Biomedical Engineering Reference
In-Depth Information
Figure 2.3
Waterdropletsinoil.Transitionfromacontinuousmicrolow(leftpartofthephoto-
graph)tomicrodrops(rightpartofthephotograph)byshearingthroughanapertureinasolidwall.
(CourtesyofHarvard.)
action of a piston (syringe or deflected membrane), or under the action of an elec-
tric field inducing an electro-osmotic effect [1]. Different apparatus are design to
monitor and actuate the flow as valves, compliance chambers, and passive or active
micromixers. One of the major difficulties is to integrate many different functions
in a miniaturized system, due to limitations brought by the fabrication process, the
various connections between the fluidic elements, and the handling and packaging
of the components.
In this section, we will not go into the details of the many microfluidics ap-
paratus, but we present the equations that govern fluid flows and we focus on the
physics and particularities of microflows.
2.2.1 Navier-Stokes (NS) Equations
The macroscopic approach for the calculation of the velocities and pressure in a
fluid is based on the continuum hypothesis, that is, in every elementary volume of
Figure 2.4
Digitalmicroluidics:photographsof(a)watermicrodropsonanelectrode,(b)awater
microdropinsiliconoilsubmittedtoapulsedelectricield,and(c)awaterdropinsiliconoilinan
EWODmicrodevice.(CourtesyofLETI/Biosoc.)
