Explaining interfacial behavior of a particle laden interface using microstructure analysis
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Abstract
The study of particle laden interfaces has increased significantly due to the increasing industrial use of particle-stabilized foams and Pickering emulsions, whose bulk rheology and stability are highly dependent on particle laden interface’s interfacial rheology, which is a function of interfacial microstructure and acting interparticle forces. Understanding the physical mechanisms that dictate interfacial rheology of particle laden interfaces requires correlating rheology to microstructure. To achieve this goal, a double wall ring interfacial rheometer has been modified to allow real time, simultaneous interfacial visualization and rheology measurements. The development of this tool is outlined, and its ability to provide novel and unique measurements is demonstrated. This tool has been used to examine the role of microstructure on the steady shear and small amplitude oscillatory rheology of densely packed, aggregated particle laden interfaces at different surface concentrations. Through examination of the rheology and analysis of interfacial microstructure response to shear, a transition from shear thinning due to aggregated cluster breakup to yielding at a slip plane within the interface has been identified. Further SAOS experimentation revealed that local microstructure determines severity of caging while capillarity determines inter-particle attraction, and that both of these elements dictate the degree of restricted particle motion, which in turn determines locally interfacial viscoelastic moduli magnitude. However, macroscale rheological behaviors such as elasticity and yield are tied to the mesostructural organization of local microstructure into large hexagonal domains. Large domains of aligned hexagonal packed particles create elastic interfaces; when these domains begin to break up, and smaller domains can flow on the interface, a transition to viscous-like behavior is observed.