Removing foreign particles from surfaces is the target of many cleaning operations. The spectrum of the definition of particles as contaminants is very broad and encompasses size, number, shape and composition. Because of the diversity of the nature of particles, the methods to detect and quantify them are diverse as well. Some particles are easily seen with the naked eye while others require complex procedures to find and evaluate. In this series there will be many references to the micron as a measure of particle size. A micron is 1/1,000,000 of a meter. I am an engineer. You would think that the concept of the size of a micron would be a cinch - but guess what - I have a real problem comprehending just how small these micron-sized things are. One million is a very big number and yet, as the world has grown to bigger numbers, one million doesn't have the impact that it did 50 or 60 years ago when, as a kid, I was first learning about numbers and their meaning. In those times, one million was about the largest imaginable number and was perceived as just short of infinity. In fact, the odometers on cars didn't even have enough digits to get there. Now we talk about hundreds of millions, billions and even trillions (usually dollars) every day and the concept of a million has lost impact. So let's bring it back home.
The moon is only 1/4 million miles from the earth. You can drive a car that far but, driving at 60 miles per hour, 24 hours a day, it would take 166 days to get there. Still, difficult to comprehend - at least for me. Here's something that works for me - - one million inches a little over 15 miles (I wonder if inchworms get tired!). A meter is just a little longer than a yardstick. Actually, 1 Meter = 39.3700787 Inches. Someday we'll talk about how THAT happened. Anyway, a Micron is to a meter as 1 inch is to 15.782828 Miles! Think of a place located about 15 miles away and then look at an inch. It's pretty amazing just how small those microns are. The illustration is a popular tool for helping drive home the concept of a micron. So microns are small. It is interesting, however, that a particle that is just 10 microns in size can actually be seen by a human being with 20/20 vision without optical aid! I'm not sure if I find this more amazing because of the small size of the particle or because the human eye is such an incredibly sensitive organ. OK, you won't see 10 micron sized particles around your house or office - if you could, it would be very disconcerting because there are an incredible number of them everywhere. In order to see one, one needs a very bright light (maybe one of those new blinding LED headlamps would do the trick) and a particle that contrasts with the surface it is resting on. White particles on a shiny black surface work best. Shining the light at an angle will help define the particle. Particles smaller than 10 microns in size are seldom seen without optical aids but may be detected in other ways. There are several tools used to detect particles on surfaces. In upcoming blogs we will explore how to find and quantify particles that contaminate cleaned surfaces.
- FJF -