A7_Microfluidics_fundamentals_I
.pdf
Diffusion in microsystems
Diffusion tends to be more relevant in microsystems, but the actual values of D are low!
if D ~ 10-5 cm2/s, U ~ 1 mm/s, l ~ 100 m (channel height) Pe ~ 100
if D ~ 10-5 cm2/s, U ~ 10 m/s, l ~ 1 m (channel height) Pe ~ 0.01
Real situations somewhere in between – there is not a characteristic order of magnitude for the Peclet number
Diffusion mixing time
δ = 2
DT
τ ~ L 2
D
MEMS-based microfluidics: the plumbing
•Microvalves
•Micropumps
•Microchannels
•Micro flow sensors
ρflow in microchannels (1-1000 µm) is laminar Re < 2000
Re is the ratio of inertial force to friction (viscous) force – in a microchannel, Re~10-3-10-5, and the dynamics are of a highly viscous fluid, such as molasses
The plumbing: micro flow channels
Glass/Silicon etching and bonding
double-T intersection etched in glass for 165 pL sample fluid plugs
glass-glass microchannel, 20 µm deep and 50 µm wide
Wafer bondingAnodic
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chuck
•bring sodium containing glass (Pyrex) and silicon together
•heat to high temperature (200-500 ºC) in vacuum, air or inert ambient
•apply high electric field between the 2 materials (V~1000V) causing mobile + ions to migrate to the cathode leaving behind fixed negative charge at glass/silicon interface
•bonding is complete when current vanishes
•glass and silicon held together by electrostatic attraction between – charge in glass and + charges in silicon
Piezoresistive pressure sensor
SiO2
p+ Si
<100> Si
glass
Microflow channels – soft lithography
PDMS micromolding
The plumbing: microvalves - concepts
Microvalves - realization
Redwood Microsystems
active normally open microvalve
active normally-closed microvalve
Micropumps
Example of the electrostatically actuated membrane pump
Fraunhofer Institute for Solid State Technology,
Munich, Germany
•An increase in volume draws liquid through the inlet check-valve
•Relaxation of the diaphragm expels the liquid through the outlet checkvalve
Micropumps
Example of the electrostatically actuated membrane pump
Fabrication process involves etching many cavities separately
in each wafer, and then bonding the individual substrates together by siliconfusion bonding
Basic electrokinetic effects
A: Electroosmotic flow (EOF), B: electrophoresis (EP), C: dielectrophoresis (DEP)
Microfluidic realization of capillary electrophoresis analysis on the electrokinetic platform. After the sample has been transported to the junction area (1) it is metered by the activated horizontal flow and injected into the separation channel (2). Therein, the sample components are electrophoretically separated (3) and read out by their fluorescence signal (4).