Earlier this month, we wrote about the “Ballantine’s Space Glass”, and its free-fall in a free-fall tower. This time we are going to discuss the basic principle that this device is exploiting in order to function.
It is called “Capillary action”, and it was first discovered by Leonardo Da Vinci.
Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper, in some non-porous materials such as liquefied carbon fiber, or in a cell. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container act to lift the liquid. In short, the capillary action is due to the pressure of cohesion and adhesion which cause the liquid to work against gravity.
The height of a meniscus of a liquid column is given by this equation, where γ is the liquid-air surface tension (force/unit length), θ is the contact angle, ρ the density of liquid (mass/volume), g is local acceleration due to gravity (length/square of time), and r is radius of tube (length).
The thinner the space in which the water can travel, the further up it goes.
Now go and solve this equation, while we are writing the next post which is the sequel of this one! We are going to discuss the formula that describes the height of the liquid in a vertical tube.
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