While hand washing has been proven effective in curbing the spread of disease and infection, the physics behind it has rarely been studied. But in Physics of Fluids, researchers at Hammond Consulting Limited describe a simple model that captures the key mechanisms of hand washing. By simulating hand washing, they estimate the time scales at which particles, such as viruses and bacteria, were removed from hands.
The mathematical model works in two dimensions, with one wavy surface moving past another wavy surface, and a thin film of liquid between the two. Wavy surfaces represent hands because they are rough on small spatial scales.
Particles are trapped on the rough surfaces of the hand in potential pits. In other words, they are at the bottom of a valley, and in order to escape, the energy of the water flow must be high enough to get them up and out of the valley.
The strength of the flowing liquid depends on the speed of the moving hands. A stronger flow removes particles more easily.
"Basically, the current tells you about the forces on the particles," said author Paul Hammond. “Then you can find out how the particles move and find out whether they are being removed.”
He likens the process to scrubbing a stain on a shirt:the faster the movement, the greater the chance that it will come out.
“If you move your hands too gently, too slowly, relative to each other, the forces created by the flowing liquid will not be great enough to overcome the force holding the particle down,” said Hammond.
Even when particles are removed, that process is not fast. Typical hand washing guidelines suggest at least 20 seconds under the tap.
Results from Hammond's model agree. It takes about 20 seconds of vigorous movement to remove potential viruses and bacteria.
The model does not take into account chemical or biological processes that occur when using soap. However, knowing the mechanisms that physically remove particles from hands can provide clues to formulating more effective, environmentally friendly soaps.
“Nowadays we need to think a little more about what happens to the laundry chemicals as they go down the sewer and into the environment,” said Hammond.
Hammond said this isn't the whole story of hand washing, but it does answer important questions and lay the groundwork for future research.
