Rheological behaviour of attractive emulsions differing in droplet-droplet interaction strength

Philipp L Fuhrmann,Swantje Breunig,Guido Sala,Leonard Sagis,Markus Stieger,Elke Scholten

doi:10.1016/j.jcis.2021.08.124

Philipp L Fuhrmann, Swantje Breunig + Show 4 more

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https://doi.org/10.1016/j.jcis.2021.08.124

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Abstract

HypothesisWe hypothesise that interaction strength between oil droplets determine the rheological properties of oil-in-water (O/W) emulsions by simultaneous formation and break-up of bonds between droplets. Using small (SAOS) and large (LAOS) amplitude oscillatory shear measurements, we aim to distinguish different classes of emulsions based on the specific microstructural evolution of the emulsions. ExperimentsConcentrated O/W emulsions differing in droplet-droplet interaction strength were obtained. Different interaction strength was obtained using different types of interactions; (a) electrostatic attraction, (b) salt bridging, or (c) crosslinking. FindingsIn line with our hypothesis, different rheological events in emulsions depend on the droplet-droplet interaction strength. Strong interactions lead to monotonous yielding, and droplets undergo jamming or densification to provide strain hardening and gel-like behaviour. Emulsions with weak interactions exhibit two-step yielding (SAOS) and continuous yielding in LAOS; indicating a soft-glassy material. In emulsions above maximum packing, and with weak interactions the rheology is controlled by cluster/cage breaking, and transient formation of new clusters. For medium-strength interactions, two-step yielding was reduced, and apparent stain-hardening occurred. The probability of two distinct time scales of yielding is hindered by stronger interactions and jamming. Overall, in concentrated emulsions, yielding is determined by network rupture and reformation, cluster rearrangement and -breaking, which in turn is influenced by interaction type and strength. We present a more differentiated categorisation of emulsions based on interaction strength.

Highlights

Oil-in-water emulsions are ubiquitous materials [1]
In the case of concentrated emulsions with purely repulsive interactions, the viscous modulus can exhibit a peak at a specific strain value [6], which is related to the temporary jamming and the subsequent structural relaxation of the oil droplets [7]
This study aimed to investigate the effect of droplet-droplet interactions in emulsions with high oil volume fractions on the rheological behaviour both at small and large deformations

Summary

Introduction

Oil-in-water emulsions are ubiquitous materials [1]. Their rheological behaviour is influenced by their composition, microstructure, and their interactions [2]. When droplets are deformable (i.e., when their Laplace pressure is relatively low), the oil volume fraction can be increased above the maximum packing fraction. In this case, the emulsion starts to show viscoelastic solid properties, with a strong dependence of the elasticity on interfacial properties. In the case of attractive interactions, the maximum packing fraction decreases, as oil droplets start to cluster, leading to network formation already at lower volume fractions. In the case of concentrated emulsions with purely repulsive interactions, the viscous modulus can exhibit a peak at a specific strain value [6], which is related to the temporary jamming and the subsequent structural relaxation of the oil droplets [7]

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