 400 [g/hr . m²], which is 4 times the reported production rate of the leading free-surface electrospinning technology. Throughput is shown to increase with increasing array size at constant density suggesting the current design can be scaled up with no loss of productivity. For the arrays tested, increased emitter density led to decreased throughput. This is likely due to a large decrease in electric field enhancement at high emitter densities and may be alleviated with the incorporation of a proximal, individually-gated extractor electrode.