Complexity at Low Reynolds Number

Surprisingly intricate flow patterns may develop under certain conditions in microfluidic channels, discovered Todd Thorsen, Stephen Quake (quake@caltech.edu, 626-395-3362) and coworkers at the California Institute of Technology. Generally, liquids flow smoothly when they have low Reynolds numbers - a parameter that takes into account the fluid density, viscosity, and velocity as well as the dimensions of the conduit. In channels only tens of microns across, the diminutive conduits lead to very small Reynolds numbers, and therefore linear, turbulence-free streams.

The researchers found, however, that at the juncture of a water-filled microfluidic channel and another channel containing an oil mixture, interactions on the boundary between the two fluids results in nonlinear flow and complex, crystalline trains of droplets. By varying the relative fluid pressures, the researchers could create elegant water droplet sequences, ranging from simple strings to elaborate helical and ribbon-like structures (see image at /png).

The discovery may lead to new microfluidic screening devices to control the flow of biochemical substances. In addition, the unexpected complexity of fluid behavior at low Reynolds numbers will likely stimulate new insights to fluid mechanics, potentially generating advances in the theory of liquids confined in micron-scale structures. (T. Thorsen et al, Physical Review Letters, 30 April 2001; text at Physics News Select.)