Capillary rise method

In the case of incomplete wetting, characterized by a nonzero value of the contact tingle 0, the earlier expression is written as 

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The capillary rise method is generally considered to be the most accurate means to measure 7, partly because the theory has been worked out with considerable exactitude and partly because the experimental variables can be closely controlled. This is to some extent a historical accident, and other methods now rival or surpass the capillary rise one in value. 

Perhaps the best discussions of the experimental aspects of the capillary rise method are still those given by Richards and Carver [20] and Harkins and Brown [21]. For the most accurate work, it is necessary that the liquid wet the wall of the capillary so that there be no uncertainty as to the contact angle. Because of its transparency and because it is wet by most liquids, a glass capillary is most commonly used. The glass must be very clean, and even so it is wise to use a receding meniscus. The capillary must be accurately vertical, of accurately known and uniform radius, and should not deviate from circularity in cross section by more than a few percent. 

The general attributes of the capillary rise method may be summarized as follows. It is considered to be one of the best and most accurate absolute methods, good to a few hundredths of a percent in precision. On the other hand, for practical reasons, a zero contact angle is required, and fairly large volumes of solution are needed. With glass capillaries, there are limitations as to the alkalinity of the solution. For variations in the capillary rise method, see Refs. 11, 12, and 22-26. 

The following values for the surface tension of a 10 Af solution of sodium oleate at 25°C are reported by various authors (a) by the capillary rise method, y - 43 mN/m (b) by the drop weight method, 7 = 50 mN/m and (c) by the sessile drop method, 7 = 40 mN/m. Explain how these discrepancies might arise. Which value should be the most reliable and why ... 

The wetting ability of the anode electrode was evaluated as the contact angle measured by the capillary rise method. The value of fractal dimension of anode electrode of MCFC was calculated by use of the nitrogen adsorption (fractal FHH equation) and the mercury porosimetry. 

Here, A is the contacting surface area of anode electrode facing with electrolyte and P is the porosity of anode electrode. The average effective radius of pore,, could be calculated from the results of the capillary rise method using ethanol, which shows a contact angle of 0° with the anode electrode. And then, the contact angle 0 could be acquired as the slope from the plot of m versus... 

One of the most common ways to characterize the hydrophobicity (or hydrophilicity) of a material is through measurement of the contact angle, which is the angle between the liquid-gas interface and the solid surface measured at the triple point at which all three phases interconnect. The two most popular techniques to measure contact angles for diffusion layers are the sessile drop method and the capillary rise method (or Wihelmy method) [9,192]. 

It is important to note that a number of different techniques are based on the Wilhelmy method. Here, we will describe only the capillary rise method (based on the Wilhelmy gravimetric plate technique) presented by Lim and Wang [101] and Wang [192]. For more detail on the other Wilhelmy methods, please refer to Mathias et al. [9]. 

The capillary-rise method was employed to measure the surface tension of aqueous solutions of disodlum alkyl phosphate at 25 °C. The cmc values of the solutions were obtained from the discontinuity in the surface tension - concentration curves(7). 

Figure 2. Surface tensions of sodium dodecylsulfonate solutions with and without polymer addition as measured by the capillary rise method.

Lim, C. Wang, C. Y. Measurement of contact angles of liquid water in PEM fuel cell gas diffusion layer (GDL) by sessile drop and capillary rise methods. Penn State University Electrochemical Engine Center (ECEC) Technical Report no. 2001 03, Perm State University State College, PA, 2001. 

The capillary rise method, althongh simple, is in practice, not as useful as the pendant drop method because of several experimental problems, snch as the need to determine the contact angle, non-sphericity of the meniscns and nneven bore of the capillary. 

The approximation that limits this analysis of capillary rise originates from neglecting the weight of the liquid in the crown of the curved meniscus. We see in Section 6.8b that the height of capillary rise can be related to surface tension without making this approximation, although the connection is somewhat unwieldy. A more detailed description of the experimental aspects of the capillary rise method can be obtained from advanced textbooks (e.g., Adamson 1990). 

Both the Wilhelmy and capillary rise methods for determining 7 have been based on the concept of surface tension as a force. While this point of view is useful for describing the experimental methods we have discussed, it is only one way of interpreting 7. An energetic interpretation is also possible that makes surface tension amenable to the powerful methods of thermodynamics. 

In many applications, powders come into contact with a liquid and we would like to quantify their wetting behavior. The usual way to do this is by the capillary rise method [233,234], In a capillary rise measurement the powder is pressed into a tube of typically 1 cm diameter (Fig. 7.7). This porous material is then treated as a bundle of thin capillaries with a certain effective radius [235-237], In order to measure this effective radius, first a completely wetting liquid is used. Either the speed of the liquid rise is measured (this technique is sometimes referred to as the capillary penetration technique [238]) or the pressure required to keep the liquid out of the porous material, is determined. This backpressure is equal to the... 

Comparing the two pressures directly leads to the contact angle. One limitation of the capillary rise method is that it averages over many particles and the actual size distribution remains unknown. In addition, it relies on the assumption that a powder can be treated as a bundle of capillaries and depends on the specific model applied [239,240],... 

Figure 7.7 Capillary rise method to quantify the wetting properties of powders or porous materials.

Contact angles are commonly measured by the sessile drop, the captive bubble, and the Wilhelmy plate method. To characterize the wetting properties of powders the capillary rise method is used. 

In practice, the capillary rise method is only used when the contact angle is zero, owing to the uncertainty in measuring contact angles... 

The capillary rise method is a classical method for determining surface tension that has important applications. For capillary rise in a narrow capillary (Figure 3.7) a force balance yields ... 

Figure 30. Rewetting data. Capillary rise method U. Baumgarte Melliand Tex-tilberichte, 49, 1306 (1968). (mm) height of the elevation of test solution. ((- -) Cto-i SAE (- -) Cn-n SAE (-Q-) C1Jf.16 SAE (-M ) NPE)...

Surface Tension. The surface tension of various ozone-fluorine mixtures was determined by the capillary rise method in the apparatus used for the viscosity measurements (1) using Equation 4. 

All these difficulties are virtually absent in the capillary rise method, which works on a very similar geometric constellation. 

The capillary-rise method is used to study the change in surface tension as a function of concentration for aqueous solutions of />butanol and sodium chloride. The data are interpreted in terms of the surface concentration using the Gibbs isotherm. 

O Connor RE, Berryman WH. Evaluation of enteric film permeability tablet swelling method and capillary rise method. Drug Dev Ind Pharm 1992 18 2123-2133. 

The surface tension of pure ozone was determined by the capillary rise method in the apparatus used for viscosity measurements. The ratio of the capillary rise of liquid ozone to that of water at 20° C. was measured and zero contact angle was assumed. Results at —183° and —195.5° C. are given in Table III. The parachor for... 

It should be noted that the pressure is always greater on the concave side of the interface irrespective of whether or not this is a condensed phase.) The phenomena due to the presence of curved liquid surfaces are called capillary phenomena, even if no capillaries (tiny cylindrical tubes) are involved. The Young-Laplace equation is the expression that relates the pressure difference, AP, to the curvature of the surface and the surface tension of the liquid. It was derived independently by T. Young and P. S. Laplace around 1805 and relates the surface tension to the curvature of any shape in capillary phenomena. In practice, the pressure drop across curved liquid surfaces should be known from the experimental determination of the surface tension of liquids by the capillary rise method, detailed in Section 6.1. 

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