Being able to measure accurately the microscopic dynamics that occur between a flowing fluid and a semi-solid, such as happens throughout the human body as blood courses through the circulatory system, could be an important research and diagnostic tool for biologists.Although optical microrheology techniques exist to measure the movement in Brownian systems driven by thermal motion, there are limited techniques 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 is reporting a video microscopy technique that could enable rheology measurements in difficult-to-access biological systems where thermal motion is irrelevant; it could measure the elasticity of tissue in vivo and the stresses being placed upon arteries. Microscopic beads embedded in a gel surface were used to trace the motion of a gel forming an interface with a liquid. As this interface was stirred, the beads followed a complicated trajectory (patterns are shown above the photos), which the researchers broke down into a range from small, fast movements to large, slow movements to determine the gel’s underlying mechanical properties. As the strength of the flow is increased (from left to right), the scale of the motion also increases. Courtesy of Erik K. Hobbie, NIST.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 occurs 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 placed a soft solid (in this case the polymer gel polydimethylsiloxane) on a quartz plate and attached 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. Rotating the plate forced the fluid 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 was gradually increased. They observed the boundary between gel and fluid becoming increasingly unstable in response to the motion of the plates and 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 would like to develop a usable system to examine interfaces between flowing fluids and soft solids, which are otherwise inaccessible, using optical microscopy.Physical Review Letters, Feb. 22, 2008, 076001.