Understanding the microscopic dynamics that occur between a flowing fluid and an elastic solid, such as those that happen in plastics manufacturing and in numerous other industrial applications, can be an important parameter in quality control.Although optical microrheology techniques exist to measure the movement in Brownian systems driven by thermal motion, only a limited number of techniques exist for non-Brownian systems where an external mechanical force — and not heat — is the catalyst that drives a fluid’s movement.A group of researchers has reported a video microscopy technique that could enable rheology measurements in difficult-to-access industrial systems where thermal motion is irrelevant. Microscopic polymer beads embedded in the surface of a polymer gel of thickness hg are optically tracked as a fluid layer of thickness hf flows past. Small fluctuations in the strength of the force that drives the flow lead to fluctuations in the position of theinterface, as measured by tracking the bead. The scale of these fluctuations, which bear some resemblance to Brownian diffusion in thermal systems, increases as the strengthof the flow increases (0.1, 0.5, 2.0, 5.0 and 10 s–1, from left to right). Information about the elasticity of the gel is contained in the position of the bead as a function of time. Reprinted with permission of Physical Review Letters.Researchers Erik K. Hobbie and Sheng Lin-Gibson of the National Institute of Standards and Technology in Gaithersburg, Md., teamed with Satish Kumar at the University of Minnesota in Minneapolis to optically measure the fast motion (or vibration) that occurred between the interface of a soft elastic solid and a flowing viscous fluid. From this, they determined the elastic properties of the soft solid itself.The investigators accomplished this by placing a soft solid (in this case, the polymer gel polydimethylsiloxane) on a quartz plate and attaching microscopic tracer particles — polystyrene latex beads measuring 3 μm in diameter — to the gel’s surface. Next they added a viscous fluid on top of the gel and lowered a quartz plate on top of the fluid. Rotating the top plate forced the fluid in between to flow. A homebuilt video microscope with a 45-μs strobe flash, a Leica 25× long-working-distance phase-contrast objective and an MTI CCD camera allowed them to record how the tracer particles moved as the speed of the top plate gradually increased. They noted that the boundary between the gel and fluid became increasingly unstable in response to the motion of the plates. The scientists used this information to extract the gel’s mechanical properties.One surprise encountered during the experiments was that the shearing motion of the plates caused the fluid to move in a nonlinear fashion, a likely consequence of the surface of the gel buckling. Hobbie said that they want to find a way to measure this buckling phenomenon in greater detail. Ultimately, the researchers hope to develop a system that could examine otherwise inaccessible interfaces between flowing fluids and soft solids using optical microscopy.Physical Review Letters, Feb. 20, 2008, 076001.