Surface shear films

Both high bulk and surface shear viscosity delay film thinning and stretching deformations that precede bubble bursting. The development of ordered stmctures in the surface region can also have a stabilizing effect. Liquid crystalline phases in foam films enhance stabiUty (18). In water-surfactant-fatty alcohol systems the alcohol components may serve as a foam stabilizer or a foam breaker depending on concentration (18). 

Film is locally removed by dissolution, surface shear stress or particle/bubble impact but it can repassivate. Erosion corrosion rate is a function of the frequency of film removal, bare metal dissolution rate and subsequent repassivation rate. 

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 surface shear viscosity of a monolayer is a valuable tool in that it reflects the intermolecular associations within the film at a given thermodynamic state as defined by the surface pressure and average molecular area. These data may be Used in conjunction with II/A isotherms and thermodynamic analyses of equilibrium spreading to determine the phase of a monolayer at a given surface pressure. This has been demonstrated in the shear viscosities of long-chain fatty acids, esters, amides, and amines (Jarvis, 1965). In addition,... 

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... 

When spread from a benzene/hexane solution on to a slightly acidic water subphase, spread films of racemic and enantiomeric STy exhibit nearly the same IT/A isotherms (Fig. 22) and surface shear viscosities (Harvey et al., 1990). The shapes of these isotherms and the apparently small differences between the compression/expansion characteristics of these fluid homochiral and heterochiral monolayers is conserved throughout the... 

The difference in the n/ A properties of these mixed chiral/achiral systems was also observed in the films dynamic properties. Figure 25 gives the surface shear viscosities of the palmitic acid/SSME systems at surface pressures of 2.5 and 5.0 dyn cm -1 at 25°C. It is clear that stereo-dependence of film flow... 

Fig. 25 Surface shear viscosity vs. film composition for the palmitic acid/stearoylserine methyl ester film system at 25°C.

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. 

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 8.12 Time-dependent surface shear viscosity rjS of bovine serum albumin (BSA) + dextran sulfate (DS) at the /7-tetradecane-water interface (pH = 7, ionic strength = 50 mM, 25 °C) ( ) 10 3 wt% DS ( ) 10 3 wt% BSA (A) 10"3 wt% DS added (11) to aqueous sub-phase below the BSA film after 24 h. Reproduced from Dickinson (1995) with permission.

Dickinson, E., Rolfe, S.E., Dalgleish, D.G. (1990). Surface shear viscometry as a probe of protein-protein interactions in mixed milk protein films adsorbed at the oil-water interface. International Journal of Biological Macromolecules, 12, 189-194. 

In addition to the film elasticity, other factors that may affect foam stability arc surface shear viscosity, bulk viscosity of the foaming liquid, and the presence of particulate matter. 

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).

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. 

Effect of Surface Shear Stress on Film Condensation on... 

The theoretical analysis indicated that asymmetric drainage was caused by the hydrodynamic instability being a result of surface tension driven flow. A criterion giving the conditions of the onset of instability that causes asymmetric drainage in foam films was proposed. This analysis showed as well that surface-tension-driven flow was stabilised by surface dilational viscosity, surface diffusivity and especially surface shear viscosity. 

Fig. 3.11. depicts the dependence of drainage time on shear viscosity for NaDoS microscopic foam films in the presence of C12H25OH. From a certain value on the drainage time steeply increases in accordance with the increased surface shear viscosity and there occurs a symmetric drainage. 

For the case of a spinning cone inclined at an angle a. to the plane of rotation, film thickness, velocity, residence time, and surface shear can be related to that for the spinning disk using the following scaling relationships ... 

Film thickness Residence time Flow velocity Surface shear... 

For a spinning disk, the standard model for falling film flow is complicated by the changing thickness and shear as the liquid flows over the disk. An approximation of this to conditions on a spinning disk surface can, however, be made by substitution of Eq. (9) for average liquid-solid surface shear into the above equation for mass transfer. If it is also assumed that the characteristic distance L traveled by the liquid is equal to that of the disk radius then an equation for the liquid-solid mass transfer coefficient ls can be written for an SDR as... 

While dilatational rheology plays an important role in short-term stability of dispersions shear viscosity may contribute appreciably to the long-term stability (Murray and Dickinson, 1996 Murray, 1998, 2002). The shear characteristics of the interfacial film are governed by the composition and structure of the adsorbed material. In addition, surface shear viscosity is a very sensitive technique to analyze the competitive adsorption of protein and water-soluble LMWE at the air-water interface (Murray and Dickinson, 1996 Murray, 1998, 2002 Bos and van Vliet, 2001). 

These phenomena have significant repercussions on surface shear properfies. Thaf is, fhe surface shear characferisfics reflecf fhe complex phenomena fhaf fake place in profein-monoglyceride mixed films under flow conditions. 

Carrera, C. and Patino, J.M.R. Surface shear rheology of WPI-monoglyceride mixed films spread at the air-water interface. Colloid Surf. B- B., 36, 57, 2004a. 

Here p is the dimensionless pressure in the gas, i.e., p /(/j,uc/s2ic) and r is the dimensionless surface shear stress, r /(p,uc/e c)- In (6-21), we have also introduced the dimensionless surface-tension function b such that a = o b. We denote the value of a at the ambient temperature (and/or in the absence of surfactants) as a0. Hence, if there are no changes in a from its ambient value, the function Vsb = 0. Once again, it will be noted that we have retained terms that include the small parameter e. These are again terms that could be responsible for the motion of the film, and in this case, the appropriate choice for uc would be either eao//j or 3 er0//b depending on whether the dominant effect is Marangoni or capillary-driven motion, respectively. 

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