About: Microchannel is a(n) research topic. Over the lifetime, 14178 publication(s) have been published within this topic receiving 270770 citation(s).
24 Feb 2006-Lab on a Chip
TL;DR: Experimental results support the assertion that the dominant contribution to the dynamics of break-up arises from the pressure drop across the emerging droplet or bubble.
Abstract: This article describes the process of formation of droplets and bubbles in microfluidic T-junction geometries. At low capillary numbers break-up is not dominated by shear stresses: experimental results support the assertion that the dominant contribution to the dynamics of break-up arises from the pressure drop across the emerging droplet or bubble. This pressure drop results from the high resistance to flow of the continuous (carrier) fluid in the thin films that separate the droplet from the walls of the microchannel when the droplet fills almost the entire cross-section of the channel. A simple scaling relation, based on this assertion, predicts the size of droplets and bubbles produced in the T-junctions over a range of rates of flow of the two immiscible phases, the viscosity of the continuous phase, the interfacial tension, and the geometrical dimensions of the device.
Abstract: A three-dimensional serpentine microchannel design with a "C shaped" repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a "square-wave" channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring.
01 Jun 1979-Nuclear Instruments and Methods
Abstract: A microchannel plate (MCP) is an array of 104-107 miniature electron multipliers oriented parallel to one another (fig. 1); typical channel diameters are in the range 10-100 μm and have length to diameter ratios (α) between 40 and 100. Channel axes are typically normal to, or biased at a small angle (~8°) to the MCP input surface. The channel matrix is usually fabricated from a lead glass, treated in such a way as to optimize the secondary emission characteristics of each channel and to render the channel walls semiconducting so as to allow charge replenishment from an external voltage source. Thus each channel can be considered to be a continuous dynode structure which acts as its own dynode resistor chain. Parallel electrical contact to each channel is provided by the deposition of a metallic coating, usually Nichrome or Inconel, on the front and rear surfaces of the MCP, which then serve as input and output electrodes, respectively. The total resistance between electrodes is on the order of 10 Ω Such microchannel plates, used singly or in a cascade, allow electron multiplication factors of 10-10 coupled with ultra-high time resolution (< 100 ps) and spatial resolution limited only by the channel dimensions and spacings; 12 μm diameter channels with 15 μm center-to-center spacings are typical. Originally developed as an amplification element for image intensification devices, MCPs have direct sensitivity to charged particles and energetic photons which has extended their usefulness to such diverse fields as X-ray) and E.U.V.) astronomy, e-beam fusion studies) and of course, nuclear science, where to date most applications have capitalized on the superior MCP time resolution characteristics). The MCP is the result of a fortuitous convergence of technologies. The continuous dynode electron multiplier was suggested by Farnsworth) in 1930. Actual implementation, however, was delayed until the 1960s when experimental work by Oschepkov et al.) from the USSR, Goodrich and Wiley) at the Bendix Research Laboratories in the USA, and Adams and Manley) at the Mullard Research Laboratories in the U.K. was described in the scientific literature. These developments relied heavily on a wealth of information on secondary electron emission) and earlier work on the technique of producing resistive surfaces in lead glasses by high temperature reduction (250-450 °C) in a hydrogen atmosphere. Finally, since most of the electrical performance characteristics of channel multipliers are not a function of channel length, l, or channel diameter, d, separately, but only a function of the ratio l/d =α, an almost arbitrary size reduction is possible. Such size reduction may be achieved by glass fiber drawing techniques which form the basis of fiber op tic device fabrication). In addition to a significant dimensional reduction resulting from these methods, a logarithmic compression of repetitive manufacturing steps is also possible, i.e., one can achieve a structure with ~10 holes requiring ~2 x 10 fiber alignment steps by a draw/multidraw technique. Prior to the application of reliable fiber drawing techniques, however, the first operational MCPs were
09 Nov 2004-Physics of Fluids
Abstract: A series of experiments is presented which demonstrate significant drag reduction for the laminar flow of water through microchannels using hydrophobic surfaces with well-defined micron-sized surface roughness. These ultrahydrophobic surfaces are fabricated from silicon wafers using photolithography and are designed to incorporate precise patterns of microposts and microridges which are made hydrophobic through a chemical reaction with an organosilane. An experimental flow cell is used to measure the pressure drop as a function of the flow rate for a series of microchannel geometries and ultrahydrophobic surface designs. Pressure drop reductions up to 40% and apparent slip lengths larger than 20 μm are obtained using ultrahydrophobic surfaces. No drag reduction is observed for smooth hydrophobic surfaces. A confocal surface metrology system was used to measure the deflection of an air–water interface that is formed between microposts and supported by surface tension. This shear-free interface reduces the ...
13 Feb 2002-Physics of Fluids
Abstract: Micron-resolution particle image velocimetry is used to measure the velocity profiles of water flowing through 30×300 μm channels. The velocity profiles are measured to within 450 nm of the microchannel surface. When the surface is hydrophilic (uncoated glass), the measured velocity profiles are consistent with solutions of Stokes’ equation and the well-accepted no-slip boundary condition. However, when the microchannel surface is coated with a 2.3 nm thick monolayer of hydrophobic octadecyltrichlorosilane, an apparent velocity slip is measured just above the solid surface. This velocity is approximately 10% of the free-stream velocity and yields a slip length of approximately 1 μm. For this slip length, slip flow is negligible for length scales greater than 1 mm, but must be considered at the micro- and nano scales.