Reply #8: Sorry [View All]

[midnight armadillo]

I was just sort of teasing ;-)

Fluid dynamics defines several quantities that are used to describe the type of fluid flow regime. The Knudsen number (http://en.wikipedia.org/wiki/Knudsen_number) relates the 'mean free path', i.e. the distance a particle will travel before bumping into anouther, to the representative length scale, or size, of the region where the fluid is flowing.

The Reynolds number (http://en.wikipedia.org/wiki/Reynolds_number) is velocity times length divided by viscosity. This characterizes the fluid flow independent of the fluids. Example: an airplane model immersed in water can exhibit the identical behavior as the real airplane in air, provided you pick your water velocity and model size to give the same Reynolds number as for the real airplane in flight.

At very low R (under say 1500), there's no turbulence, and little mixing goes on, aside from diffusion. So, in microfluids you have very small length scale (a channel maybe a few hundreds of nanometers across) and very low velocity, so R is very small. So it's not that there's no time for turbulence, it's just that it won't happen.

Another way to characterize such flow is via the Knudsen number. Low Knudsen number indicates a traditional fluid. High Knudsen number indicates the flow of free molecules in which normal fluid-like behavior is rare, since it'll act like a stream of BB's instead of a fluid. Microfluids (especially flows of gases through microscopic channels) often exhibit highish Knudsen numbers, so they don't act like normal fluids do.

Let me know if this helped, I can try again but I don't know your background, and can try to be more or less technical.