Water in water emulsions: phase separation and rheology of biopolymer solutions

Partially miscible polymers in solution do not separate into two macroscopi c phases; in general they behave as viscoelastic fluids containing droplets of the minority phase dispersed into a continuous majority phase (emulsion type systems). Both phases contain two types of polymers and solvent in va riable amounts. With time, the smaller droplets tend to merge into larger o nes and eventually sediment. Provided the time stability of the emulsion is long enough and the size of the droplets does not exceed a few tens of mic rons, the emulsion can be characterized by conventional rheological methods as an effective medium, both in the linear regime (viscoelasticity) and un der flow. We investigated a ternary system composed by two biopolymers, a p rotein (caseinate) and a polysaccharide (alginate) in aqueous solution and established an analogy between these phase separated solutions and immiscib le blends of polymers. We first characterized the biopolymers and determine d the phase diagram at room temperature that we interpreted within the fram ework of the Edmond and Ogston model. For the rheological investigations, s tarting with an initial composition of the system, we separated the two pha ses by centrifugation. The individual phases were then characterized throug h their viscoelastic and flow behaviors. By recombining variable amounts of these phases, thereby varying only their volume fractions, we were able to prepare stable emulsions with constituents having constant compositions. T he effect of shear on these emulsions was investigated. After different she aring protocols, the size of the droplets was derived from the Palierne mod el and the flow curves were analyzed. The droplet sizes were compared to th e critical capillary numbers and coalescence predictions. The flow curves a nd the dynamic viscosities of the emulsions were interpreted with a model r ecently proposed by Kroy et al. that extends earlier work of Oldroyd (1953) , Schowalter et al. (1968), and Frankel and Acrivos (1970).