Why are drops of liquid always spherical in shape?

Every molecule of liquid attracts its neighboring molecules towards it, and more of them will get drawn towards every neighboring molecule. It is only the molecules that are on the surface which are not drawn externally. Beyond the surface, there are no further molecules that have the same structure as the water, hence the reason why the surface or solid molecules are not being pulled by liquid molecules in the outward direction. The result is that every molecule on the surface is facing an equal pull inward. It is as if there is a common center in the droplet that is exerting an equal pull on each of its molecules and draws them towards it. Thus, liquid naturally takes a spherical shape as a droplet is formed, although not every droplet is necessarily a perfect sphere. The form ultimately depends on the size of a droplet and the number of molecules. For example, the picture below shows droplets of mercury.


The first one is a perfect sphere since every droplet “tries” to keep the surface area at a minimum. As the drops get bigger and bigger, they are starting to flatten out. Due to the large size of the water droplets, the pull between the molecules is not enough to hold them together tightly. The same applies to water droplets as well, which we see sliding on the leaves of plants without wetting the surface.

Surface Tension

All of these processes are caused by surface tension, and this has a different effect on liquids than solids or gases. Water is made up of multiple molecules, and each of them has the same structure and components. As mentioned previously, the molecules on the water are bonded together via intermolecular attraction, and this connection enables these molecules to form water. On a body of water like a sea or lake, the intermolecular attraction often exerts a stronger pull to the molecules that are at the top, and this process forms the water’s flat surface. This is called surface tension.

Solids and gases are also made up of molecules, with solids having a more tight connection that bonds molecules together more closely, and gases having a more “free” structure where the molecules are far away from each other. Furthermore, if thousand or even millions of molecules are attached together tightly, the object that they form becomes visible to the naked eye, and because air is a kind of gas, we cannot see it because its molecules don’t really have a connection.

The molecular structure of liquids is in between the ones found on solids and gases since water molecules tend to bond together, although not as tight as solid molecules. It is important to note that if the molecules have a tighter connection, they will be able to carry more weight than liquids, which is the reason why we can’t stand on water. However, there are certain animals that exploit the loopholes in surface tension to let them stand on water. One of these animals is the water strider, which has long legs that enable the animal to distribute its weight on water evenly so that it won’t disturb its surface tension.

When water molecules are connected in one body of water, they are usually moving around their space if there is an external force that allows them to do so. For example, whenever you move your hand from left to right on the water’s surface, you would see that the water moves in a pattern similar to how you moved your hand, and these movements tend to go forwards. This is an indication that the water molecules have moved.

As for the droplets of water discussed in this article, the surface tension of the ground disables the droplet to get through it. Because the water molecules at the bottom of the droplet cannot pass through the solid molecules, they are prevented from moving, thus creating a solid-like bottom wherein the molecules at the tops exert more force downwards, causing the bottom molecules to look similar to how solid molecules are connected. Think of the droplet of water to be the upside-down version of surface tension on a lake, as the molecules at the bottom have more force than the ones at the top.