Hand washing is a proven strategy for preventing disease spread, yet the physics underlying it has received little attention. Researchers at Hammond Consulting Limited, publishing in Physics of Fluids, developed a simple model that reveals the key mechanisms of hand washing. Their simulations estimate how long it takes to remove particles like viruses and bacteria from hands.
The two-dimensional mathematical model simulates one wavy surface sliding past another, with a thin liquid film in between. Wavy surfaces mimic the natural roughness of skin.
Particles lodge in the microscopic valleys of hand surfaces. Escaping requires water flow energetic enough to lift them out, like climbing out of a valley.
The liquid flow's strength depends on hand-rubbing speed—faster motion generates stronger flows for better particle dislodgement.
"Basically, the current tells you about the forces on the particles," explains lead author Paul Hammond. "Then you can find out how the particles move and whether they are being removed."
He compares it to scrubbing a stubborn stain from a shirt: quicker rubbing boosts removal odds.
"If you move your hands too gently or slowly relative to each other, the forces from the flowing liquid won't overcome what holds the particle down," Hammond notes.
Even with vigorous rubbing, removal isn't instant—standard guidelines recommend at least 20 seconds under running water.
The model confirms this: about 20 seconds of energetic hand movement clears most potential pathogens.
While soap's chemical effects aren't modeled, understanding physical removal aids development of effective, eco-friendly soaps.
"We need to consider what happens to cleaning chemicals as they enter sewers and the environment," Hammond adds.
This work doesn't cover all aspects of hand washing but addresses core questions and sets the stage for further studies.
