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The world's first flow visualization representation (top) is a sketch of a free water jet issuing from a square hole into a pool, drawn by the hands of Leonardo da Vinci, circa 1500. Da Vinci wrote, "Observe the motion of the surface of the water, which resembles that of hair, which has two motions, of which one is caused by the weight of the hair, the other by the direction of the curls; thus the water has eddying motions, one part of which is due to the principal current, the other to the random and reverse motion." The bottom photograph was taken close to five centuries after that of da Vinci. Laser-induced fluorescence is used to reveal a side view of a low-Reynolds-number lifting surface undergoing a pitching maneuver in a water towing tank. Flow is from left to right, and the argon laser sheet is generated using a rotating mirror located above the wing; hence the shadow seen below.
Prashanta Dutta of Texas A&M University presented numerical simulations of liquid flows in microchannels with variable depth. He suggested the use of electro-osmotic flow to prevent or enhance recirculation flow in a gap. By applying different voltages across the gap, they obtained different flow patterns without recirculation and with non-symmetric vortices inside the gap. The implication is that such designs can reduce or enhance mixing in micro-devices. Sandra Troian of Princeton presented a new design for injecting and driving fluids in miocrochannels using temperature-driven Marangoni effects. She also suggested a self-assembly technique at microscales by inducing fluid fingering on a surface with periodic stripes of wetting and non-wetting materials. The large audience and the creative and preliminary nature of the reported work suggest that this field will blossom in the near future. Separation, mixing, dispersion reduction, driving mechanism, mutli-phase transport and reaction problems in microdevices will be the focus for the next few years.
During a session on electro-hydrodynamics: Paul Todd of the University Colorado studied the demixing of an emulsion of poly(ethylene glycol) in a phosphate-buffered solution in the presence of an electric field. Recirculation of liquid inside the drop was shown to be very important to its mobility, thus invalidating the infinite viscosity approximation.
Andreas Acrivos of CCNY reported particle separation in the presence of anelectric field when flowing in a wavy square channel. His experiments apply a high electric field across the channel to assemble the particles in wide parts of channel and removes particle in narrow parts, creating almost periodic packets of particles.
John Anderson of Carnegie Mellon University studied cluster formation near an electrode due to electrokinetic flow. Gravity keeps the particles on the electrode and the electrode field induces an electrokinetic flow that creates an attractive force between particles against their double-layer repulsion. This attraction can lead to cluster formation. These novel separation and clustering techniques could result in new devices, including microdevices.
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