American scientists have built a mathematical model of the mechanics of liquid when washing hands. The results showed that the smallest particles, such as viruses and bacteria, are washed out of microscopic depressions on the surface of the skin only after 20 seconds of intense friction of wet palms against each other. The article is published in the journal Physics of Fluids of the American Institute of Physics.

Thorough hand washing is recognized as an effective means of combating the spread of diseases and infections, but the physics of this process has not yet been studied. Researchers from Hammond Consulting Limited, led by Paul Hammond, built a model reflecting the key mechanism of handwashing and estimated the length of time during which particles such as viruses and bacteria are washed off the hands.

The mathematical model is based on the interaction of two wavy surfaces that move relative to each other in two dimensions, while there is a thin film of liquid between them. The particles are trapped on the rough surfaces of the hand in potential pits, and to extract them from there, the energy of the water flow must be high enough.

The authors note that a stronger flow removes particles more easily, and its strength depends on the speed of movement of the hands.

“In fact, the flow force is a characteristic of the forces acting on particles,” Paul Hammond said in a press release from the American Institute of Physics. “By assessing the strength of the flow, you will be able to understand how the particles move and find out if they are moving away.”

The scientist compares hand washing with the process of erasing a stain of dirt from a cloth: the faster the movement, the more likely it is that the dirt will come out.

“If you move your hands too gently and slowly relative to each other, the forces created by the flowing liquid will not be sufficient to overcome the force holding the particles,” Hammond explains.

The model results showed that it takes about 20 seconds of vigorous movement to wash away potential viruses and bacteria from your hands. This is fully consistent with the American Centers for Disease Control and Prevention recommendations, which indicate the need to wash your hands under the tap for at least 20 seconds.

Hammond notes that his model is purely physical; it does not consider the chemical and biological processes that occur when using soap. However, the scientist is confident that the identified physical mechanisms will allow manufacturers of cosmetics and household chemicals to create more effective and environmentally friendly detergents and antiseptics.