Solid surface contact angle

By assuming spherical geometry it is possible, knowing the solid surface contact angles to determine suitable radii of curvature, i.e. 

Another aspect to be considered when measuring contact angles is the phenomenon of contact angle hysteresis. This term describes the observation that different values of 6 may be obtained depending upon whether the liquid drop is advanced or withdrawn across the solid surface. Contact angle... 

Neumann, A.W. and Good, R.J., Hope, C.J. and Sejpal, M. (1974). An equation od state approach to determine surface tension of low energy solids from contact angles. J. Colloid. Interface Sci. 49, 291-303. 

FIGURE 5.2 The state of equilibrium between the surface tensions of liquid (GL)-solid (GS)-liquid/solid (GLS)-contact angle (CA). 

A number of studies have examined fibril formation in the presence of different solid nonbiological surfaces, as summarized in Table 1. While many of these studies have focused on the formation of fibrils, the wettability and RMS of some surfaces have been characterized. Typical surface contact angles are also presented in Table 1 to aid comparison between these surface-based experiments. 

The problem here consists in listing the material parameters because, in activated sludge, they fluctuate strongly (much in contrast to ore flotation). Surely, the degree of hydrophobicity (wettability) of the particle surface (contact angle, ) will be of importance. Furthermore, the pH value, the concentration of the flocculant (poly-electrolyte), cF, the portion of solids, [Pg.30]

The pore size distribution is derived, assuming a cylindrical pore model, from the intrusion volume-pressure curve using the Washburn law dp = -Ay cos0) / P, where y is the surface tension of mercury (484 mN/m), 6 the solid/mercury contact angle (130°) and P the pressure exerted by the mercury. 

APc depends on the liquid surface tension, y, the liquid-solid (membrane) contact angle, 0 (greater than 90° for hydrophobic membranes), the membrane pore radius, r, and the pore geometry co-efficient, B, in accordance with the Laplace equation [Eq. (1)]. B has a value of 1 for cylindrical pores. The higher the liquid surface tension, the larger the contact angle, and the smaller the pore radius, the greater the pressure required for pore intrusion by the liquid. 

If the surface tension of the liquid is significantly lower than that of the solid, the contact angle may go to 0°. Viscosity permitting, such a liquid will spontaneously spread over the surface of the solid. This level of wetting offers good adhesion potential. 

In view of the close chemical similarity between paraffin and polyethylene, the large difference in the estiniates of the solid surface tensions is rather unexpected and deserves comment. This difference is, by virtue of the assumptions and related estimates used in the present approach, reflected in the rather different contact angles observed for water on these two solids. This contact angle difference has been confirmed and commented on by Adam and Elliott [1]. They attribute the difference from the expected behavior for polyethylene to the presence of traces of polar impurities. 

Since the time that study was made, a large body of experimental data has accumulated which shows that regular and predictable changes in solid-liquid contact angles result from changes in the outermost layer of atoms in the solid surface. A "wettability spectrum" has been published [14],and the utility of the "critical surface tension of wetting" as an index of solid surface energy has been well established. 

Estimation of surface energies of solids from contact angle data, J. Phys. Chem. 64 561 (1960). 

The wettability of solid surfaces is a veiy important properly of surface chemistiy, which is controlled by both the chemical composition and the geometrical microsttuc-ture of surface [21-23], When a liquid droplet contacts a solid surface, it will sptead or remain as droplet with the formation of angle between the liquid and solid phases. Contact angle (CA) measurements are widely used to characterize the wettability of solid surface. Surface with a water CA greater than 150° is usually called superhydrophobic surface. On the other hand, when the CA is lower than 5°, it is called superhy-drophilic surface. Fabrication of these surfaces has attracted considerable interest for both fundamental research and practical studies [23-25]. 

The wettabihty of a sohd surface is defined in terms of the contact angle 9 made by a hquid drop resting on the solid surface. The angle 6 can take on values ranging from 0 to 180°. If the sohd surface is idealized as smooth, chemicaUy homogeneous. 

When three phases are present, three different interfaces are possible, one for each pair of fluids. Sometimes all three interfaces meet, the jimction forming a curve known as a contact line. If one phase is a sohd, the contact line lies along its surface. In this case, the angle that the fluid interface makes with the solid surface is called the contact angle. Since it reflects the wetting properties of liquids on solids, the contact angle is a second fundamental property important in interfacial phenomena. 

Bascom, W.D., Cottington, R.L., and Singleterry, C.R., Dynamic surface phenomena in the spontaneous spreading of oils on solids, in Contact Angle, Wettability, and Adhesion, Advances in Chemistry Series, vol. 43, American Chemical Society, Washington, D.C., 1964, p. 355. 

Sessile drop This method involves measurement of the contact angle optically and is used to estimate the wetting properties of a localized region on a solid surface. The angle between the baseline of the drop and the tangent at the drop boundary are determined. This technique is ideal for curved samples or when one side of the sample surface has different properties than the other side. 

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