Particles can be produced by many different sources. Inert (nonliving) particles usually arise from the rubbing of one item against another, such as the dust produced when you saw through a piece of wood. Humans shed lots of inert particles, as in the continuous sloughing off of dead skin. Electric motors give off particles where the commutator is rubbed by wire brushes. Plastic disintegrating slowly in ultraviolet light sheds particles in a light breeze. Viable particles are living microorganisms such as bacteria, viruses and fungi. Humans shed large quantities of viable particles.
Particles can be classified as organic (arising from living matter, though not necessarily alive themselves) or inorganic (arising from matter that was never alive). A dead skin cell is an inert organic particle. A protozoan is a viable organic particle. A grain of copper dust is an inert inorganic particle.
Excerpted by permission of Particle Measuring Systems, Boulder, CO, from the “Basic Guide to Particle Counting” – Download the “Basic Guide to Particle Counting”. Although the guide focuses on cleanroom environments, it is of interest from a general cleaning-science perspective.
Among the tiniest particles are atoms. Next in size are molecules, or groups of atoms. These are still too small to be considered industrial microcontamination.
The particles that particle counters typically monitor for clean manufacturing range in size from well under a micron (abbreviated µm, 1/1000 of a millimeter) to about 100 microns (1/10 of a millimeter.) Particles larger than this can be seen with the naked eye. Particles smaller than this (e.g., 0.01 µm) are of little or no consequence to modern manufacturing processes [although they may have human health impacts]. To put this in perspective, a typical human hair is about 50 -150 µm in diameter.
Some particles can change in size. Take, for example, a viable organic particle like a paramecium. A paramecium is a microorganism that, like most animals, is made mostly of water. If the paramecium becomes desiccated (dries up) it will be much smaller than it was when it was hydrated (full of water).
Why is the size of a particle of interest to a manufacturer with a cleanroom? Depending on the “clean process”, particles in a particular size range may be of interest because they could do specific kinds of damage to the process. If you are buying a filter, you need to know how small the pores in the filter media need to be.
Particles can be made of virtually any substance. Metals, plastics, fibers, animals, sea salt, smoke, fumes, and dust are all examples of particle sources. Almost anything can generate particles under the right circumstances. In a cleanroom, the most prolific particle generators are usually the people who work inside, shedding skin cells, breathing, sneezing, etc.
Behavior: How Do Particles Act?
Particles exhibit certain tendencies. They move through the air (and other fluids) by means of diffusion and ballistic forces, and they accumulate on surfaces through gravity and electrostatic adhesion. In liquids, particles may adhere to air bubbles, cling to the walls of a duct or container, or agglomerate into a larger mass.
If red dye is dumped into a bucket of clean water, the entire bucket of water eventually turns a uniform red color. This spreading-out action is called diffusion and takes place even when a gas or liquid seems to be still. Particles suspended in a fluid (liquid or gas) are moved by several forces: currents, thermal variation, and Brownian motion.
Currents are the laminar (smooth) and turbulent (rough) movements of a fluid. Currents are a result of pressure differences, with the fluid always moving from an area of higher pressure to an area of lower pressure. Particles suspended in a laminar flow tend to remain in that part of the fluid. In air, a lateral (side-to-side) movement is called advection; a vertical (up and down) movement is called convection.
Thermal Variation (Thermophoresis)
Temperature differences in a fluid contribute to currents, particularly convective (vertical) currents. Warming a fluid will also increase Brownian motion. This causes the molecules to be more energetic, and consequently they collide more frequently and are farther apart. This is why warm air is less dense than cold air and tends to rise.
Air is chock-full of particles, ranging from visible dust to non-visible gas molecules, that are continually colliding and bouncing off of each other (and into other particles). The same thing is true of liquids. Over time, Brownian motion results in a more-or-less random distribution of particles. The distance a particle can travel in a straight line before it bounces off another particle is its mean free path.
Particles can be ejected from a tool or process causing them to move against the prevailing air flow. This is one reason that particles are seldom found in a truly random distribution.
There are several ways a particle can be taken out of its free (diffused) state. These primary adhesive forces are electrostatic adhesion, agglomeration, accretion, and friction.
Particles can carry static electricity the same way a balloon rubbed against your hair can. This causes particles to be attracted and stick to a surface that carries the opposite charge.
In liquids, particles tend to agglomerate around (stick to) gas bubbles.
Particles can stick to each other. This can be the result of electrostatic adhesion or other “sticky” forces. Under certain conditions, it is common for two particles to stick together forming a doublet.
A particle can get caught on a rough surface where the movement forces are not strong enough to dislodge it. This mechanism, along with electrostatic adhesion, is the basis for most types of filtration.
Movement and Adhesion Cycle
Diffusion and adhesion coexist in a continuous cycle, such that a particle circulates, is trapped, breaks free, circulates, etc. Because of this the number and size of particles in a given fluid volume is always changing.
Why Important in Cleanroom Environments
Many of our modern, high-technology clean manufacturing practices demand cleanliness. Specifically, they demand an absence of particle contamination. To ensure the area is clean, particle counters are required.
Let’s look at a concrete example, semiconductors, commonly referred to as “microchips.” A microchip is a flat piece of silicon on which very small traces (flat wires) are etched, forming transistors and other components. This allows the manufacturer to create a very tiny electronic circuit.
Some traces are so close together (0.3 µm apart) that a particle lying across two of them would cause a short circuit. Because of this, the manufacturer wants to filter out any particles in the air that are 0.3 µm or larger. Particles smaller than this are not big enough to cause a short circuit. A particle counter is needed to ensure the particles were filtered properly so that the product is protected.