Application Note – Clean Room N.2 – “Physical laws” that regulate the diffusion and deposition of particles in the air
“Physical laws” that regulate the diffusion and deposition of particles in the air
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.
• Comments
Complexity of particles behaviour
As you may understand from this multitude of physical factors, the behavior of the particle diffusion in a confined space is quite complex. The phenomenon is more complicated and difficult to understand when viable particles like micro-organisms, that are not following the “math rules”, are involved.
Importance of the capture of “viable micro-organisms” and not “dead particles”
In this contest, the “capture capacity of micro-organisms” for a microbiological air sampler should be evaluated in function of its capacity of capturing and guarantee the multiplication of viable germs and not “dead particles”.
Air sampling with vacuum
Considering the microbiological air samplers, if the vacuum is used to transfer the air from the confined space (e.g.: Clean Room or isolator) to an external central control unit – and the consequent need to use long tubing , the particle behavior is posing doubts on the true analytical value of the system. The official documents 2009 EC GMP Annex 1 point out how important and difficult is the evaluation of the correct length, bending, diameter to transfer air into tubings from a closed environment to a counter.
