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Microfluidic networks (µFNs) are passive, self-filling devices which can be used for depositing lines of biomolecules on surfaces at high resolution. We have explored the use of µFNs replica molded in poly(dimethylsiloxane) (PDMS) or micromachined in Si. The density of biomolecules patterned on a substrate can be controlled through the channel geometry and surface chemistry, and through the volume and concentration of injected biomolecules. For instance, hundreds of micrometer long density gradients of adsorbed biomolecules could be formed. Further, we used µFNs to perform bioassays in an array-format with a spot size of 20 × 20 µm2. We present advanced µFNs and a sealing strategy relying on free surface energy. Hard Si µFNs can thus be sealed onto a hard substrate by simply pressing the two surfaces against each other.

Experiments are described to investigate local scour at bridge abutments. The experiments were performed in a two-stage channel using abutments that extended different distances onto the floodplain including right up to the edge of the main channel (Melville, Type III). To ensure the largest scour depths the conditions on the floodplain upstream of the abutment were close to critical conditions for the bed material. The time evolution of the scour and the ultimate scour depth were measured. The time development of the local scour corresponded well with the theories of Ettema and Franzetti and the theory of Whitehouse for scour at horizontal cylinders in the marine environment. Melville has suggested that scour at abutments on floodplains can be approximated by scour in rectangular channels if an imaginary boundary is assumed, separating the flow in the main channel from that on the floodplain. The experimental results confirm the validity of Melville's suggestion for the configurations tested in the experiments.

Recently, applied research activities on the miniaturization of conventional forming processes have received attention in search for mass production methods for micro-scale metallic parts. This study investigated the coining of micro-features by numerical analyses and experiments. Prior to conducting the experiments, finite element simulation was used to understand the effects of channel aspect ratio, workpiece thickness, channel number, channel geometry, and friction on the coining process. Experiments using pure copper specimens were performed to confirm the findings from the simulation and to study the effects of preform shape and grain size. It was found that the channel depth, workpiece thickness, and channel geometry had a significant effect on the final load, while the channel number did not influence the final load. It was also concluded that certain preform shapes induce favorable flow of the material and result in enhanced formation of the micro-protrusions. For the conditions used in the experiments, however, effects from the grain size were not observed.Copyright © 2006 by ASME

Mass transfer from single CO2 Taylor bubbles in vertical minichannels was measured for various channel hydraulic diameters Dh. The effects of channel geometries on the mass transfer were also investigated by using square ducts and circular pipes. Bubble rising velocities, vB, in the ducts were much faster than those in the pipes due to large liquid flow areas in the corners of the ducts. The values of mass transfer coefficients in the pipes were almost the same as those in the ducts, in spite of a large difference in vB. Sherwood numbers, ShD, using Dh as a characteristic length, are well-correlated in terms of the Eötvös number. The proposed ShD correlation can well predict a long-term dissolution process of a Taylor bubble.

Microfluidic channels are powerful means of control of minute volumes such as droplets. These droplets are usually conveyed at will in an externally imposed flow which follows the geometry of the micro-channel. It has recently been pointed out by Dangla et al. [“Trapping microfluidic drops in wells of surface energy,” Phys. Rev. Lett.107(12), 124501 (2011)] that the motion of transported droplets may also be stopped in the flow, when they are anchored to grooves which are etched in the channels top wall. This feature of the channel geometry explores a direction that is usually uniform in microfluidics. Herein, this anchoring effect exploiting the three spatial directions is studied combining a depth averaged fluid description and a geometrical model that accounts for the shape of the droplet in the anchor. First, the presented method is shown to enable the capture and release droplets in numerical simulations. Second, this tool is used in a numerical investigation of the physical mechanisms at play in the capture of the droplet: a localized reduced Laplace pressure jump is found on its interface when the droplet penetrates the groove. This modified boundary condition helps the droplet cope with the linear pressure drop in the surrounding fluid. Held on the anchor the droplet deforms and stretches in the flow. The combination of these ingredients leads to recover the scaling law for the critical capillary number at which the droplets exit the anchors Ca★∝h2/R2 where h is the channel height and R the droplet undeformed radius.

We study viscous fingering of an air-nematic interface in a radial Hele-Shaw cell when periodically switching on and off an electric field, which reorients the nematic and thus changes its viscosity, as well as the surface tension and its anisotropy (mainly enforced by a single groove in the cell). We observe undulations at the sides of the fingers which correlate with the switching frequency and with tip oscillations which give maximal velocity to smallest curvatures. These lateral undulations appear to be decoupled from spontaneous (noise-induced) side branching. We conclude that the lateral undulations are generated by successive relaxations between two limiting finger widths. The change between these two selected pattern scales is mainly due to the change in the anisotropy. This scenario is confirmed by numerical simulations in the channel geometry, using a phase-field model for anisotropic viscous fingering., completely rewritten version, more clear exposition of results (14 pages in Revtex + 7 eps figures)

Investigations of channel geometry in a headwater stream in Cornwall, England, reveal that systematic variations of channel width occur in conjunction with the oscillations of bed topography in the riffle-pool sequence. The channel tends to be about 15 percent wider in the riffle section, where central bars of coarse bed material divert the flow against the banks. This is reflected in the occurrence of distinct downstream trends of width as a function of bankfull discharge in riffle and pool sections. There is some evidence that the widest channel occurs just downstream from the summit of the riffle, which indicates that the width oscillation lags behind the profile oscillation, presumably because it is related to flow characteristics induced by bed topography.

It is shown that the MOSFET channel length measurement techniques of Terada and Muta, Peng et al., Whitfield, Suciu and Johnston, and De La Moneda et al. are actually equivalent, i.e. merely different expressions of the same formula for channel length in terms of measured resistance, and that some of the transresistance methods of Jain, although not equivalent, are also related to the same formula. The accuracy of this formula is evaluated for the general case and related to the error components due to source and drain resistance asymmetry, short channel geometry effect, and variation of series resistance with bias. No independent error component due to field-induced mobility degradation is found. Finally the errors in the methods of Terada and Muta, Chen et al., Sheu et al., Wordeman et al. and Jain, are determined and compared. The gate transresistance technique is found to be the most accurate method.