Do you know why you need to wash your hands for 20 seconds, according to physics?

 Physicists have figured out the perfect method of hand washing to clear out particles of bacteria and viruses, including the novel coronavirus. They found that it takes about 20 seconds to dislodge viral or bacterial particles from our hands. That should sound familiar — it's in line with what most public health experts recommend.

For the study, published Tuesday (Aug. 16) in the journal Physics of Fluids, researchers created a simple mathematical model to simulate the movement of particles (such as viral or bacterial particles) during hand washing. In the model, hands are represented by two rough surfaces that move past each other (to mimic hands scrubbing together), separated by a thin film of liquid.

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The model showed that particles are attracted to the rough surfaces, and a certain amount of energy is required to enable the particles to escape into the fluid. A faster motion of the hands creates a stronger flow of fluid and removes the particles more easily, the authors said.

"If you move your hands too gently, too slowly, relative to one another, the forces created by the flowing fluid are not big enough to overcome the force holding the particle down," study author Paul Hammond, a scientific consultant at Hammond Consulting Limited in the United Kingdom, said in a statement. Hammond likened the situation to removing a stain from a shirt; a faster scrubbing action removes the stain more easily.

Using reasonable estimates for the variables, including the speed of hand movement, the model revealed that about 20 seconds are needed for the particles to escape. That is in keeping with recommendations from the Centers for Disease Control and Prevention (CDC), which advises people to wash their hands with soap and water for 20 seconds, or about as long as it takes to sing "Happy Birthday" twice. 

The CDC's recommendation is not based on physics modeling, but rather studies of the levels of microbes that remain on hands after washing for certain time periods.

Hammond noted that the new study did not take into account the biological action of soap. Soap not only helps lift dirt and germs from hands, but it also disrupts the membrane surrounding the viral or bacterial particles, thus destroying them.