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).
- Israelachvili, Jacob. Intermolecular & Surface Forces. London: Academic Press, 1991.