Biomedical Engineering Reference
In-Depth Information
Figure 2.1
Continuouslowmicroluidics:microchannelsdesignedforliquid-liquidextraction.Two
immiscible liquids are moving in parallel, separated by pillars aligned in the middle. Micro- and
nanoparticlesormacromoleculescarriedbyoneoftheliquidmigratethroughtheinterfaceintothe
otherliquid.Ifthelowrateofthislastliquidissmall,theparticlesareautomaticallyconcentrated.
(PhotocourtesyofN.Sarrut,LETI.)
For example, alternate plugs of liquid separated by air bubbles are used to create
microemulsions or sprays.
A similar, but slightly different, category of flow results from the breakup of one
phase into droplets (Figure 2.3). It is still a multiphase flow, but the discontinuous
phase is more or less dispersed in the continuous phase. This way of proceeding
may be interesting to concentrate and isolate some biological targets in very small
volumes of buffer fluid.
Finally, microdrops can be considered as separate entities and manipulated indi-
vidually (Figure 2.4). This type of fluid motion is often called
digital microfluidics.
.
The aim here is to manipulate with precision extremely small quantities of fluid—of
the order of tens of nanoliters—containing the biological target (DNA, cells, and
so forth).
In this chapter, we deal with continuous and two-phase microflows. Digital
microfluidics and droplet microfluidics will be treated in Chapters 3 and 4.
2.2 Single-Phase Microflows
As we have seen in the preceding section, the first category of microflows is the
single-phase continuous microflow. It is the most widely used in biotechnology for
transporting biological targets and detection probes. In biochips, the fluid is flow-
ing under the effect of a driving pressure—imposed by a micropump—or under the
Figure 2.2
Liquidplugsinacapillarytube.Alternateplugsofbuffersolution—separatedbyimmis-
ciblesiliconoilplugs—circulateinsideacapillarytube.(PhotocourtesyofLETI.)