Biomedical Engineering Reference
In-Depth Information
Table 3.2
Surface Tension of Various Liquids in Contact with Air (
γ
la
)
Surface Tension,
γ
la
(dyne/cm)
(mN/m)
Liquid
Temperature (°C)
Water
0
75.64
Water
25
71.97
Water
50
67.91
Water
100
58.85
NaCl.6.0.M.aqueous.solution
20
82.55
Sucrose.(55%.wt..in.water)
20
76.45
Ethanol
20
22.27
Ethanol.(11.1%.wt..in.water)
25
46.03
Ethanol.(40%.wt..in.water)
25
29.63
Isopropanol
20
21.7
Methanol
20
22.6
Glycerol
20
63
Acetone
20
23.7
Acetic.acid
20
27.6
HCl.17.7.M.aqueous.solution
20
65.95
n
-Hexane
20
18.4
Mercury 15 487
Source:
.
Lange's Handbook of Chemistry
,.11th.ed.,.1973,.John.A..Jean.(Ed.),.
McGraw-Hill.
Wetting is not a new phenomenon, yet its applications to BioMEMS seem endless. Scientists
have observed for centuries how luids will spontaneously enter thin tubes (a phenomenon
termed “
capillary action
”) and wick porous media such as paper and sand—entire micro-
luidic devices are now made integrally in paper (see Section 3.5.1.6). When a drop of water
is placed at the inlet of a microchannel (provided its walls are hydrophilic), the water will
spontaneously enter the channel, but at a speed that gradually slows down. Emmanuel
Delamarche's group at IBM Zurich, in Switzerland, alongside many others, have developed
pumps and valves based on capillary action (see Section 3.8.4.1). In the last decade, there have
been many demonstrations of devices that exploit selective wetting as a strategy to move lu-
ids or particles and to conduct assays on a microscale. George Whitesides and colleagues at
Harvard irst used self-assembled monolayers (SAMs) of alkanethiolates (of various terminal
groups, with diferent hydrophilicity) to produce “liquid patterns” on surfaces. A collaborative
efort between David Beebe's group and Jefrey Moore's group at the University of Illinois at
Urbana-Champaign produced microluidic devices in which the luids were prevented from
contacting the sidewalls by stripes of hydrophobic SAM and were guided to stay in the middle
of the channel by a stripe of hydrophilic SAM—so the luid would “magically” low between
the loor and the ceiling of the channel without any walls (the air-luid interfaces inside the
channel would act as “virtual walls”). Similarly, it is possible to use electrical ields to modulate
