10.8.1 Surface Shear
Application of a two-dimensional shear stress can be done in various ways,
for instance as depicted in Figure 10.33a; here an annular surface or
interface is sheared, and the torque on the disc (or, alternatively, on the
vessel) is measured. Naturally, also the bordering liquids are sheared, and to
obtain true surface parameters, the torque measured in the absence of
surfactant has to be subtracted.
Most workers determinesurface shear viscosityZSS, defined as the
(two-dimensional) shear stress over the shear rate. To be sure, most surface
layers are viscoelastic and shear rate–thinning, and one thus determines an
apparent viscosity ZSSa; often, the surface shear rate applied is of order
0.1 s^1. The value obtained has been called ‘‘film strength,’’ a very
misleading term. It is questionable whether a monolayer can be called a
film, since this word generally refers to a far thicker layer that has two
surfaces. More important, the property measured is not a strength, which
would be the stress needed for the adsorbed layer to break or maybe to
yield. In fact, also a surface shear modulusESScan be measured and, for a
large strain, yielding or fracture can possibly occur but systematic
experiments in that direction appear to be lacking.
FIGURE10.33 Principles of interfacial rheological measurements. (a) In shear. A
thin disc (D) is at an O–W interface and is made to rotate (or oscillate); the torque on
the disc can be measured, e.g., via a torsion wire (T). (b) In expansion/compression.
Barriers (B) at an O–W interface are moved, thereby increasing or decreasing the
interfacial area between them; the interfacial tension is measured by means of a
Wilhelmy plate (P). Both kinds of measurement can also be made at A–W and A–O
surfaces.