Microfluidics – Where the Surface Dominates
Bettina Frohnapfel, Karlsruhe Institute of Technology (KIT), Germany

Microfluidics refers to the transport of fluids in geometries with characteristic dimensions below one millimeter. The subject receives enormous attention due to a combination of novel manufacturing techniques for this length scale and the growing quest for portable and cheap analytical tools in chemical, biological and medical applications. The potential impact of microfluidics to large-scale automation in chemistry and biology is often compared to the introduction of microfabricated integrated circuits which revolutionized computations by vastly reducing space, labor and time required for calculations.
A significant characteristic of microfluidic devices are the large surface area to volume ratios which lead to a dominance of surface effects that are typically negligibly small in “conventional” fluid mechanics. The micro-world of fluid mechanics is therefore often counterintuitive. The most obvious fact about life at low Reynolds numbers (i.e. flows in small dimensions and with low velocity) is that a laminar flow state prevails and thus all the challenges related to the inability to analytically treat turbulent flows can be ignored. At the same time the lack of turbulence slows down mixing tremendously and much research in the field is dedicated to the question of how to generate laminar chaos and mix the small fluid volumes at hand efficiently. One possible solution is given by structured surfaces that introduce transverse flows which are felt in the entire channel due to its small dimensions. This dominance of the surface has a very undesirable effect for pumping of fluid: it largely increases the required pumping power. The design of novel fluid transportation mechanisms is therefore another main research topic in microfluidics. Driving forces like capillary effects, electric or magnetic fields, rotation and sound are employed to replace the pressure gradient we typically consider for pumping.

For extremely small sample amounts found in e.g. prenatal diagnostics or forensic analysis the hydrodynamic dispersion that smears out the sample while it is guided through a micro channel is a limiting factor. A solution can be provided by “digital microfluidics” where samples are wrapped in individual droplets that are again manipulated through surface effects.

For further miniaturization the fluid dynamics world moves from being counter-intuitive to “new physics”. Once the characteristic dimensions of the fluidic device reach the order of the mean free path between molecules the continuum hypothesis which forms the basis for the governing equations of fluid mechanics does not hold anymore and the single molecule starts to matter. This fact is primarily important in gas und not fluid flows. Therefore, nano-scale gas dynamics is the field that follows microfluidics on its small end.

Further Reading:

[1] Stone HA, Strook AD, Ajdari A (2004): Engineering flows in small devices: microfluidics toward a lab-on-a-chip. Annual Review of Fluid Mechanics 36: 381-411.

[2] Squires TM, Quake SR (2005): Microfluidics: Fluid physics at the nanoliter scale. Reviews of Modern Physics 44: 977-1026.

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