Surface shear measurements

The subject of surface viscosity is a somewhat complicated one it has been reviewed by several groups [95,96], and here we restrict our discussion to its measurement via surface shear and scattering from capillary waves. 

The surface shear stress t is a consequence of the velocity difference between the metal surface and the fluid velocity. For tubular geometries it can be obtained from pressure drop measurements or calculated ... 

Fig. 6 Detail of the Verger film balance modified for measurement of surface shear viscosity. Reprinted with permission from Harvey et al, 1988. Copyright 1988 American Chemical Society.

The latter point is illustrated by the surface shear viscosities of the homochiral and heterochiral films at surface pressures below the monolayer stability limits. Table 7 gives the surface shear viscosities at surface pressures of 2.5 and 5 dyn cm -1 in the temperature range given in Fig. 19 (20-40°C). Neither enantiomeric nor racemic films flow under these conditions at the lower temperature extreme, while at 30°C the racemic system is the more fluid, Newtonian film. However, in the 35-40°C temperature range, the racemic and enantiomeric film systems are both Newtonian in flow, and have surface shear viscosities that are independent of stereochemistry. These results are not surprising when one considers that (i) when the monolayer stability limit is below the surface pressure at which shear viscosity is measured, the film system does not flow, or flows in a non-Newtonian manner (ii) when the monolayer stability limit is above the surface pressure... 

An unusually extensive battery of experimental techniques was brought to bear on these comparisons of enantiomers with their racemic mixtures and of diastereomers with each other. A very sensitive Langmuir trough was constructed for the project, with temperature control from 15 to 40°C. In addition to the familiar force/area isotherms, which were used to compare all systems, measurements of surface potentials, surface shear viscosities, and dynamic suface tensions (for hysteresis only) were made on several systems with specially designed apparatus. Several microscopic techniques, epi-fluorescence optical microscopy, scanning tunneling microscopy, and electron microscopy, were applied to films of stearoylserine methyl ester, the most extensively investigated surfactant. 

As an illustration of the fact that the two sign conventions give the same results, the equivalents of equations 1.46 to 1.53 can be written for the positive sign convention. In the positive sign convention, the shear stress acting on the outer surface of the element is measured in the positive x-direction and that on the inner surface is measured in the negative x-direction. This is the opposite of the directions shown in Figure 1.17. The force balance, equivalent to equation 1.46 is now... 

Figure 8.12 illustrates the effect of complex formation between protein and polysaccharide on the time-dependent surface shear viscosity at the oil-water interface for the system BSA + dextran sulfate (DS) at pH = 7 and ionic strength = 50 mM. The film adsorbed from the 10 wt % solution of pure protein has a surface viscosity of t]s > 200 mPa s after 24 h. As the polysaccharide is not itself surface-active, it exhibited no measurable surface viscosity (t]s < 1 niPa s). But, when 10 wt% DS was introduced into the aqueous phase below the 24-hour-old BSA film, the surface viscosity showed an increase (after a further 24 h) to a value around twice that for the original protein film. Hence, in this case, the new protein-polysaccharide interactions induced at the oil-water interface were sufficiently strong to influence considerably the viscoelastic properties of the adsorbed biopolymer layer. 

Figure 24. A comparison of the data obtained from a range of surface rheological measurements of samples of /3-lg as a function of Tween 20 concentration. ( ), The surface diffusion coefficient of FITC-jS-lg (0.2 mg/ml) at the interfaces of a/w thin films (X), the surface shear viscosity of /3-lg (0.01 mg/ml) at the o/w interface after 5 hours adsorption ( ), the surface dilational elasticity and (o) the dilational loss modulus of /3-lg (0.2 mg/ml).

It was of interest to compare the results obtained with the FRAP technique with those obtained with classical surface rheological techniques. Our detailed knowledge of properties of solutions of /3-lg containing Tween 20 made this an ideal system on which to compare the methods. Firstly, surface shear viscosity measurements were performed on the Tween 20//3-lg system [47] using a Couette-type torsion-wire surface rheometer as described previously [3,48]. All the experiments were carried out at a macroscopic n-tetradecane-water interface at a fixed protein concentration of O.Olmg/ml. In the absence of Tween 20, the surface shear... 

