Surface viscosity liquid crystalline phases

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). 

This has been verified for polydimethylsiloxanes added to crude oils. The effect of the dilatational elasticities and viscosities on crude oil by the addition of polydimethylsiloxanes is shown in Table 21-1. Under nonequilibrium conditions, both a high bulk viscosity and a surface viscosity can delay the film thinning and the stretching deformation, which precedes the destruction of a foam. There is another issue that concerns the formation of ordered structures. The development of ordered structures in the surface film may also stabilize the foams. Liquid crystalline phases in surfaces enhance the stability of the foam. 

Viscosity and surface tension measurements were taken for isotropic and anisotropic (liquid crystalline phase) phases. 

Another important factor for control of biological efficacy is the formation of "deposits after evaporation of the spray droplets, which ensure the tenacity of the particles or droplets of the agrochemical. This will prevent removal of the agrochemical from the leaf surface by falling rain. Many microemulsion systems form liquid crystalline structures after evaporation, which have high viscosity (hexagonal or lamellar liquid crystalline phases). These structures will incorporate the agrochemical particles or droplets and ensure their stickiness to the leaf surface. 

A maximum in emulsion stability is obtained when three phases exist in equilibrium, and it was therefore proposed that the lamellar liquid crystalline phase stabilizes the emulsion by forming a film at the 0/W interlace. This film provides a barrier against coalescence. This is illustrated in Fig. 5.12 which shows that the lamellar liquid crystalline phase exhibits a hydrophobic surface towards the oil and a hydrophilic surface towards the water. These multilayers cause a significant reduction in the attraction potential cuid they also produce a viscoelastic film with much higher viscosity than that of the oil droplet. In other words, the multilayers produce a form of "mechanical barrier against coalescence. 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [27], For example, in some cases, the addition of a small amount of nonionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [21, 126], To the extent that viscosity and surface viscosity influence emulsion and foam stability one would predict that stability would vary according to the effect of temperature on the viscosity. Thus, some petroleum industry processes exhibit serious foaming problems at low process temperatures, which disappear at higher temperatures [21],... 

Conoscopy provides an extremely sensitive method with which to determine the degree of biaxiality. By the early 1990 s, conoscopic measurements had already indicated the presence of phase biaxiality in a nematic side-on liquid crystalline side-chain polymer [9]. However, the method s sensitivity is also its weak point because surface effects may induce optical biaxiality in an actual uniaxial system. For this reason, deuterium NMR was used to confirm phase biaxiality in a liquid crystalline polymer system similar to the one investigated with conoscopy by Leube [11-13]. Due to the fairly high viscosity of the polymeric samples, the tilt experiment, employed by Yu and Saupe to show phase biaxiality in a lyotropic liquid crystal [4], was used. The results obtained in this way are in good agreement with observations of optical textures in a biaxial cholesteric copolymer [16], where phase biaxiality disturbs the smooth optical periodicity of the cholesteric phase structure. 

As we have seen in Section 6.6.1 such confined liquids may behave quite differently from the bulk lubricant. Near the surfaces, the formation of layered structures can lead to an oscillatory density profile (see Fig. 6.12). When these layered structures start to overlap, the confined liquid may undergo a phase transition to a crystalline or glassy state, as observed in surface force apparatus experiments [471,497-500], This is correlated with a strong increase in viscosity. Shearing of such solidified films, may lead to stick-slip motions. When a critical shear strength is exceeded, the film liquefies. The system relaxes by relative movement of the surfaces and the lubricant solidifies again. 

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