Interfacial Tension and Dilational Stress Checklist
This list will serve as a trouble shooting guide for interfacial tension and dilational stress measurements. Further discussion of these suggestions is found in the other FAQ articles.
Always build confidence in your technique by performing simpler, but similar, measurements first. For example, before you do an upside down pendant drop measurement (say an air bubble up in surounding liquid), do a normal hanging down pendant drop measurement first with the same liquid. Make static measurements before you make perturbation measurements with time-varying volumes.
If you have trouble keeping the drop (or bubble) on the needle, you may need a larger diameter needle.
Lower interfacial tensions require larger needles. This is counter-intuitive, but nevertheless true.
Remember any drop to be analyzed for interfacial tension using the Laplace-Young equation must be tall enough that it is noticeably distorted by gravity. It must not be spherical in shape. Exactly how tall the drop must be, and therefore what its volume must be, is determined by the difference in density between the drop and the surrounding media. The greater the density difference, the shorter the drop and the smaller the volume can be. The only recourse with small drops is to forgo the accuracy of the Laplace-Young equation for an estimate of surface tension based on contact angles and known substrate surface energies. If larger drops do not stay on the needle (large enough for the Laplace-Young equation to be satisfied), you must move to a larger needle.
If the drop or bubble keeps expanding after the pump stops, the hydraulic system is not tight enough. There is excess air in the system. Sometimes, of course, you want air in the system, as when you do a bubble-up pendant drop experiment. The problem arises because air is compressible and therefore acts like a spring. This affects the dynamics of the system. When the pump moves, it causes pressure changes and these move through the system. Excess air acts to retard the motion, but the pressurized air relaxes after the pump stops. You can mitigate these effects by a) moving more slowly or, b) minimizing the air volume through the use of shorter or smaller tubing, etc. This is a trial and error process. The secret is to mimimize air volume to the extent possible.
If your surrounding medium is not clear, say with a liquid/liquid measurement, you may have to work with the brightness and contrast settings to obtain an image that can be analyzed. Unfortunately this must be done with the Live video, as opposed to a Movie you already have. With minor amounts of interference, you may be able to use the region of interest box (ROI) in the Movie to mask out problems. Otherwise, try the following:
- open the lens aperture so the depth of focus is minimized; this will make objects outside the focal region blend in.
- increase electronic brightness and decrease contrast on the Video window to force the smaller particules to blend in while maintaining the drop image.
You will have to recalibrate after making these brightness and contrast changes. Make sure the drop itself remains in focus and has reasonable contrast. The edge of the drop must be sharp and the surrounding media free of particules for 10 pixels or so.
Your needle size and how tight your hydraulic system is will determine how much you can perturb the drop volume during dilational stress measurements. We do all our work with a most simple 2-step pump program: a) pump linearly out at constant rate then, b) pump linearly in at the same constant rate. As discussed eleswhere, this results in an excellent approximation to a sine way for the surface area of the drop, which is what counts.