• Joseph Diong

Aerosols and the physics

Mosquito spray, whipped cream cans and insect repellent have one thing in common: they are all packaged and delivered in aerosol cans. The press of a button can deliver a mist of a product you want to apply to an area almost instantaneously. However, how do they work?

Fluids are made of particles that can move freely, and as such, gases and liquids are classified as fluids. The stark difference between gases and liquids is that liquids have a fixed volume. Volumes of gases can increase, and decrease based on the amount of energy and pressure in a gas. With enough energy applied to a liquid, particles can further separate and become a gas. The temperature where a liquid change into a gas to it is called a boiling point. With a high amount of pressure, a gas can be converted into a liquid. The opposite holds true, where a lower amount of pressure can change a liquid into a gas.

The aerosol contains 2 types of fluids: one that has a boiling point lower than room temperature, and one that has a boiling point higher significantly higher than room temperature. The latter is usually the product to be applied and used, and typically a liquid, and the former is called a propellent. These 2 fluids are contained in a closed can, and a small tube is inserted into the can. This small tube is attached to a valve and a nozzle. The valve prevents backflow, while the nozzle breaks up the liquid into tiny drops, creating a mist. Inside the headpiece, the button you press down to spray, there is a spring which pushes up the headpiece to block the contents of the can from the outside with a tight seal.

The propellent exists as a liquid. This occurs because it is pumped with high pressures into the can, forcing the particles of the propellent to stick close together and forming a liquid. However, when you press the headpiece down, the contents are now exposed to the outside. The pressure inside the can lowers, and with less pressure, the propellent changes from a liquid to gas. The resulting gas pushes down on the product to be applied, forcing it through the tube [1]. As it passes to the outside, the product is passed through the nozzle, creating a mist.

And there you have it! Next time when you pick up an aerosol can, you can think about the physics happening inside the humble metal can.

[1] T. Harris, "How Aerosol Cans Work", HowStuffWorks. [Online]. Available: https://science.howstuffworks.com/innovation/everyday-innovations/aerosol-can3.htm. [Accessed: 03- Jul- 2020].