Wash your hands for 20 seconds: physics shows why

DOI:10.1063/5.0060307

Commentary

This physics study was designed to simulate hand washing and estimate the time scale over which particles such as viruses and bacteria are removed from the hands.

The mathematical model works in two dimensions, with one wavy surface passing through another wavy surface and a thin film of liquid moving between the two, representing a hand because wavy surfaces are rough on small spatial scales.

Particles are trapped in the rough surface of the well hand, and for them to escape, the energy from the water flow must be high enough to move them in and out of the trough.

The strength of the flowing liquid depends on the speed of the moving hand. The stronger the flow, the easier it is to remove the particles.

And even if the particles are removed, the process is not fast enough.

The Centers for Disease Control and Prevention guidelines suggest at least 20 seconds under the faucet. Consistent with the results of this study, it would take about 20 seconds of intense movement to remove potential viruses and bacteria.

However, this model does not take into account the chemical and biological processes that occur when using soap.