Linear and nonlinear microrheometry of small samples and interfaces using microfabricated probes

We describe a microrheological strategy that enables sensitive surface shear rheology measurements of surfactant-laden interfaces, with the capacity to simultaneously visualize deforming interfaces. This technique utilizes a ferromagnetic microbutton probe pinned to a fluid-fluid interface, and actively torqued or forced with externally controlled electromagnets. Various modes of operation are possible: Small-amplitude oscillatory rotations, which provide frequency-dependent viscoelastic shear moduli; controlled torque (analogous to fixing shear stress); controlled rotation rate (analogous to fixing strain rate); and imposed force (analogous to active, translational microrheology). The circular shape of the probe ensures pure shear strains (when driven to rotate). We describe the experimental apparatus, its measurement limits and sources of error. We then highlight its versatility and capabilities with measurements on a variety of qualitatively distinct systems, including purely viscous monolayers, block-copolymer interfaces, aging and evolving interfaces, colloidal monolayers, and bulk rheometry of Newtonian and viscoelastic materials, with sample volumes as small as 2 μl.