© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

9.2.7.5 Fluid Mixing

In nanofluidic systems (e.g., pipes, chambers and junctions) of size L <~ 100 nm, even with strictly laminar flow, mixing of separate fluid streams is rapid because the small-molecule diffusion time t <~ 10 microsec (Eqn. 3.1 and Table 3.4). At the other extreme, systems of characteristic size L <~ 1000 microns with v_flow >~ 1.4 m/sec may readily establish a turbulent flow with N_R >~ 2000 in water at 310 K (Eqn. 9.29), and again mixing of multiple fluid streams can be quite rapid.

However, for systems of scale 1000 microns > L > 100 nm, diffusion times range from 10^-5­10³ sec for small molecules, and even longer for large molecules. Nonlaminar flow would demand unreasonably high fluid velocities, and diffusive stirring (Section 3.2.3.1) yields only modest improvements in mixing. To achieve rapid mixing in this intermediate size range, a micromixer chamber is first filled with one liquid, and then the other liquid is injected into the chamber volume simultaneously through a large number of very small nozzles. In the simplest micromixer designs the mixing nozzles form a regular coplanar array.¹²¹⁹ In more complex designs, mixing nozzles will form a three-dimensional array possibly using a branching tree or fractal geometry with tip spacing ~1 micron to allow fully intimate mixing to be completed in ~1000 microsec; a ~100 nm tip spacing allows complete mixing in ~10 microsec.

Last updated on 20 February 2003