Interfacial microrheology

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Interfacial microrheology
Todd Squires, UCSB Chemical Engineering
While centuries have been devoted to
equilibrium interfacial properties (e.g. surface
tension), their dynamic properties (e.g.
interfacial rheology) have been more elusive
it is difficult to measure the viscosity and
elasticity of a molecularly thin interface
without getting swamped by the bulk materials on
either side. We have developed a novel
microrheological technique for interfacial
rheometry, which employs micro-scale
ferromagnetic disks as probes. The disks large
perimeter to area ratio makes them highly
sensitive to interfacial drag, and their small
size enables local measurements in
heterogeneous interfaces, enables direct
visualization of the flowing interface, and
measurements with very small sample volumes.
Microdisks can also be torqued steadily, here in
a colliodal monolayer at a clean water/decane
interface, to probe nonlinear rheology
(shear-rate dependent viscosity). Interfacial
strain can be directly vizualized.
Electromagnets apply a torque (green) to the
microdisk, whose orientation is simultaneously
tracked (red), from which rotational drag (and
thus interfacial viscoelasticity) is determined.
Monodisperse 20 micron diameter, 1 micron tall
ferromagnetic disks with tailorable surface
chemistry are used as microrheological
micro-probes.