Wick flow

Another factor and potential problem that can affect mobility is wick flow. During electrophoresis heat evolved because of the passage of current through a resistive medium can cause evaporation of solvent from the electrophoretic support. This drying effect draws buffer into the support fi om both buffer compartments. If significant, the flow of buffer from both directions can affect protein migration and hence the calculated mobility. 

The effects of surface tension on sessile and pendent drops or lenses are but a simple manifestation of capillary hydrostatics. The field of capillarity can be far more extensive, principally when coupled with electromagnetic forces and also for liquid interfaces in motion, or in the motion in liquid interfaces that may result from local variations in surface tension as may be caused, for example, by local variations in temperature, or by the localized introduction of surfactants (interfacial tension modifiers), or by localized space-delimited chemical reactions. Wicking flows (as in heat pipes ) and flows in porous media (as in petroleum reservoir displacement) are a few of many other examples in which interfacial forces play a predominant role. ... 

Heat enters the evaporator end of the heat pipe via conduction to the vessel and to the fluid-filled wick. The liquid in the wick changes into its liquid phase. Inside the heat pipe, there is a small pressure differential between the evaporator end and the condenser end, caused by the small temperature differential. The pressure differential causes the vapor to flow toward the condenser area where it condenses within the wick and releases heat. The released heat is conducted through the heat-pipe walls to the ultimate heat sink. The condensed liquid in the wick flows back to the evaporator area via capillary action. 

In this section, we will address situations in LCM processes where capillary effects (wicking flows) might be important and put forth approaches on how to model and account for them, when conducting macro or meso level... 

Nowadays, numerical simulations of the LCM filling phase can already account for the wicking flow [73], for the voids and dry spots formation, and changes [73] (LCMFlot, LIMS) for the sink term (filling of the fiber tows) with capillary action included (LIMS or with the new name SLIMS) [77,86]. Thus the problem which is still not satisfactorily resolved is the correct determination of saturation curves and relative permeabilities for particular preform geometries. 

The Washburn model is consistent with recent studies by Rye and co-workers of liquid flow in V-shaped grooves [49] however, the experiments are unable to distinguish between this and more sophisticated models. Equation XIII-8 is also used in studies of wicking. Wicking is the measurement of the rate of capillary rise in a porous medium to determine the average pore radius [50], surface area [51] or contact angle [52]. 

Two different types of dynamic test have been devised to exploit this possibility. The first and more easily interpretable, used by Gibilaro et al [62] and by Dogu and Smith [63], employs a cell geometrically similar to the Wicke-Kallenbach apparatus, with a flow of carrier gas past each face of the porous septum. A sharp pulse of tracer is injected into the carrier stream on one side, and the response of the gas stream composition on the other side is then monitored as a function of time. Interpretation is based on the first two moments of the measured response curve, and Gibilaro et al refer explicitly to a model of the medium with a blmodal pore... 

More recent versions of this type of probe include some refinements, such as the provision of a wick to aid evaporation of the solvent and matrix from the probe tip (Figure 13.5). Such improvements have allowed greater flow rates to be used, and rates of 1 to 10 ml/min are possible. For these sorts of low flow rates, minibore LC columns must be employed. 

Several wick stmctures are in common use. First is a fine-pore (0.14—0.25 mm (100-60 mesh) wire spacing) woven screen which is roUed into an annular stmcture consisting of one or more wraps inserted into the heat pipe bore. The mesh wick is a satisfactory compromise, in many cases, between cost and performance. Where high heat transfer in a given diameter is of paramount importance, a fine-pore screen is placed over longitudinal slots in the vessel wall. Such a composite stmcture provides low viscous drag for Hquid flow in the channels and a small pore size in the screen for maximum pumping pressure. 

The cross-sectional area of the wick is deterrnined by the required Hquid flow rate and the specific properties of capillary pressure and viscous drag. The mass flow rate is equal to the desired heat-transfer rate divided by the latent heat of vaporization of the fluid. Thus the transfer of 2260 W requires a Hquid (H2O) flow of 1 cm /s at 100°C. Because of porous character, wicks are relatively poor thermal conductors. Radial heat flow through the wick is often the dominant source of temperature loss in a heat pipe therefore, the wick thickness tends to be constrained and rarely exceeds 3 mm. 

Regardless of which, or which combination, of the above mechanisms is responsible for adhesion in a given case, intimate molecular contact between the adhesive and adherend is required. This means that the contact angle of the liquid adhesive against the adherend surface should be as low as possible, and preferably 0°. For the case of contact adhesion, this is immediately evident, but in cases where mechanical interlocking is the primary mechanism for adhesion it is also the case because the adhesive must first be able to flow or wick into the pores of the... 

Dukler, A. E. Moye Wicks III, and Cleveland, R. G. <4. I. Ch. E. Jl. 10 (1964) 38. Frictional pressure drop in two-phase flow. A comparison of existing correlations for pressure loss and holdup. 

Dukler, A. E., M. Wicks, and R. E. Cleveland, 1964, Frictional Pressure Drop in Two Phase Flow, B. 

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