Shear banding in concentrated Na-caseinate emulsions

Abstract

We report on the phase space and rheological behaviour of concentrated soybean oil/Na-caseinate/water emulsions. Four domains exist within the phase space spanned by 50–55 wt% soybean oil and 1–5 wt% Na-caseinate. These domains are distinguished by both their macroscopic stability and their microstructure. At low Na-caseinate concentration (<1.3 wt%) bridging between oil droplets occurs due to insufficient emulsifier coverage. Upon increasing protein concentration full coverage of the oil droplet surfaces is achieved, emulsion stability is enhanced and individual droplets can be visualised. Further incrementing protein concentration (>2.3 wt%) reduces emulsion stability to its lowest levels due to the inducement of depletion flocculation. The final emulsion domain occurring at the highest Na-caseinate and soybean oil concentrations is the most stable. Here the aqueous continuous phase is comprised of a three-dimensional self-assembled protein network. Upon increasing Na-caseinate and soybean oil concentrations the viscosity of the emulsions is increased. Despite the considerable variation of macroscopic stability and emulsion microstructure, mechanical rheometry measurements in a cone-and-plate geometry indicate that all emulsions within the phase space exhibit shear-thinning behaviour at low shear rates (∼5 × 10⁻³ s⁻¹). At higher shear rates a stress plateau (between ∼0.02 and 1 s⁻¹), indicative of shear banding, is reflected in the steady state flow curves before a Newtonian response is achieved. No dependence of the shear rates delineating these regions in the flow curve on varying either soybean oil or protein concentration is observed, however, the stress response of the plateau region increases monotonically with Na-caseinate concentration. Despite this dependence, no demarcation is observed corresponding to the occurrence of the underlying emulsion phase transitions. Switching soybean oil for either palm oil or tetradecane does not alter the form of the flow curves, or the characteristic shear rates defining the plateau. Inducement of the shear-banded state is therefore not dependent on volume fraction or the strength of the interdroplet interactions. NMR velocimetry measurements in a Couette geometry revealed the subdivision of fluids into high and low shear bands at applied shear rates along the stress plateau. The onset of shear banding is the result of a switch in dominance of the restructuring and destruction rates at a critical shear rate.