Background: What is a meniscus?
The meniscus is most easily seen in a clear narrow tube, partially filled with liquid. If you look through the side of the tube, you notice that the surface of the liquid is curved (either upwards or downwards, depending on what materials the liquid and the tube happen to be). The meniscus is the point at which that curvature is horizontal, found at the center of the surface. To make it more visible, people will sometimes place a sheet of white paper behind the tube, or even a fancy “meniscus reader” that has black and white sections, so that the contrast makes the surface easier to view. For chemists, the meniscus is very important, as this is the point at which volumetric measurements are made.
Explanation: Why does a meniscus form?
All the matter that we see and interact with is made up of molecules – individual chemical structures that add up to make everything in the world around us. A glass of water, for instance, is filled with water molecules. We do not see the individual molecules, for they are far too small, but the properties of water result from the interactions of those molecules with one another, and everything around them.
The meniscus, too, is the result of interactions between molecules. To begin, let us stick with the most familiar case of water in a glass tube, such as a test tube or burette (buret). Water (H-O-H) molecules have polar chemical bonds, and are bent, so that they have a permanent dipole. In simpler language, they have positive and negative ends. The positive ends are attracted to the negative ends of other molecules, and vice-versa. This includes other water molecules, so one property of water is that it tends to “stick” to itself. Glass, while not a specific molecule, is full of silicates – a group of chemicals which all have a common component – silicon bonded to oxygen (-Si-O). When placed in water, the silicate will pick up a hydrogen atom from the water, becoming a “silanol” (-Si-O-H) which you might notice looks similar to water on one end. The silanol group is fixed in place, as part of the glass, but it is even more polar than the water molecules, so as a result, the water molecules are attracted even more strongly to the glass than they are to one another. The water molecules move towards the glass to increase their contact with it, so much so that the level of water in the center of the tube dips noticeably. The water is, quite literally, climbing the walls of the tube. At some point, gravity strikes a balance in the system, so tubes of water don’t spontaneously overflow, but the result is quite observable. In very narrow tubes (capillary tubes) the effect is so great that water can climb inches within the tube before it is stopped by gravity. In a larger tube, like a glass of water, the effect is much smaller, though still visible if you look carefully.
Glass and water is only one of many possible combinations of tubes and liquids. Not all solids are more attractive to water than water itself is. Consider a tube made of wax. Waxes are less polar than water, and so water is not very attracted to it. Water would prefer to stick to itself. As a result, water bunches together, and pulls away from the wax. If you have waxed your car lately, you have probably seen the same result. The water forms a dome – it “beads up” as it tries to maximize contact with other water molecules. Again, gravity puts a limit on this, pulling the drop flat against the car (or down into the tube). If, however, you were out in zero-gravity space, a drop of water would be able to pull itself completely into a sphere, maximizing its contact with itself, and minimizing contact with less polar surfaces.
The liquid mercury is also a popular illustration of the “upside-down” meniscus. Mercury is non-polar, and is more attracted to other non-polar chemicals, like itself, than it is to polar substances, like glass. As a result, when mercury is placed in a glass tube, it bunches up to form a dome.
All in all, the meniscus is the result of intermolecular forces. (There’s a Helium article on intermolecular forces just waiting for you to read more.) In general, if the liquid and solid have a stronger interaction than the liquid molecules do with themselves, then the meniscus will be the low point of the dip formed in the surface of the liquid. If the liquid molecules are more strongly attracted to one another than to the solid of the container, the meniscus will be the high point of the dome formed by the liquid’s surface. In either case, it should be easily recognizable, and found towards the center of the tube.