Do I Need A Contact Angle Analyzer?
This note is a guide to the uses of contact angle analyzers. It assumes almost no background in surface chemistry, just a little familiarity with application areas (semiconductor processing, medical plastics, printing, plating, coating and so forth). After discussing the uses, we'll discuss briefly the various types of analyzers, from quality-control systems to research instruments.
What's the basis of the method?
You can see lots of contact angle phenomena in everyday life. On a clean glass surface, for example, water doesn't "bead up" in drops. If you take your car through a car wash, though, and order the spray wax option, the glass surface of the car windows becomes coated with an agent that will cause water to form distinct hemispherical drops. Water on a Teflon surface, similarly, makes distinct drops that "curve under" at the edges.
It would be a challenge to calculate the contact angle theoretically in advance in this situation, but changes in the angle from case to case provide lots of useful information.
Are contact angles sensitive enough?
The sensitivity of contact angles to many substances is extremely high, being able to detect a fraction of a monolayer on a surface. Contact angles are extremely sensitive to the details of surface, rather than bulk, composition. For inspection of modified surfaces, contact angles are typically sensitive enough to detect tiny changes in modification results.
Are contact angles specific?
Do they respond to things other than the treatment under investigation? This is sometimes the case. Contact angle measurements respond to bonding energy rather than specific chemical compounds. Different molecules on a surface can have similar bonding energies, so this must be checked. So while contact angles determine surface energy, they don't specify chemical composition, but typically it's a relatively straightforward matter to devise a set of quick experiments to sort out composition issues.
Is the spatial resolution satisfactory?
Contact angle measurements are made by placing a drop of fluid on a surface. This drop has a definite physical size, and may be too large for the feature you wish to investigate. The basic FTÅ200 systems are "macro drop" systems and deal with microliter quantities which form drops measured by one or more millimeters. The more expensive FTÅ4000 systems are "microdrop" systems which can deal with nanoliter (and smaller) volumes and achieve spatial resolutions below 100 microns.
Is the timing resolution satisfactory?
Absorbent materials and surfaces which experience hydration call for systems with sufficient time resolution to follow these dynamic phenomena. The FTÅ125/135 quality systems make measurements several times per second, whereas the FTÅ200 and 4000 systems operate at much higher speeds, with at least .016 second resolution. These systems can study absorption phenomena on all kinds of surface. Here's an example of a time-slice from a study of water drops on an open-weave surface, following absorption at 1/60 second per video frame. In this case, the absorption rate is a function of chemical treatment of fibers in the weave.
The computer in the measuring system directs the placement and volume of the drop, and uses sophisticated software to obtain the contact angle from the drop image. As you move from the simplest contact angle systems to the more advanced, you get more control over drop sizes (ranging down to fractions of a microliter), placement rate, and shape analysis.
The FTÅ200 systems provide the researcher with tools to carry out a variety of experiments, approaching a problem in different ways. They are appropriate to academic and industrial research laboratory environments.
Solving Your Problems: Some First Questions:
What if I don't have a clearly-defined contact angle problem?
Give us a call. We've worked on all sorts of applications, from printing to biotechnology to semiconductor manufacture. We're well-versed in the realities of industrial process control and in developing simple experimental protocols for problem solving. Contact angles have proved useful in many non-classical, less-than-ideal situations. In this figure, contact angle is used to monitor the cleanliness of the metal in a wire grid.