Liquid viscosity stabilizing effect

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). 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

The correlation derived by Dombrowski and Johns covers a large range of liquid viscosity and agrees favorably with experimental results. Crapper et al.[236] further applied second order and large amplitude theories to achieve better predictions. In addition, the effects of surface tension, and viscosity of a liquid sheet as well as the radial spreading and the resultant changes in the sheet thickness on the stability have been examined by Weihs.[257]... 

The effects of liquid viscosity on tray efficiency have been studied by Drickamer and Bradford(58) and () Co nt ij. 59- and these are discussed in Section 11.10.5. Surface tension influences operation with sieve trays, in relation both to foaming and to the stability of bubbles. 

Fig. 11. Effect of density difference at various liquid viscosities on particle Reynolds number evaluation at lower critical particle diameter, (a) Solid-liquid fluidized beds [a = 3.0, Cv = f(s), pi = 1000 kg/m ]. (b) Gas-solid fluidized beds [a = 3.0, Cy = /(e), po = 1 kg/m ]. (c) Unified stability map of particle Reynolds number vs density difference for different values of transition hold-up solid-liquid fluidized beds [a = 3.0, Cy = f(s), p-l = 1 mPas, pi = 1000 kg/m ].

The first requirements for foam formation are thus surface tension lowering and surface elasticity. A greater elasticity tends to produce more-stable bubbles. But if the restoring force contributed by surface elasticity is not of sufficient magnitude, then persistent foams may not be formed because of the overwhelming effects of the gravitational and capillary forces. These foams are termed evanescent foams. Important properties that determine the stability will include bubble size, liquid viscosity, and density difference between gas and liquid. More-stable foams may require additional stabilizing mechanisms. 

The normal liquid chlorinated paraffins used as plasticizers for PVC have viscosities ranging from 100 to 40,000 MPa.s at 20°C. Products with chlorine contents ranging from 30 to 70% are on the market. Compatibility with PVC increases with increasing chlorine content but the plasticizing effect is reduced. The low viscosity products (chlorine content 30%—40%) are used as secondary plasticizers for PVC. They have a stabilizing effect on viscosity in plastisols. Chlorinated paraffins can be used up to a maximum of 25% of the total plasticizer content of the PVC plastisol without the risk of exudation. As chlorine-containing substances, these plasticizers also have a flame-retarding effect. 

Viscosity of the liquid phase also plays an important role in bubble stability. Martin et al. (2010) argued that high viscosity liquids can absorb substantial part of the inertial energy associated with the eddies reaching the bubble surface. Thus, viscosity has a dampening effect on the stress transmitted to the bubble surface, which in turn curtails bubble breakage. For Rushton turbine, Martin et al. (2010) have given Equation 7A.11 for the effect of the liquid viscosity on the critical Weber number ... 

One of the earliest methods for reducing coalescence is to use mixed surfactant films. These will increase the Gibbs elasticity and/or interfacial viscosity. Both effects reduce film fluctuations and, hence, reduce coalescence. In addition, mixed surfactant films are usually more condensed and hence diffusion of the surfactant molecules from the interface is greatly hindered. An alternative explanation for enhanced stability using surfactant mixture was introduced by Friberg and coworkers [67] who considered the formation of a three-dimensional association structure (liquid crystals) at the oil/water interface. These liquid crystalline structures prevent coalescence since one has to remove several surfactant layers before droplet-droplet contact may occur. 

The liquid PEG grades mainly used for the production of cellulose films have the following valuable properties as humectants or plasticizers balanced hygroscopic-ity complete nonvolatility, absolute transparency, peptizing effect on resin components of the viscose, anchoring effect on nitrocellulose lacquers, and the imparting of dimensional stability. 

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