(221d) Active and Nonlinear Microrheology of Fluid/fluid Interfaces

We describe a novel microrheological technique to measure the rheological properties of fluid/fluid interfaces, which can dramatically affect the flow properties and dynamics of multiphase materials (emulsions, foams, cells and organs). Such measurements can be particularly challenging, as one needs to measure the influence of molecularly thin, two-dimensional layers but be insensitive to the three-dimensional bulk fluids on either side. However, dimensionality helps here: interfacial forces on a probe are exerted along a contact perimeter, whereas the bulk forces are exerted on the contact area. Smaller probes, increase the perimeter/area ratio, and therefore the relative sensitivity to interfacial viscoelasticity. Our technique is capable of measuring frequency-dependent linear viscoelasticity, nonlinear (shear-dependent) viscosity and yield stresses. Furthermore, it is capable of measuring local properties (eg on heterogeneous interfaces) and of simultaneously visualizing the interface, using Brewster-angle microscopy for water/air systems (including lung surfactant monolayers) as well as bright-field and fluorescent microscopy. We demonstrate the capabilities of our system with fluid/fluid interfaces, including two-dimensional colloidal monolayers, lung surfactant and other phospholipid monolayers, and others.