Hand-washing is helpful in preventing disease spread, although its mechanics are rarely researched. Scientists at Hammond describe a simple model that captures the basic mechanics of hand-washing.
Simulating hand-washing allowed them to predict particle removal time scales, such as viruses and germs.
The mathematical model operates in two dimensions, with one wavy surface passing through another, separated by a thin film of liquid. Because wavy surfaces are rough on tiny spatial scales, they represent hands.
Particles become caught in potential wells on the rough surfaces of the hand. In other words, they are at the bottom of a valley, and to escape, the energy contained in the flowing water must be sufficient to propel them upward and out of the valley.
The force exerted by the flowing liquid is proportional to the speed of the moving hands. A greater flow efficiently eliminates particulates.
“Basically, the flow tells you about the forces on the particles,” said author Paul Hammond. “Then you can work out how the particles move and figure out if they get removed.”
He compares it to scrubbing a stain out of a shirt: the faster the motion, the more probable the stain will come out.
“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,” said Hammond.
Even when particles are eliminated, this is a slow process. Hand-washing guidelines often used, such as those from the Centers for Disease Control and Prevention, recommend at least 20 seconds under the faucet.
The results of Hammond’s model concur. Dislodging possible viruses and germs requires around 20 seconds of vigorous agitation.
The model does not take into account the chemical or biological reactions that occur when soap is used. However, understanding the mechanisms by which particles are physically removed from hands may provide insight into developing more effective, ecologically 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.
Hammond stated that while this study does not provide the whole story of hand-washing, it does address critical concerns and establish the groundwork for future research.
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