After more than a year of the pandemic, it is clear to all of us: hand washing is necessary and effective to prevent the spread of diseases and infections. Now, a curious study published in the scientific journal Physics of Fluids explains the physical bases of this phenomenon.
The authors designed a mathematical model to simulate hand washing and estimated the time scales in which very small particles such as viruses and bacteria are removed from the same. The simulation works in two dimensions: two wavy surfaces that move one on top of the other (and that represent the hands, since these, seen on a very small scale, have a rough texture), and a thin film of liquid (water) between both.
Small particles such as viruses and bacteria would be trapped in the potential valleys that form on rough surfaces. In order to escape from there, the energy of the water flow would have to be high enough to lift them from the bottom of the valley.
How long do you have to rub?
On the other hand, the force of the flowing liquid depends on the speed of the moving hands: a stronger flow removes trapped particles more easily. The authors compare this process to rubbing stains on a shirt: the faster the movement, the more likely it is to remove the stain. “If you move your hands too gently or too slowly, the forces created by the flowing fluid are not great enough to overcome the force that, in turn, holds the particle down,” explains Paul Hammond, researcher at Hammond Consulting. Limited and author of the work.
On the other hand, the process is not fast. The usual guidelines for hand washing suggest that we should at least have them under the tap for twenty seconds. The results of the Hammond model are consistent with this recommendation: it takes about 20 seconds of vigorous movement to remove potential viruses and bacteria.
Towards less polluting soaps
On the other hand, the model does not take into account the chemical or biological processes that take place when soap is used. However, knowing the mechanisms that physically remove particles from your hands can provide clues to formulating more effective and environmentally friendly soaps. “We must take into account what happens to chemical washing products when they go down the drain and their impact on the environment”, reflects the scientist.
Referencia: Hammond, 2021. “Will we ever wash our hands of lubrication theory?” Physics of Fluids. (DOI: 10.1063/5.0060307)
Fuente: AMERICAN INSTITUTE OF PHYSICS
