Simultaneous interfacial rheology and microstructure measurement of densely aggregated particle laden interfaces using a modified double wall ring interfacial rheometer.
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. To understand 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 shear rheology measurements. The development of this tool is outlined, and its ability to provide novel and unique measurements is demonstrated on a sample system. This tool has been used to examine the role of microstructure on the steady shear rheology of densely packed, aggregated particle laden interfaces at three 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. Interestingly, it is found that aggregated interfaces transition to yielding well before they reached a jammed state. Furthermore, these systems undergo significant shear induced order when densely packed. These results indicate that the mechanics of these interfaces are not simply jammed or unjammed and that the interfacial rheology relationship with microstructure can give us significant insight into understanding how to engineer particle laden interfaces in the future. By examining both rheology and microstructure, the mechanisms that dictate observed rheology are now understood and can be used to predict and control the rheology of the interface.