Elucidation of fast chemical reactions such as protein folding requires resolution on a submillisecond time scale. However, most quench-flow and stop-flow techniques only allow chemical processes to be studied after a few milliseconds have elapsed. In order to shorten the minimum observation time for quench-flow experiments, we designed a miniaturized mixer assembly. Two “T” mixers connected by a channel are etched into a 1 cm × 1 cm silicon chip which is interfaced with a commercially available quench-flow instrument. Decreasing the volume of the mixing chambers and the distance between them results in an instrument with greatly reduced dead times. As a test of submillisecond measurements, we studied the basic hydrolysis of phenyl chloroacetate. This reaction proceeds with a second-order rate constant, k = 430 M-1 s-1, and shows pseudo-first-order kinetics at high hydroxide concentrations. The chemical reaction data demonstrate that the silicon device is capable of initiating and quenching chemical reactions in time intervals as short as 110 μs. The performance of these mixers was further confirmed by visualization using acid−base indicators.

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Microfilters for collecting micron-size airborne biological agents are designed and fabricated using a micro-electro-mechanical-system (MEMS) fabrication technology. The thickness of the microfilter ranges from 1 μm–3 μm, and the hole diameter from 5 μm–12 μm. Iterations between experimental and numerical studies are carried out to attain efficient microfilter designs with low pressure drop. Two orders of magnitude reduction of viscous power consumption have been achieved. A design rule of the filter in a low Reynolds-number range was first derived from numerical simulations. Highly accurate measurements of the three-dimensional (3-D) geometry, side-wall profile, and diameter of the micron-size holes are critical in validating and modifying the design rule. The effect of the surface slip is found to be small in the tested Knudsen-number range.

We report herr several particle membrane filters (8 x 8 mn2) with circular, hexagonal and rectangular through holes. By varying hole dimensions from 6 to 12 pm, opening factors from 4 to 45 ?4 arc achieved. In order to improve the filter robustness, a composite silicon nitridelparylene membrane technology is developed. More importantly, fluid dynamic performance of the filters is also studied by both experiments and numerical simulations. It is found that the gaseous flow through the filters depends strongly on opening factors, and the measured pressure drops are much lower than that from numerical simulation using the Navier-Stokes equation. Interestingly, surface velocity slip can only account for a minor part of the discrepancy. This suggests that a very interesting topic for micro fluid mechanics research is identified.

We report herr several particle membrane filters (8 x 8 mn2) with circular, hexagonal and rectangular through holes. By varying hole dimensions from 6 to 12 pm, opening factors from 4 to 45 ?4 arc achieved. In order to improve the filter robustness, a composite silicon nitridelparylene membrane technology is developed. More importantly, fluid dynamic performance of the filters is also studied by both experiments and numerical simulations. It is found that the gaseous flow through the filters depends strongly on opening factors, and the measured pressure drops are much lower than that from numerical simulation using the Navier-Stokes equation. Interestingly, surface velocity slip can only account for a minor part of the discrepancy. This suggests that a very interesting topic for micro fluid mechanics research is identified.

Technologies for fabricating silicone rubber membranes and integrating them with other processes on silicon wafers have been developed. Silicone rubber has been found to have exceptional mechanical properties including low modulus, high elongation, and good sealing. Thermopneumatically actuated, normally open, silicone rubber membrane valves with optimized components have been designed, fabricated, and tested. Suspended silicon nitride membrane heaters have been developed for low-power thermopneumatic actuation. Composite silicone rubber on Parylene valve membranes have been shown to have low permeability and modulus. Also, novel valve seats were designed to improve sealing in the presence of particles. The valves have been extensively characterized with respect to power consumption versus flow rate and transient response. Low power consumption, high flow rate, and high pressure have been demonstrated. For example, less than 40 mW is required to switch a 1-slpm nitrogen flow at 33 psi. Water requires close to 100 mW due to the cooling effect of the liquid. [427]

Previously, we reported a thermopneumatic silicone rubber membrane valve [l].This valve combined thermopneumatic actuation with a low modulus silicone rubber membrane. However, the leakage of the working fluid through the membrane rendered the valve unusable in a day or two. Here, we present extensive optimization and characterization of a redesigned valve structure. This new design has a suspended membrane heater optimized for low power consumption, a composite silicone rubber on Parylene membrane that exhibits low permeability and modulus, and a novel valve seat designed to improve sealing in the presence of particles. The valve has been extensively characterized with respect to power consumption vs. flow rate and transient response. Very low power consumption has been demonstrated. For example, less than 40 mW is required to switch a one slpm nitrogen flow at 33 psi. Water requires close to 100 mW due to the cooling effect of the liquid. The previously reported valve required more than 280 mW to switch a similar air flow.

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