Contact area development

M. M. Yovanovich, Transient Spreading Resistance of Arbitrary Isoflux Contact Areas Development of a Universal Time Function, AIAA-97-2458, AIAA 32nd Thermophysics Conference, Atlanta, GA, June 23-25,1997. 

In both of these pieces of apparatus, isothermal operation and optimum membrane area are obtained. Good temperature control is essential not only to provide a value for T in the equations, but also because the capillary attached to a larger reservoir behaves like a thermometer, with the column height varying with temperature fluctuations. The contact area must be maximized to speed up an otherwise slow equilibration process. Various practical strategies for presetting the osmometer to an approximate n value have been developed, and these also accelerate the equilibration process. 

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

There are, however, technical limitations to substitution. Some materials are used in ways not easily filled by others. Platinum as a catalyst, liquid helium as a refrigerant, and silver on electrical contact areas cannot be replaced they perform a unique function - they are, so to speak, the vitamins of engineering materials. Others - a replacement for tungsten for lamp filaments, for example - would require the development of a whole new technology, and this can take many years. Finally,... 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

A very practical way to infer the contact area was later developed by Carpick et al. [65] and Lantz et al. [66]. In these experiments, a small (up to nanometer) lateral modulation, djc, is applied to the sample, and torsion of the cantilever is monitored with a lock-in amplifier to detect the lateral force response, dF (Fig. 5). In this way, the lateral stiffness, [51], given by... 

All the models of mixed lubrication developed previously were based on a traditional idea, as schematically shown in Fig. 2, that the nominal contact zone, O, has to be divided into two different t3q>es of areas the lubricated area, 0,1, where two surfaces are separated by a lubricant film and the asperity contact area, where two surfaces are assumed to be in direct contact. The present authors and Dr. Zhu [16,17] proposed a different strategy for modeling... 

Traction zone or actual contact zone This region is the rear part of the contact area, beginning with the end of transition zone. It is the zone where most of the traction or skid resistance is developed. Here, the lubricating water him has been totally or substanhaUy removed, and vertical equilibrium of the tread elements on road surface has been attained. 

The study of transport between immiscible phases requires a device which (1) yields contacting surfaces with known contact area, (2) has a stationary interface, and (3) has known hydrodynamics. Stowe and Shaewitz [6] developed and evaluated a cell which meets these criteria. Each liquid is stirred with a rotating disk. As noted by these investigators, if the fluids are rotated in opposite directions by the rotating disks, then the torque on the interface cancels. The criterion for operation of the stirred cell with a stationary fluid interface is... 

Falling-film column. A falling-film column (Figure 24.1(d)) is also an empty vessel with liquid, introduced at the top, flowing down the wall as a film to con-tad an upward-flowing gas stream. Ideal flow for each phase is PF. Since neither liquid nor gas is dispersed, the interfacial area developed is relatively small, and gas-liquid contact is relatively inefficient This type is used primarily in the exper-imental determination of mass transfer characteristics, since the interfacial area is w ell defined... 

Another approach has been developed to fabricate electrodes with loading as low as 0.1 mg Pt/cm (32). The electrode structure was improved by increasing the contact area between the electrolyte and the platinum clusters. The advantages of this approach are that a thinner catalyst layer of 2 to 3 microns and a uniform mix of catalyst and ionomer are produced. For example, a cell with a Pt loading of 0.17 to 0.13 mg/cm has been fabricated. The cell generated 3 A/cm at > 0.4V on pressurized O2 and 0.65 V at 1 A/cm on pressurized air (32,... 

This method, which has been developed and extensively utilized in Russia [22,23], deals with nonstirred liquid phases and is based on measurements of the electrical conductivity of the aqueous phase and of its changes during extraction. In the experimental device, the two phases are initially kept separated inside a theimostated cell and then instantaneously contacted through a known contact area. At this moment, the concentration variation in one phase is recorded as a function of time, generally using conductometry. 

Australia, and scaled up by BP Solar in Spain, the heterojunction with intrinsic thin layer (HIT) cells developed by Sanyo by replacing the diffused P-doped emitter with an amorphous silicon layer and the back contact cells developed by Stanford University for use in concentrator technology and now converted to a large area for flat plate use. All three use single-crystalline silicon, while the majority of screen-printed cells use multicrystalline silicon wafers. 

In the in situ consolidation model of Liu [26], the Lee-Springer intimate contact model was modified to account for the effects of shear rate-dependent viscosity of the non-Newtonian matrix resin and included a contact model to estimate the size of the contact area between the roller and the composite. The authors also considered lateral expansion of the composite tow, which can lead to gaps and/or laps between adjacent tows. For constant temperature and loading conditions, their analysis can be integrated exactly to give the expression developed by Wang and Gutowski [27]. In fact, the expression for lateral expansion was used to fit tow compression data to determine the temperature dependent non-Newtonian viscosity and the power law exponent of the fiber-matrix mixture. 

Another osmometer design is shown assembled in Figure 3.3b and in detail in Figure 3.3c. The membrane is pressed between two grooved faces that contain solvent on one side and solution on the other. The grooves are attached to filling tubes and to capillaries in which the pressure head develops. This apparatus permits a large contact area between liquids with a minimum volume of liquids involved. 

Other problems might include kinetics, contact area, polymer losses, and regeneration or disposal of the contaminated polyols. [Note We are not suggesting that these problems are not addressed and mitigated by the fine researchers at Alabama. We are making a case for the development of an alternative.]... 

Once the wear track was formed, the coefficient of friction increased two to three times due to two factors. One was the increase in contact area between the ball and the conforming wear track. A second was the development of wear debris which was... 

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