Multi-scale rheology for emulsion stability investigation: Bulk-, interfacial-, and micro-perspectives

Emulsions are hierarchically structured complex fluids, whose physical stability plays an imperative role in their production and application. The rheology at different length-scales (macroscopic: bulk rheology; mesoscopic: interfacial rheology; microscopic: microrheology) are powerful tools to comprehensively investigate the emulsion stability from a multi-scale point of view. A lack of systematic knowledge on the suitability of all the three rheological techniques, which have been progressing rapidly in recent years, for emulsion stability characterization underscores the necessity of this review. Herein, we begin with a concise review of the common destabilization mechanisms in food emulsions. Then we critically summarize various rheological techniques for emulsion stability investigation, covering the thorough testing modes of each technique. The qualitative and quantitative relationships between different rheological parameters and emulsion stability are described to promote a mechanistic understanding, taking into account both classical theoretical models and state-of-the-art analytical protocols. The strengths and weaknesses of each method are also evaluated to determine the suitability of each rheological technique in investigating emulsion stability. Finally, we point out the future trends by analyzing the current knowledge and technical gaps to further advance this field. We hope this review can provide detailed guidance for the rational formulation and processing control of emulsions for higher stability performance.