Biomedical Engineering Reference
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Figure 3.3  Simplified scheme of molecules near an air/water interface. In the bulk, molecules have
interaction forces with all the neighboring molecules. At the interface, half of the interactions have
disappeared.
This relation shows that surface tension is important for liquids with a large
cohesive energy and a small molecular dimension. This is why mercury has a large
surface tension, whereas oil and organic liquids have small surface tensions.
Another consequence of this analysis is the fact that a fluid system will always
act to minimize surface areas. The larger the surface area, the larger the number of
molecules at the interface and the larger the cohesive energy imbalance. Molecules
at the interface always look for other molecules to equilibrate their interactions. As
a result, in the absence of other forces, interfaces tend to adopt a flat profile, and
when it is not possible due to capillary constraints at the contact of solids, they take
a convex rounded shape, as close as possible to that of a sphere. Another conse-
quence is that it is energetically costly to create or increase an interfacial area.
The same reasoning applies to the interface between two immiscible liquids,
except that the interactions with the other liquid will usually be more energetic than
a gas and the resulting dissymmetry will be less. For example, the contact energy
(surface tension) between water and air is 72 mN/m, whereas it is only 50 mN/m
between water and oil (Table 3.1). Interfacial tension between two liquids may be
zero. Fluids with zero interfacial tension are said to be miscible. For example, there
is no surface tension between fresh water and saltwater. Salt molecules will diffuse
freely across a boundary between fresh water and saltwater.
The principle applies for a liquid at the contact of a solid. The interface is just
the solid surface at the contact of the liquid. Molecules in the liquid are attracted
towards the interface by van der Waals forces, but usually these molecules do not
 
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