Particle tracking using confocal microscopy to probe the microrheology in a phase-separating emulsion containing nonadsorbing polysaccharide

Brownian diffusion of fluorescent microspheres (0.21, 0.5, and 0.89 mu m diameter) in conjunction with confocal microscopy has been used to monitor the microrheology of phase-separated regions in a protein-stabilized oil-in-water emulsion containing various low concentrations of a nonadsorbing polysaccharide, xanthan gum. The sensitivity and reliability of the technique has been demonstrated in test experiments on (i) aqueous glycerol solutions and (ii) concentrated surfactant-stabilized emulsions (30-60 vol % oil, 1-2 wt% Tween 20). From particle tracking measurements on the caseinate-stabilized emulsions (30 vol % oil, 1.4 wt% sodium casemate, pH 7) containing xanthan (0.03-0.07 wt %), the apparent viscosity in the oil-droplet-rich regions has been estimated to be up to 10(3) times higher than that in the phase-separated xanthan-rich regions. This means that our previously determined shape relaxation times for xanthan-containing blobs in the same systems can be attributed to the dominant viscoelasticity of the surrounding regions of concentrated oil droplets and not to the rheology of the xanthan-rich blobs themselves. These data provide clear and unequivocal evidence for the dominant role of the interconnected depletion-flocculated network of oil droplets in the physicochemical mechanism by which hydrocolloid thickeners control the creaming instability of concentrated oil-in-water emulsions.