How can an aerosol can keep spraying as it empties?

Inside many aerosol cans, the product is mixed with a liquefied propellant. Some propellant is liquid and some is vapor in the headspace. Pressing the actuator opens a valve. Higher internal pressure then drives the liquid mixture up a dip tube and through the outlet.

As material leaves, the headspace grows. If liquid propellant remains, more can evaporate and replenish the vapor. At a fixed temperature, an ideal pure substance with liquid and vapor in equilibrium has a characteristic saturation pressure. Changing the amounts of the two phases does not itself change that pressure.

For the illustrative fixed temperature, suppose this pressure is 300 kPa absolute and outside pressure is 100 kPa. The difference is 200 kPa whether the headspace is 50, 150 or 250 mL, as long as both phases remain available to equilibrate. This pressure difference drives flow; it does not determine droplet size by itself.

Real aerosol formulations are mixtures, and spraying can cool them through evaporation. Pressure can therefore change with temperature and composition. Once liquid propellant is exhausted, the two-phase buffering ends. Compressed-gas aerosols and bag-on-valve designs work differently. At the outlet, propellant evaporation and nozzle geometry help produce a spray or foam.