Contact, receding

Asymptotic Refers to not meeting. An example is a straight line that is a limiting position of a tangent to a curve as its point of contact recedes indefinitely along an infinite branch of the curve. 

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 Wilhelmy slide has been operated in dynamic immersion studies to measure advancing and receding contact angles [59] (see Chapter X). It can also... 

Fig. X-7. Advancing and receding contact angles of octane on mica coated with a fluo-ropolymer FC 722 (3M) versus the duration of the solid-liquid contact. The solid lines represent the initial advancing and infinite time advancing and receding contact lines and the dashed lines are 95% confidence limits. (From Ref. 75.)...

Fig. X-12. Advancing and receding contact angles of various liquids [water (circles), Gly = glycerol (squares), Form = formamide (diamonds), EG = ethylene glycol (circles), BN = abromonapthalene (squares), BCH = bicyclohexyl (diamond), HD = hexadecane (circles)] on monolayers of HS(CH2)i60R having a range of R groups adsorbed on gold and silver (open and filled symbols respectively). (From Ref. 171.)...

One other cause of hysteresis remains to be mentioned. As was pointed out earlier (p. 177) the contact angle may be different as the mercury is advancing over or receding from a solid surface, and it depends also on the chemical and physical state of the surface the mercury may even react with the surface layer of the solid to form an amalgam. A change in 9 of only a few degrees has a significant effect on the calculated value of pore radius (cf. Table 3.15). 

In a similar procedure, the atomizer test, which depends on the behavior of an advancing rather than a receding contact angle, a fine mist of water is apphed to the metal surface and the spreading of water is observed. On a clean surface, water spreads to a uniform film. With oleic acid as the test soil, the atomizer test can detect the presence of 10 mg of soil per cm, less than a monomolecular layer (115). For steel that is to be electroplated, the copper dip test is often employed. Steel is dipped into a cupric salt solution and the eveimess of the resulting metallic copper deposit is noted. 

The JKR equation enables values for surface energy to be obtained both when the surfaces are advancing into closer contact, and when they are receding further apart, These two values would be expected to be the same, as indeed they some times are. In many cases, however, there is hysteresis with > > a-Israelachvili and his colleagues have studied this phenomenon in some detail [23,24,86,87]. 

Figure 2.6 Advancing and receding contact angle for a droplet slowly moving from left to right on a solid surface.

Angle to the main flow direction Surface coverage Advancing contact angle Dynamic contact angle Receding contact angle Density Eluid density Residual... 

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