Time and again during the pandemic, from the beginning to the later stages we were thoroughly advised to wash our hands regularly, whether or not we went out or came in contact with someone. Handwashing must be done at a steady pace to shake off any viruses and bacteria; and now according to fluid dynamics should last approximately around 20 seconds, which surprisingly comes in par with the information advertised by doctors, news reports and even the government. It was a protective measure that got us through the Covid-19 pandemic and the practice is still prevalent.

Although the science behind this was simple logic reasoning to prevent spread of infection through contact, the physics behind it was rarely even grazed upon. Nevertheless, as stated in the journal Physics of Fluids- “Will we ever wash our hands of lubrication theory?” by Paul S. Hammond, dated 17 August 2021, by AIP Publishing, researchers from Hammond Consulting Limited has come up with a straightforward mathematical model, to simulate the movement of particles (such as viral or bacterial particles) during hand washing, which thus explains the key mechanism of hand-washing.

In the 2D model, one wavy surface moves past another wavy surface, with a thin film of liquid separating the two. Wavy surfaces represent hands because they are rough on small spatial scales, thus simulating the feel of hands scrubbing together.

The model demonstrated that particles are trapped onto the rough surfaces and a particular amount of force would be needed to detach these particles, so they can be passed into the fluids. According to the author, fast hand movement ensured a stronger flow of fluid which removes the particles more easily.

"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.  

Even when particles are removed, that process is not fast. Typical hand-washing guidelines, like those from the Centers for Disease Control and Prevention, suggest at least 20 seconds under the faucet. Which concurs with the Hammond’s model, it takes about 20 seconds of vigorous movement to dislodge potential viruses and bacteria.

Yet, the model does not take into account the chemical or biological processes that occur when using soap. But, by knowing the mechanisms that physically remove particles from hands may provide clues to formulate more effective, environment friendly soaps.

“Nowadays, we need to be a bit more thoughtful about what happens to the wash chemicals when they go down the plughole and enter the environment,” said Hammond.

Although, Hammond informed that this is not the whole story of hand-washing, but it does answer crucial questions and lay the foundation for future research.