We survey progress over the past 25 years in the development of microscale devices for pumping fluids. We attempt to provide both a reference for micropump researchers and a resource for those outside the field who wish to identify the best micropump for a particular application. Reciprocating displacement micropumps have been the subject of extensive research in both academia and the private sector and have been produced with a wide range of actuators, valve configurations and materials. Aperiodic displacement micropumps based on mechanisms such as localized phase change have been shown to be suitable for specialized applications. Electroosmotic micropumps exhibit favorable scaling and are promising for a variety of applications requiring high flow rates and pressures. Dynamic micropumps based on electrohydrodynamic and magnetohydrodynamic effects have also been developed. Much progress has been made, but with micropumps suitable for important applications still not available, this remains a fertile area for future research.

Ultra-high-speed video microscopy and numerical modeling were used to assess the dynamics of microbubbles at the surface of urinary stones. Lipid-shell microbubbles designed to accumulate on stone surfaces were driven by bursts of ultrasound in the sub-MHz range with pressure amplitudes on the order of 1 MPa. Microbubbles were observed to undergo repeated cycles of expansion and violent collapse. At maximum expansion, the microbubbles' cross-section resembled an ellipse truncated by the stone. Approximating the bubble shape as an oblate spheroid, this study modeled the collapse by solving the multicomponent Euler equations with a two-dimensional-axisymmetric code with adaptive mesh refinement for fine resolution of the gas-liquid interface. Modeled bubble collapse and high-speed video microscopy showed a distinctive circumferential pinching during the collapse. In the numerical model, this pinching was associated with bidirectional microjetting normal to the rigid surface and toroidal collapse of the bubble. Modeled pressure spikes had amplitudes two-to-three orders of magnitude greater than that of the driving wave. Micro-computed tomography was used to study surface erosion and formation of microcracks from the action of microbubbles. This study suggests that engineered microbubbles enable stone-treatment modalities with driving pressures significantly lower than those required without the microbubbles.

The behavior of n-and p-Si as electrodes for the reduction and oxidation of several redox systems (e.g., benzonitrile, 9,10-diphenylanthracene, anthraquinone, benzoquinone) in acetonitrile solutions in the dark and under illumination was investigated. The results were correlated with the relative energy levels of the valence and conduction bands of the semiconductor and those of the redox couples in solution, estimated by determination of the flat-band potential of the semiconductor-solution system by photopotential methods. The electrode behavior of Si strongly depended upon the electrode pretreatment and the absence of ideal semiconductor electrode behavior for this relatively small band gap material suggests the importance of surface states and breakdown phenomena for these electrodes. A unique electrogenerated chemiluminescence (ECL) experiment with a p-Si electrode is described.

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Factors affecting the lifetime of ECL devices, including decomposition of starting materials, formation of side products which react with radical ions or quench excited states, and filming of the electrodes are discussed. The mode of operation of the ECL device is shown to be important in determining its behavior and lifetime; operation of. (i) three-electrode pulsed potentiostatic, (ii) two-electrode pulsed voltage, (iii) two-electrode pulsed constant current, and (iv) thin-layer cells is discussed and illustrated, in studies of the Ru(bip)8 ~+ system in acetonitrile and the rubrene system in benzonitrile. Experimental Purification of the solvents, acetonitrile (ACN) and benzonitrile (BZN), the supporting e.~ectrolyte, tetran-butylammonium perchlorate (TBAP), and the reactants followed previous practice (7, 11). All experiments were conducted in a Vacuum Atmospheres (Los Angeles, California) Glove Box equipped with a Model MO 40-1 Dri-Train.