Contact angle measurement indirect measurements

It is hence essential to be able to measure and modify the surface free energy of the solid if criteria other than trial and error are used to predict the wettability (and the colloidal stability, for that matter) of a solid drug, since the surface tension of the liquid is readily measurable by any well-establi.shed method (Wilhelmy plate, du Nouy ring, drop shape analysis, etc. see. e.g. Ref. 37). In the case of solids, only indirect methods are available to estimate Ys and Ysl here we shall give details on the simplest one. contact angle measurements, and the reader is referred to other chapters of this volume for details on other methodologies. The technique is based on Young s equation (37) ... 

Equilibrium contact angles can be measured very simply from the profiles of liquid drops (Figure 2a) or bubbles (Figure 2b) resting on a plane surface. These methods are known as the sessile drop and captive bubble methods respectively. The contact angle may be measured indirectly by... 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

Many different techniques have been developed for the measurement of contact angles 17.8). Of these, the three most useful methods are the Wilhelnty technique, the technique of capillary rise at a vertical plate, and the drop shape methods. These techniques require the solid surface to be flat and smooth. Direct measurement of contact angles on fibers (of uniform thickness) can also be performed using the Wilhelmy technique. For nonflat surfaces or particles, indirect methods such as capillary penetration into columns of powders, sedi-... 

The measurement of contact angles on solids in powder or particle form is a challenging task, and numerous methods have been proposed 7. In this section, three successful indirect approaches are discussed capillary penetration into columns of powders, sedimentation volume of particles, and solidification fronts of particles. These methods are indirect because they provide the solid surface tension rather than the contact angle. For the sake of completeness, various direct approaches are also summarized briefly. 

The Contact Angle When the substrate is a solid, the spreading coefficient is usually evaluated by indirect means, since surface and interfacial tensions of solids cannot easily be measured directly. The method of doing this involves measuring the contact angle the substrate makes with the liquid in question. 

The contact angle may be calculated indirectly from measurement of the dimensions of a sessile drop. In order to carry out such a calculation, the drop should be small enough so... 

Indirect angle measurement on four contact points... 

Contact angles between solids and liquids of known surface tension can provide an indirect measurement of solid surface free energy, allowing a comparison between different surfaces. Just as liquids have surface tensions or surface energy, so do solid surfaces by virtue of the fact that they are surfaces. Surface tension of solids goes unnoticed because they are usually too rigid to be visibly distorted by the interatomic, rather than intermolecular, forces... 

There are a variety of simple and inexpensive techniques for measuring contact angles, most of which are described in detail in various texts and publications and will be mentioned only briefly here. The most common direct methods (Fig. 17.4) include the sessile drop (a), the captive bubble (b), the sessile bubble (c), and the tilting plate (d). Indirect methods include tensiometry and geometric analysis of the shape of a meniscus. For solids for which the above methods are not applicable, such as powders and porous materials, methods based on capillary pressures, sedimentation rates, wetting times, imbibition rates, and other properties, have been developed. 

The surface tension of solid plastics cannot be measured directly and is thus determined indirectly, usually by contact angle methods. The problem with the direct measurement of surface tension arises from the difficulty in the reversible formation of a solid surface. 

There are several approaches to the estimation of surface tensions of solids, but most are indirect measurements based on contact angle measurement. The measurement of contact angles of pure liquids, with known surface tension, on a given solid surface is the general way of estimating the surface tension of the solid material. Various approaches are summarized hereafter. 

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