Hydrodynamic focusing occurs when multiple flows with substantially different flow rates come into contact. The most common configuration is a 3-inlet device that allows rapid mixing of the contents of a small "core" stream with a bulk “sheath” flow. The center flow stream is “pinched” between two sheath streams, thereby shrinking the core stream width. This doubles the area of the diffusion interface and greatly reduces the diffusion distances and has been used to study of rapid (bio)chemical kinetics. In conventional microfluidic devices, hydrodynamic focusing is achieved through careful control of flow rates by multiple pumps and/or pressure sources. In the image below, we used a 3-leg paper network to recreate classic hydrodynamic focusing in a paper device.
The width of the hydrodynamically focused stream of blue liquid is controlled by the relative flow rates of the three streams.
The width of the focused stream (blue) is controlled via the relative fluidic resistance of the outer (“sheath”) legs and the inner (“sample”) leg. Here, fluidic resistance was varied by changing the relative leg lengths – each leg acts as a “resistor” with flow inversely proportional to length.
For more information see:
“Microfluidics without pumps: reinventing the T-sensor and H-filter in paper networks,” Lab on a Chip (2011).
Jennifer L. Osborn, Barry Lutz, Elain Fu, Peter Kauffman, Dean Y. Stevens, and Paul Yager.
Department of Bioengineering, University of Washington, Seattle, WA, USA
Example of hydrodynamic focusing in microfluidics:
“Hydrodynamic Focusing on a Silicon Chip: Mixing Nanoliters in Microseconds,” Knight, Vishwanath, Brody, and Austin. Physical Review Letters 80, 3863–3866 (1998).