Emulsion/Surface Interactions from Quiescent Quartz Crystal Microbalance Measurements with an Inverted Sensor

Interactions of three oil-in-water emulsion types with polystyrene-coated quartz crystal microbalance (QCM) sensor surfaces were probed with the QCM cell in both the conventional orientation (i.e., polystyrene surface on the bottom, "looking up") and the inverted orientation (polystyrene on top interior surface of sensor chamber, "looking down"). With the conventionally oriented QCM sensors, the adsorption of soluble and/or dispersed species quickly gave steady-state frequency and dissipation outputs. By contrast, the inverted sensors gave changing responses at long times because of the gravity driven buildup of a viscous consolidation layer next to but not necessarily bound to the sensor surface. Three emulsion types (a simple hexadecane/phosphatidylcholine emulsion, 2% homogenized milk, and a diluted commercial ophthalmic emulsion) displayed a wide range of behaviors. We propose that quiescent QCM measurement made with an inverted sample chamber is a new approach to probing emulsion behaviors near solid surfaces.