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.
Water drops on plastic surfaces can take on a great variety of shapes,
depending on the plastic's composition and its history of chemical or
electrical surface treatment. Anti-static electrical treatment of a
polyethylene surface, for example, causes water to adsorb on the surface.
So before antistatic treatments, water would bead up on the plastic
surface, but after treatment the "beads" are essentially flat.
That means you could effectively follow the course of anti-static treatment by stopping at different points in mid-treatment, and checking the shape of water drops on the surface at each point. For anti-static treatment, the normal practice is to drive the process to completion. But for many similar treatments, the goal is to bring the surface to some intermediate state between water-repellent (hydrophobic) and completely wettable. For painting or printing on plastic, or gluing together dissimilar plastics, or for obtaining cell adhesion to plastic tissue culture plates, usually an intermediate treatment value is desirable. Measuring the contact angle (see above figure) is an easy and accurate way to determine these intermediate values.
On a plastic surface being treated, the contact angle goes from a value near 90 degrees (the contact tangent line is practically straight up) to a value near zero degrees (the line is lying almost flat). By recording the contact angle, you can monitor the course of a surface treatment, or design new types of treatment. Of course, for process development work, it's most convenient if you have an instrument that measures and records contact angles automatically. That's the kind of instrument we design and build.
What can I do with contact angle measurements?
Contact angles offer an easy-to-measure indication of the chemical bonding of the uppermost surface layers of a solid. This bonding determines wettability and adhesion, and also allows prediction of coating properties and detection of trace surface contaminants. Contact angles are an easily seen physical manifestation of the more fundamental concepts of surface energy and surface tension. In a theoretical approach, you can calculate surface energy values from contact angle data, or you can take an empirical approach and deal only with contact angles. Either approach may be more useful for your situation--the important concept is that contact angles are related to surface energy and tension.
Contact angles, surface energy, and surface tension allow you to put numbers on what have been qualitative and rule-of-thumb descriptions. Numbers allow you to say "how good" and "how much to spare" in describing surface cleanliness, effect of surface treatments, and properties of coatings. Numbers also give you a way of comparing today's results with tomorrow's, or next year's.
With automated equipment, it's also easy to implement more challenging applications. Because contact angle systems can retain and analyze a continuous video record, you can study absorption rates of ink into a paper surface or blood into a medical dressing material. Because systems can set down a grid of microdrops for analysis, you can study uniformity of treatment, on the surface of a silicon wafer or a plaque of acrylic plastic treated for adhesion of proteins or cells. If you're interested in testing or modifying a surface, you're interested in contact angles. Very often, contact angles provide a reliable, repeatable index of surface properties--even of powders or finely-spaced grids--when issues of surface energy or more sophisticated parametrization are unworkably complex.
How can I get started in contact angle measurement?
If you are already familiar with contact angle measurements and want a single sample run to demonstrate instrument capabilities, FTÅ will do this without charge.
If you would like multiple samples run to determine sensitivity or specificity, FTÅ will perform this service at $500 per half day in our factory laboratory. This service includes discussing the nature of the specimens with you, running the experiments, and providing a written report.
If you would like to explore the use of contact angle measurements in your facility, you can rent the FTÅ125 system for $795 per month. This is an easy-to-use system that will have you making measurements quickly.
How do I pick the right system?
The FTÅ125/135 systems are designed for quality assurance and quality control, where simplicity and economy are most important. These systems provide totally automatic analysis, so that operator training takes only a few minutes. However, they provide only the most basic contact angle measurement, whereas research systems are more flexible and provide several kinds of measurement.
In this figure, you can see the basic video contact angle measurement.
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.
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.
How much application knowledge is required?
The FTÅ125/135 quality systems require very little study or training, since they report a contact angle and the basic surface energy value without any operator intervention once a sample is in place. You can profitably use these instruments to sort surfaces or fluids into "good" or "bad" with respect to a process by correlating other process attributes to contact angle ranges. These systems offer the most reliable contact angle/surface tension measurements available for the factory floor. The FTÅ200 and 4000 systems are, by comparison, complex and make more sensitive and sophisticated measurements. These require an understanding of surface chemistry to fully appreciate.
On a positive note, FTA would be delighted to help provide enough surface chemistry information to help you adapt an instrument to your application. We expect that, as you progress through a series of basic experiments, you'll find a practical use for contact angle data in a fairly short time.