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October 2012 |
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| Ten Things Your Goniometer Won't Tell You | |||
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We'll be the first to admit it: our line
of contact angle instruments are amazing, powerful and easy to learn and
use. But alas they are machines and have not been programmed to talk back. If they could, here are
10 things they would tell you: 1. You would get better results if you leveled the stage first. We're not sure why, but there are some folks who think that leveling the stage is optional. It's not. If you're not savvy about how to level the stage, watch the four minute video below to learn how to do it right. Alternately, point your browser to: http://youtu.be/scIFT2c-8pg.
2. Your measurements are all wrong because you didn't calibrate me properly. There's a reason why rulers aren't made out of rubber. It would make accurate measurements a challenge. The same is true with measuring contact angle. When you take a linear or angular dimension, the software must have a frame of reference. The method that ramé-hart uses for establishing a reference to something with known dimensional values is a calibration routine that measures an object of precise dimensional value - e.g., a sphere or cylinder. If the calibration is successful, then the software will know the size of each pixel in both the horizontal and vertical directions. This value is then used to calculate linear and angular dimensions as measurements are taken. However, if the calibration is wrong, then the results you are collecting will also be wrong. In order to avoid this problem, consider checking the calibration on a periodic basis (e.g., daily) but only do a new calibration if the deviation between the stored values and the new values is greater than 1%. Consider keeping a log of your calibration results so you can compare your current calibration results with prior results. A significant change in values can signal a red flag and trigger a reexamination of your current calibration. Consider using the Measure Distance command to check the width and height of your calibration ball after calibration. Lastly, use a fixed sessile drop calibration reference tool to verify the integrity of both your calibration as well as your measuring method. 3. I'm measuring a water contact angle higher than it should be because your sample is dirty. Contaminants on a surface will lower the surface energy and will result in poorer wetting and higher contact angle readings. By cleaning the surface thoroughly prior to measuring contact angle, you can ensure that the surface is free of debris and that the resulting contact angles are an accurate indicator of the wetting that is occurring on the surface and not as a function of contamination. 4. Your results would be a lot more reliable if your test liquid were not contaminated. Just as a contaminated surface can affect the contact angle, so too can a contaminated liquid. If you are using pure deionized water for the drop phase, for example, the surface tension should be about 72 mN/m (or so, depending on the ambient temperature). However, if there are contaminants in the water (perhaps due to poor cleaning of the syringe or needle), then the surface tension can be much lower. This, in turn, results in a lower contact angle. To test this theory we started with a clean glass slide and pure deionized water. We measured a contact angle of 48°. We then purposefully contaminated the water by adding some dye. This lowered the surface tension from 72 to 45 mN/m. We then measured the water contact angle on the glass slide and it dropped to 35°. Surfactants, of course, are purposefully used to lower contact angle -- which is why they are often called wetting agents as they promote or improve wetting. If your instrument includes DROPimage Advanced, you can use your tool to measure the surface tension of your test liquid prior to dispensing it for contact angle measurements. 5. You're so slow that by the time you get around to taking a measurement, the drop has lost 5 degrees of contact angle due to evaporation. The smaller the drop, the more true this is. Generally speaking, it's best to wait one or two seconds for the drop to reach equilibrium before taking a measurement. But, in most cases, the change in water contact angle after five seconds is mostly attributed to evaporation. Waiting a long time between drop disposition and taking the measurement can lead to results which reflect a lower contact angle than you should be getting. If you need more time to get things setup, consider doing a test drop, getting things all set up, then move over, dispense a new drop, and measure right away. 6. Your surface energy results are not very good because you're using the wrong liquids. If you are using the Acid-Base Tool to measure surface energy, for example, you should be using one apolar liquid and two polar. Recommended test liquids are methylene iodide or bromonaphthalene for the apolar liquid and a polar liquid pair of either water and glycerol or water and formamide. If you choose other liquids or if you attempt to use a trio of liquids that do not consist of one apolar and two polar, then your results may be invalid. Likewise, if you are using the Two Liquid Tool, the recommended liquids are water and methylene iodide or diiodomethane. Other pairs may or may not work as well. Consult the DROPimage User Guide and on-line help menu for additional direction regarding these and the other surface energy tools. 7. I would work a lot better if you didn't have me in full window mode. DROPimage is a complicated program with many potential dialog boxes that pop up and go away at various times during your work session. We try to hard to get the window priorities logically correct. But since there is a nearly infinite number of combinations of dialog boxes open, occasionally a window will slip under another one. The best way to avoid this is to simply work with the main DROPimage window in windowed mode (not full screen) and sized just big enough to hold the image window. This will allow you plenty of room on the desktop to position other dialog boxes for getting things done. One additional related tip: when you're done with one task (for example, the Contact Angle Tool), be sure to close it before moving on to the next task (such as Calibration or running a methods-based experiment). The more dialog boxes you have up, the more things there are that can go wrong. So, if you're done with something or not using it, close it. 8. You should keep me away from the window. While we're on the topic of windows, it's best to keep your instrument away from the windows which sun passes through. Excessive side light can result in poor images, cast shadows and reflections, and otherwise impair you and your instrument's ability to collect meaningful data. 9. You turned my light up too high (or down too low). While sidelight from a window can cause problems, too much or not enough backlight can also be an issue. The best way to get your light set correctly is to put the calibration tool on the stage and focus on the ball. The ball should be 100% black and the background should be 100% white. If it's not, adjust the light intensity on your light source. In the case of Model 190, use the software controls as detailed in the User Guide. 10. I can't measure correct surface
energy values if you don't first add the liquid properly using the phase
editor. If you are only measuring contact angle, you can safely
forget about entering into the Phase Editor all of the parameters of the
solids and liquids being used. However, if your goal is to measure
surface energy, the accuracy of your results is dependant on the
accuracy of the phase data. Whenever possible, try to use liquids that
are already in the database - and even then ensure that the liquids are
pure and fresh. If you are adding your own liquids, make sure you have
the correct density and surface tension values. If you have DROPimage
Advanced, you can measure the surface tension on your instrument. |
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| We're in Fifty Countries | |||
ramé-hart is an international company.
We're pleased to announce that we now have modern camera-based
instruments installed at locations in over fifty different countries
worldwide. In many of those countries, we have dedicated agents. Currently one third of our sales are shipped to locations outside of the
United States. Below is a list of countries (sorted alphabetically)
where our instruments are installed.
a 1 to 4 instruments installed
in past 10 years. |
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Regards,
Carl Clegg |
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