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.
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.
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