Biomedical Engineering Reference
In-Depth Information
Figure 3.5
Sketch of interface motion induced by a thermal gradient between two regions of
the surface. The motion of the interface propagates into the bulk under the action of the viscous
forces.
cooler region (larger value of the surface tension). This surface motion propagates
to the bulk under the action of viscosity.
If the temperature source is temporary, the motion of the fluid tends to homog-
enize the temperature and the motion progressively stops. If a difference of tem-
perature is maintained on the interface, the motion of the fluid is permanent; this is
the case of a film of liquid spread on a warm solid.
3.2.2.2 The Effect of Surfactants
Surfactant is the short form for surface active agent. Surfactants are long molecules
characterized by a hydrophilic head and a hydrophobic tail and are for this reason
called
amphiphilic molecules
. In biotechnology, very often surfactants are added to
biological samples in order to prevent the formation of aggregates, maintain parti-
cles in suspension, and prevent target molecules from adhering to the solid walls of
the microsystem (remember that microsystems have extremely large ratios between
the wall surfaces and the liquid volumes). Like any small-sized particles, surfactants
diffuse in liquids; when they reach an interface, they are captured because their
amphiphilic nature prevents them from escaping easily from the interface. Conse-
quently, they gather on the interface as in Figure 3.6, lowering the surface tension
of the liquid.
As the concentration in surfactants increases, the surface concentration in-
creases too. Above a critical value of the concentration, called CMC (critical micelle
concentration), the interface is saturated with surfactants and surfactant molecules
in the bulk of the fluid group together to form micelles.
The surface tension decreases with the concentration in surfactants as shown
in Figure 3.7. At a very low concentration, the slope is nearly linear. When con-
centration approaches the CMC, the value of the surface tension drops sharply.
Above CMC, the value of the surface tension is nearly constant [1]. For example,
