Contamination theories — airborne particle dispersion
Particles spread within a volume because they are affected by an external force, such as air volume mass flow (diffusion and
or convective spreading, i.e., through vortices or turbulence), gravitational deposition, electrostatic forces or thermal
Gravity creates a downward motion on particles based on mass, and there is a ratio between the mass weight of the particle
and the settling time. Free-floating particles within an air volume will also have a gravity effect against each other; however
this force is very small and, in a pharmaceutical environment, it can be neglected.
Particles and surfaces can be positively or negatively charged via static electricity. Particles with a positive charge will be drawn to a particle or surface with a negative charge. The potential
for static charge also depends on the material; for example, glass has fewer tendencies to be charged than plastic materials.
A material's tendency to develop static electrical charge is an important issue to consider when it comes to using them in
a cleanroom. A surface with a positive charged material can attract negatively charged particles, which will accumulate on
the surface. If a surface gets grounded, particles may also come loose and spread to the surroundings.
The energy in airflow influences particles to move parallel with the airflow vector, i.e., particles are spread in the same
direction as the air movement (Figure 1). This knowledge is commonly used in the pharmaceutical industry to create clean areas and air barriers. Unidirectional airflow
(UDF) units have a parallel airflow such that airborne particles are transported away in the flow vector direction. The flow
in a UDF unit is either laminar or turbulent. Ljungqvist and Reinmüller tell us that "it is assumed in a parallel flow field
that, next to surfaces along the main flow direction, there is a thin sub layer (boundary layer) in which the transfer of
momentum is dominated by viscous forces, and the effect of weak turbulent fluctuations can be neglected. The situation is
quite different for particle diffusion. In this case, even weak fluctuations in the viscous sub layer contribute significantly
Figure 1: Convective deposition of particles.