Water Ball
A liquid in space will always assume a spherical shape.
Figure 1
Observe Figure 1. This is water (with blue food coloring added) floating onboard the International Space Station. We notice that the object is floating in almost a perfect sphere. This sort of phenomenon is indeed familiar on earth despite gravity, as demonstrated by bubbles and drops. The ISS experiment begs us to stop and think about why the object is in the form of a sphere. To understand this, let's learn about surface tension.
Figure 2
When an object is partially submerged in water, a buoyant force pushes up on the object. Sometimes, when this force is equal to the force of gravity on an object, it floats! On the other hand, very light objects can float on a liquid's surface without being submerged, as demonstrated by Figure 2. This implies an absence of buoyant force, but there must be a force to balance out gravity. This force is supplied by surface tension. It is important to note that though "Tension" usually refers to a force, surface tension is not a force. It is the surface force "F" per unit length "L" over which it acts. It is represented by "š¯›¾" and is equal to "F/L." It can be easily derived that surface tension is equivalent to energy per unit area. Figure 3 provides a quick dimensional confirmation of this statement.
Figure 3
Let's try to connect these ideas with more intuitive processes that we're familiar with. Let's take some simple definitions. We know gravity is a force that pulls objects towards the ground. Gravitational potential energy is an energy that increases as an object gets farther away from the surface of the earth and decreases to 0 when an object reaches the surface. Notice that the potential energy decreases when an object moves in the direction of the force. It brings us to the principle that objects in nature will tend to minimize a quantity called action. This definition calls upon rigorous mathematics as action is defined as the time integral of the Lagrangian (total KE - total PE). We would need to take derivatives with respect to multiple variables to obtain a minimum (Euler-Lagrange Equation and Lagrangian Mechanics if you're interested). Figure 4 provides a visual explanation for the principle of least action. However, for our purposes, it suffices to understand that a fundamental principle of motion is that objects will try to minimize their potential energy.
Figure 4: The red path describes the motion when action is minimized. Blue paths are possible paths with greater action. The particle will follow the red path.
We now understand that the ball of water in space will have surface tension, quantified as energy per unit area. This implies that a low surface area will mean low potential energy. Objects want to minimize their potential energy, so the object will take the shape of the least surface area for the given volume. The proof in the video on the right shows that the object with the least surface area will be a sphere. As a result, the shape of the ball in water in space will be a sphere!