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Physics explains the 20 second handwashing rule

The act of handwashing masks some complex physics – and this all relates to lubrication theory.

Hand washing with soap and water at a sink. Image by Lars Klintwall Malmqvist, via Wikimedia (Public Domain)
Hand washing with soap and water at a sink. Image by Lars Klintwall Malmqvist, via Wikimedia (Public Domain)

Handwashing is of great importance in relation personal hygiene. According to the U.S. Centers for Disease Control and Protection (CDC): “Washing hands can keep you healthy and prevent the spread of respiratory and diarrheal infections from one person to the next.”

The agency, along with other health authorities around the world, recommends washing hands with hot soap and water for 20 seconds. Such advice has been in the public domain for the past two years in the context for the COVID-19 pandemic.

But where has the 20 seconds come from? While there is a biological basis and 20 seconds appears to be effective, it remains that no one – unt now – has looked in detail at the dynamics of the 20 second process.

Researchers from the non-biological domain have looked at the 20-second rule, drawing on the laws of physics to explore what is happening at the micro-level. The physics behind hand washing has behind has rarely been studied.

The new approach comes from scientists working at Hammond Consulting Limited. Here researchers have described a model that captures the key mechanics of hand-washing. This model helps to establish the optimal time – and this is confirmed as a minimum of 20 seconds – provided the washing activity is vigorous. With a gentler washing, there is insufficient physical activity with which to disassociate particles from skin.

This was shown by simulating hand-washing through a series of experiments. In doing so, the  researchers have estimated the time scales upon which particles, including viruses and bacteria, were removed from hands.

The mathematical model developed acts in two dimensions. This is with one wavy surface moving past another wavy surface, where a thin film of liquid between the two. The wavy surfaces represent hands because they are rough on small spatial scales. This forms part of a centry-old branch of physics called ‘lubrication theory’.

The study showed that particles are trapped on the rough surfaces of each hand in potential wells (this is described by the research team as a little like lying at the bottom of a valley). In order for particles to escape, the energy from the water flow must be high enough to get them up and out of the valley.

Furthermore, the strength of the flowing liquid depends on the speed of the moving hands. In contrast, a stronger flow removes particles more easily. In other words, the faster the motion, then the more likely particles are to be released).

This means if a person moves their hands too gently, or 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.

It also stands that even when particles are removed, that process is not fast. Typical hand-washing guidelines suggest at least 20 seconds under the tap (faucet). It follows that it takes about 20 seconds of vigorous movement to dislodge potential viruses and bacteria, using soap. The research appears in the journal Physics of Fluids, where the research paper is titled “Will we ever wash our hands of lubrication theory?”

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Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, business, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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