Aerosol Spray Actuator & Cap

This is done by filling the can/tank/reservoir with two different fluids: the first is a high-pressure propellant of lower density and the second is the desired product to be dispensed (this is your paint, shaving cream, WD-40, etc).

At rest, the high-pressure propellant pushes up on the stem and mounting gasket, keeping the system sealed, while also pushing down on the product. When the actuator is pressed, the dip tube path is opened to the outside; this creates a large pressure differential (inside the can → high pressure, ambient air → low pressure). The system wants to move to equilibrium, and so the product is forced through the dip tube and out the actuator, where the atomizer finely disperses the product into an aerosol spray. It is an elegant device, as the aerosol valve acts as a switch, a filling port, an atomizer, and a hermetic seal, all at once.