If the cohesive forces between molecules of a liquid are stronger than
the adhesive forces between the molecules of a liquid and those of the
container then you will see a "convex meniscus." (In other words,
that's if the liquid molecules stick to each other better than they
stick to the container.) Surface tension, which arises due to the
cohesive forces in the liquid, likes to minimize the surface area of
the liquid, prefering a spherical shape. Adhesion between the liquid
and the container, also known as "wetting", encourages as much liquid
as possible to be in contact with the container.
When a convex miniscus starts to form, the initial effect is to
decrease the surface between the liquid and the container and increase
the surface between the container and the gas by the same amount. Only
when the curvature gets large does the area of the liquid-gas surface
go up much. So the tradeoff that determines whether the miscus is
convex or concave is whether the gas-conatiner surface or the
liquid-container surface is more 'costly'. The units of the cost are
something called free energy, which is what falls to a minimum value in
equilibrium. The more things lower their free energy when they're in
contact, the more they stick together.
Mercury has very small adhesive forces with most container
materials, and strong cohesive forces. Little drops of mercury will
form into almost spheres when spilled on most surfaces (gravity will
bend them out of shape).
Water on the other hand, is made up of electrically polar
molecules. Glass is electrically polarizable, and attracts charged
objects, so water sticks to glass.
Some containers may attract water molecules much less. Many kinds
of plastics, called "hydrophobic", (e.g. teflon) have very small
adhesive forces with water , and if you make a container out of these
you will get a convex meniscus with water, too.
Tom (and mike)
(published on 10/22/2007)
