Meniscus

Sample menisci with water on the left (concave meniscus), and mercury on the right (convex meniscus). (Image from http://www.learnchem.net/)

Definition

A meniscus is a curvature in the surface of a fluid (e.g. water) as a result of molecular interactions with a container or object. If the meniscus is convex, then the molecules have a stronger attraction to themselves than the container or neighboring object (e.g. mercury which is non-polar thus not attracted to its glass container). If the meniscus is concave, then the molecules have a stronger attraction to the container or neighboring object than themselves (e.g. water which is polar and attracted to a glass container). With a concave meniscus, capillary action in a container will result in pulling the fluid upward to increase the contact area between the interfaces (energetically favorable).

Flexible Surfaces

Research continues into the behavior of menisci in unusual equilibrium scenarios. Kwon et al. investigated the equilibrium of an elastically confined liquid drop and computed theoretical predictions concerning where a meniscus should form if a liquid drop is placed between a rigid substrate and a flexible hydrophilic glass surface. They found that the surface tension due to the drop can be sufficient to pull the flexible sheet toward the rigid plate until one of three cases occurs. Either (I) the top plate is not long (or, equivalently, flexible) enough to come into contact with the substrate, (II) the top plate is long enough to come into contact with the substrate but not long enough to lie tangent to it, or (III) the top plate is long enough that its free end lies tangent to the substrate.

Theoretically predicted plate shapes and meniscus locations for the three cases discussed by Kwon et al. The predicted geometries are supported by experimental results.

In dimensionless parameters, they found that each of these three conditions corresponds to a different meniscus location, as indicated in the figure at right, due to different physical and geometric boundary conditions. Experimental data supports their theoretical predictions.

References

• http://en.wikipedia.org/wiki/Meniscus
• Israelachvili, Jacob. Intermolecular & Surface Forces. London: Academic Press, 1991.
• The 'Cheerios Effect'