Unlike in three dimensions, where liquids are often considered incompressible, a surfactant monolayer can be expanded or compressed over a wide area range. Thus, the dynamic surface tension experienced during a rate-dependent surface expansion, is the result of the surface dilational viscosity, the surface shear viscosity, and elastic forces. Often, the contributions of shear and/or the dilational viscosities are neglected during stress measurements of surface expansions. Isolating interfacial viscosity effects is difficult because, since the interface is connected to the substrate on either side of it, the interfacial viscosity is coupled to the two bulk viscosities. 

Adamson [15] and Miller et al. [410] illustrate some techniques for measuring surface shear viscosity. Further details on the principles, measurement and applications to foam stability of interfacial viscosity are reviewed by Wasan et al. [301,412], It should be noted that most experimental studies deal with the bulk and surface viscosities of bulk solution rather than the rheology of films themselves. 

Recent surface force measurements revealed a similar trend (20). Comparing steam-treated to flame-treated silica sheets using site-dissociation/site-binding model, a decrease in silanol surface sites and apparent decrease in average pKa was observed upon heat treatment. Furthermore, a repulsive force other than double-layer and van der Waals forces was observed 15 A from the surface. This repulsion was attributed to hydration of the surface and was found to be independent of surface treatment and electrolyte concentration. In Bums treatment, an arbitrary plane of shear was introduced to provide a best model fit (l 3). A value of 9 A from the surface for the plane of shear was determined from electro-osmosis measurements. 

Our understanding of miniemulsion stability is limited by the practical difficulties encountered when attempting to measure and characterize a distribution of droplets. In fact, most of the well-known, established techniques used in the literature to characterize distributions of polymer particles in water are quite invasive and generally rely upon sample dilution (as in dynamic and static laser light scattering), and/or shear (as in capillary hydrodynamic fractionation), both of which are very likely to alter or destroy the sensitive equihbrium upon which a miniemulsion is based. Good results have been obtained by indirect techniques that do not need dilution, such as soap titration [125], SANS measurements[126] or turbidity and surface tension measurements [127]. Nevertheless, a substantial amount of experimental evidence has been collected, that has enabled us to estabhsh the effects of different amounts of surfactant and costabihzer, or different costabilizer structures, on stabihty. 

Some work has shown a direct correlation between shape factor and the flow properties of powders. The flowability of fine powders, as measured by a shear-cell as well as by Carr s method, was found to increase with increasing sphericity, where the sphericity is indicated by a shape index approaching one, as measured by an image analyzer. Huber and co-workers derived an equuation in which flow rate was correlated to the volume specific surface as measured by laser diffractometry. Reasonable predictions were made for individual powders as well as binary and ternary mixtures. 

Figure 3.40. Principle of a surface creep measurement for a viscoelastic monolayer. At t = 0 a constant shear stress is (instantaneously) applied and maintained till t = The shear strain Ax/Ay is followed eis a function of time. At t = t this stress is instantaneously removed. Idealized behaviour.

In the situation described above, the dynamic experiment was cturied out in dilation the resulting complex modulus was divided into a real ( elastic ) and an imaginary ( viscous ) part. As a counterpart, the experiment can also be carried out in shear, resulting in a complex surface shear viscosity G°, consisting of a real (viscous) part, the surface shear viscosity G° and the surface shear loss viscosity, G"" identical to the elasticity. This inversion of method is formally identical to measuring complex dielectric permittivities instead of complex conductivities, discussed in sec. I1.4.8a. In that case, flg. 3.26 is modified in that panel (b) describes G°, panel (c) G " and jianel (d) the sum, with - tan 0 = G" /G. ... 

An Interfacial Shear Rheometer (ISR-1) is also offered by Sinterface Technologies for measuring interfacial shear properties, in the frequency range 0.02 to 0.2 Hz, dependent on the measurement system, in the temperature range 10 to 50°C. The measurement ranges of the rheometer include surface shear viscosity... 

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