BEHAVIOR OF DROPLETS IN SIMPLE SHEAR-FLOW IN THE PRESENCE OF A PROTEIN EMULSIFIER

The effect of two structurally diverse protein emulsifiers, beta-lacto globulin and beta-casein, upon the break-up behaviour of a single aque ous droplet in a Couette flow field has been studied over a wide range of protein concentrations for bulk viscosity ratios between 8 x 10(-5 ) and 0.05. It has been found that beta-casein and low concentrations (less than or equal to 10(-2) gl(-1)) of beta-lactoglobulin cause the droplets to be at least as stable as expected from conventional theori es on the basis of their equilibrium interfacial tension. In such case s the presence of the emulsifier at the deforming interface is thought to enhance the interfacial elasticity. This effect can be characteriz ed by an effective interfacial tension, which is higher than the equil ibrium value. High concentrations of beta-lactoglobulin (greater than or equal to 10(-1) gl(-1)), on the other hand, have been shown to caus e droplets to be less stable than would have been predicted from the e quilibrium interfacial tension. It is thought that an interfacial prot ein network is formed, which limits the droplet deformation and makes the droplet interface rigid with respect to tangential stresses. As a result, the critical deformation and capillary number are found to be essentially independent of the viscosity ratio. It is proposed that th e interfacial structure may be probed using a combination of interfaci al shear and dilational rheological measurements. From this type of an alysis it may be possible to predict the break-up stability of droplet s. (C) 1997 Elsevier Science B.V.