(353b) The Effects of Polydisperse Particle Populations On the Shear Rheology of Solid Stabilized Interfaces

When particles stabilize drop interfaces in an emulsion, a “Pickering Emulsion” is formed.  Pickering Emulsions provide stability through increased mechanical protection of an interface and enhanced surface viscoelasticity, They are used in food science, cosmetics, drug delivery, and energy applications.  Many of these applications use polydisperse particle populations.  However, there have been no systematic studies of the effects of polydispersity on the mechanical properties of solid stabilized interfaces.  Changes in particle properties affect the interfacial rheology of a solid stabilized interface which in turn affects emulsion stability; therefore it is of great importance to elucidate the effects of polydispersity on surface rheology.

In order to examine the effects of particle polydispersity on packing and interfacial rheology, an AR-G2 rheometer is used with a double wall ring geometry. This tool is capable of measuring complex surface moduli down to 10^-5 Pa m of both oil/water or air/water interfaces, which is sensitive enough to measure the shear rheology of solid stabilized interfaces. Furthermore, we have developed a system to allow real-time imaging of interfaces during testing.  This unique setup allows the interfacial shear rheology to be directly tied to structural changes observed in solid stabilized interfaces.  The result is an interfacial shear rheometer with the ability to visualize an interface and capture the dynamics of individual particles, of ~1 micron or larger, as they are sheared.  We are able to examine of the effects of particle size, polydispersity, surface coverage and applied deformation on packing structure of an interface, and correlate changes in ordering to interfacial shear moduli.  Using 2D analogs of standard rheological tests for suspensions, we examine the effects of polydispersity on polystyrene spheres at an air/oil interface at surface concentrations large enough to create hexagonal packing.

In suspensions of solid spheres, particle polydispersity affects particle ordering and maximum packing fraction, reducing the viscoelasticity of the system. Polydispersity has similar effects on 2D surfaces. As polydispersity increases the maximum packing fraction increases, long range order decreases, and particle mobility on the interface increases in comparison to monodisperse systems. As surface concentration increases, particle packing and structure becomes more ordered for all systems.   However, polydisperse systems continue to show less order than monodisperse ones.  We observe decreasing interfacial moduli and viscosity with increasing particle polydispersity over a range of surface concentrations.  For all systems as maximum packing fractions is approached, the interfacial moduli and viscosity increase; however, the interfacial moduli and viscosity are lower for polydisperse systems in comparison to monodisperse systems at all surface concentrations.  Polydispersity not only affects the magnitude of the measured interfacial moduli and viscosity, but also decreases surface relaxation times.

This work is the first to systematically study the shear rheology of interfaces stabilized with polydisperse particle populations.  It shows the importance of particle population uniformity on the interfacial rheology of solid stabilized interfaces, which could affect a number of known applications.