The tests described in the last couple of blogs are rudimentary ways to assess the surface activity of the surface being tested. In fact, they are all based on the concept of contact angle. A drop of water resting at equilibrium on a surface will intersect that surface at a measurable angle called the contact angle. The less active the surface, the greater the contact angle. Devices used to measure contact angle are not that complex. They usually consist of a "stage" or platform where the test piece is placed, a device to apply a drop of water in a precise way (often a syringe) and, finally, a light source and optical aid aligned with the stage to show a cross section of the water droplet with a reticule to allow measurement of the contact angle. Although simple, the contact angle measuring device would not be considered "portable" and should be used under controlled conditions such as in a laboratory. Some years ago, a somewhat more portable, and simpler to interpret test was devised based on the fact that the contact angle can be changed by using liquids having different surface tensions. The contact angle is, in fact, based on the differential in the surface activity of the liquid and the surface and is not determined by the activity of the surface alone. Ultimately, as the surface tension of the liquid used to form the test droplet is reduced, a point will be reached where the liquid will have a contact angle approaching zero. When this happens, the test droplet will spread out or "wet" the surface. The test involves the use of a number of different solutions each formulated to have a specific surface tension measured in Dynes/cm. The formulas for these solutions and their surface tensions was published in the October 1985 edition of Products Finishing Magazine and is shown below.
A single solution can be used to perform a go-no go test (if it spreads out or not) or the solutions can be applied sequentially to establish a quantitative value for the activity of the surface under test which may then be translated into a measure of cleanliness. As you might imagine, mixing of the test solutions described above and maintaining them in a fresh and uncontaminated form is tedious and time consuming. To simplify things, several companies now sell these (or similar) test solutions in a premixed form which are readily available. However, handling and maintaining the solutions remains a task because they are easily contaminated in repeated use under varying conditions. Today, the "dyne test" as it has come to be known (an outgrowth of the contact angle test) is frequently performed using precisely manufactured and controlled "Dyne Pens." These pens are actually felt tip markers loaded with "inks" of varying surface tensions. They are used to deposit a line of ink on the surface being tested. If the ink remains as a continuous film, the surface clean to the degree indicated by the Dynes/cm rating of the pen used. If the ink beads up, the surface is not clean to the degree associated with that pen. Dyne pens are readily available today and are in common use in many applications. They are easily portable and not susceptible to contamination. For those wishing to learn more about contact angle and how it is applied as a test for cleanliness I would recommend http://www.astp.com/PDFs/ContactAnglePrinciples.pdf.
- FJF -