The model showed that particles are attracted to the rough surfaces, and a certain amount of energy is required to enable the particles to escape into the fluid. A faster motion of the hands creates a stronger flow of fluid and removes the particles more easily, the authors said.
“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.
Using reasonable estimates for the variables, including the speed of hand movement, the model revealed that about 20 seconds are needed for the particles to escape. That is in keeping with recommendations from the Centers for Disease Control and Prevention (CDC), which advises people to wash their hands with soap and water for 20 seconds, or about as long as it takes to sing “Happy Birthday” twice.
The CDC’s recommendation is not based on physics modeling, but rather studies of the levels of microbes that remain on hands after washing for certain time periods.
Hammond noted that the new study did not take into account the biological action of soap. Soap not only helps lift dirt and germs from hands, but it also disrupts the membrane surrounding the viral or bacterial particles, thus destroying them.
Future research should take into account this “chemical attack” from soap, but the current study lays the groundwork, Hammond said.