“Leggi fisiche” che regolano la diffusione e la sedimentazione delle particelle nell’aria
Per meglio comprendere il meccanismo di come il particolato ed i microrganismi si diffondono in un ambiente confinato è importante conoscere alcune leggi fondamentali della fisica ed alcune definizioni:
– Brownian motion
As the particles migrate through a body of air, random impacts from individual molecules will cause them to veer from course.
- Drag coefficient
It is the ratio of the force of gravity to the inertial force on a particle in fluid. It indicates how a particle will resist any force that could cause a change in the particle velocity. Smaller particles have smaller drag coefficients due to their lesser masses.
– Deposition velocity
It is also called “settling velocity” and it is the ratio of particle flux (distance per unit time for sedimentation to occur) relative to the ambient particle concentration.
– Diffusion forces
This force on a particle varies inversely with the particle’s radius. Therefore smaller particles are more prone to interaction due to diffusion.
– Electrostatic forces
It varies with the particle’s electrical charge and the strength of the electrical field in which the particle is located. Electrostatic charge can develop as a particle slips through the air stream.
- Gravitational forces
It varies with particle mass and the difference between particle and air density. The larger the particles, the greater the interaction.
– Relaxation time
It is the time for a particle initially in equilibrium with a moving fluid to match a change in fluid velocity. Large particles have a long relaxation time.
- Stopping distance
A related term of “relaxation time” is “stopping distance” which is defined as the distance for a particle initially moving with a gas stream to come to a stop when the gas flow is halted, as by an obstacle.
– Stokes number
It is the ratio of a particle’s ratio to the dimension of an obstacle in fluid flow. This is an important factor in determining when a particle in motion will be collected by any obstacle or will pass around it. An obstacle could be a filter fiber or the sample inlet.
– Temperature gradient
The temperature gradient is another factor to be considered in the movement of the small particles.
- Viscous forces
The fluid dynamic force from a moving fluid stream or the viscous nature of an air stream will “pull” particles along that flow path. In a unidirectional laminar flow, other forces act upon the larger particles encouraging settling and deposition; smaller particles remain buoyant on a laminar flow. In a turbulent flow stream, the larger particles are re-entrained back into the air flow and the smaller particles are more prone to additional forces acting upon them.
Conclusioni
- Complessità del comportamento del particolato
Come si può comprendere da questa moltitudine di fattori, il comportamento della diffusione del particolato in un ambiente confinato a contaminazione controllata, da un punto di vista fisico, è alquanto complesso. Il fenomeno diventa ancora più complesso quando si tratta di particelle microbiche che coinvolgono “esseri viventi”, come i microrganismi, che non seguono “regole matematiche”.
