May 2009

Last month we discussed interfacial tension which is measured between two immiscible liquids. This month we're going to take a look at surface tension which is measured when the external phase is a gas - such as air. Surface tension is measured using a tensiometer. At ramé-hart we currently offer two tools with surface tension capabilities, Model 250-F1 and 500-F1, both with DROPimage Advanced and both are tensiometers as well as goniometers.

Surface tension is described as an attraction between the molecules on the surface of a liquid. Inside and away from the surface of a liquid, all of the (blue) molecules are pulled by neighboring molecules with an equal force as shown in the graphic above. At the surface, however, the (green) molecules are pulled inward with a force greater than their attraction to molecules in the neighboring medium - whether it be a gas or an immiscible liquid. This results in the propensity for the surface of the liquid to be smooth and for the surface area to be minimized. In short, the surface of the liquid behaves like a thin film.

The effect of surface tension can be seen with the element mercury which has a very high surface tension value. As a result, mercury will bead up and join together forming smooth drops.

Surface tension is measured using a variety of methods:

The Wilhelmy Plate Method requires a thin plate of a fixed perimeter to be lowered into the test liquid. As it is pulled up, a precision scale is used to measure the force that results from wetting. This method has been around for a long time (Ludwig Wilhelmy died in 1864) and requires a correction for buoyancy. There are many legacy instruments based on this method. 

The Du Noüy Ring Method uses a small ring, usually with a perimeter of about 2-2.5 inches suspended by 3 fine wires, which is lowered into the liquid. As the ring is raised forming a lamella, a precision scale measures the force. This value is then corrected to determine surface tension. The setup for this method is tedious, the ring is fragile and must be cleaned between measurements, and measurements can be time-consuming. The ring is usually made from Platinum and is subject to errors from distortion and damage.

A new version based on the Du Noüy Ring Method, called the Du Noüy-Padday Method,  uses a small diameter metal rod in lieu of the ring. A super high sensitivity balance measures the force required to pull the test probe out of the test liquid. This method does not require as much volume as the ring method, setup time is much easier, and measurements can be taken in fewer than 30 seconds. Calibration of a Du Noüy-Padday instrument is done using a liquid with a known surface tension, such as water. This method is highly repeatability offering high sensitivity. Instruments based on this method typically are much more affordable than traditional ring and plate tools, a correction for buoyancy is not required (it is for the ring and plate methods). The diameter of the rod is really not important -- as long as the instrument is recalibrated when changing to a new rod. Lastly, the Du Noüy-Padday method works well with viscous liquids such as paints, oils, and polymers -- liquids that the ring and plate methods cannot accommodate.

The Axisymmetric Drop Shape Analysis (ADSA) Method also known simply as the Pendant Drop Method calls on the Young-Laplace equation to calculate the surface (or interfacial) tension based on various dimensions and contact angle data. This method requires a high resolution profile image and a custom software program suitable for the intense computational algorithms that are required. The results of this method are excellent but the technology necessary has only become mainstream in the last 15 or so years. ramé-hart has been a pioneer in drop shape analysis with our award-winning DROPimage Advanced program. All methods require contaminant-free environments; in the case of the drop shape method, there are no plates, rings, capillaries, or other potential sources of contamination, just a dispensing needle or disposable tip. Additionally, the drop shape method is particularly suitable for testing systems that change over time.

The Bubble Pressure Method (also called the Jaeger Method) measures surface tension from the maximum pressure required to cause a gas to flow from a capillary tube which is immersed in the test liquid. The range of values that can be measured using this method is limited.

Similarly, the Capillary Rise Method requires the end of a capillary to be immersed in the liquid. The height that the liquid reaches and the diameter of the inside of the tube is used to calculate surface tension. Since it's nearly impossible to measure the contact angle (θ), this method is only used when θ=0. The capillaries must be totally clean (typically with chromic sulfuric acid). A correction must be made for the shape of the meniscus.

A similar method that uses a specialized capillary tube is called the Stalagmometric Method. The tube, called a Stalagmometer, has a section with a wider diameter. The liquid is dispensed through the top. When it reaches the wide section, drops form and release. By counting the drop frequency, the surface tension can be estimated. The principle relies on Tate's Law.

There are various other less-common methods: the Oscillating Jet, the Spinning Drop , the Drop Volume Methods. These methods are primarily for measuring interfacial tension and thus will not be explained here.

With all methods, including those detailed above, one of the most important factors is cleanliness. Not only does the apparatus (ring, plate, needle, capillary, etc) and test liquid need to be completely clean but the external air or gas phase needs also to be contaminant-free. ramé-hart offers several different environmental chambers and an environment fixture -- not only to reduce contamination -- but also to control humidity and temperature or provide a non-air external gas phase.

Conclusion

Our research and experience with surface tension lead us to conclude that in the majority of cases, the (2) best methods for measuring surface tension are the Pendant Drop and the Du Noüy-Padday Methods. The Pendant Drop Method requires a clean environment and sophisticated software such a DROPimage Advanced but is fast, repeatable, and requires very little test liquid. Pendant Drop is suitable for a wide range of application including dynamic and over-time studies. The Du Noüy-Padday Method is also fast, easy to use, very sensitive, useable with a variety of viscosities, and does not require software. It does however require a low-noise high precision balance and of course a very clean contaminant-free environment. It is fast and well-suited for high-volume unattended multi-channel applications and is more cost effective than traditional ring and plate based tensiometers.
 

Over a span of 40 years, ramé-hart manufactured thousands of the classic Model 100-00 NRL Goniometers. This is the same tool that many surface scientists first learned on and, in fact, many still use today. About five years ago when the benefits of software-driven camera-based technology completely overwhelmed the simple microscope-based design of the Model 100-00, we discontinued its production. From time to time, we accept trade-ins from customers who have a legacy Model 100-00 but wish to move up to a new Advanced Goniometer Model 300-F1 or 500-F1. This leaves us with an inventory of used legacy goniometers. In our spare time, we like to rebuild them. The unit shown below was once a legacy Model 100-00 with microscope. We upgraded the backlight to use our current halogen-based fiber optic illuminator system. We installed a new leveling stage and microsyringe fixture. The microscope was replaced with our high-speed F1 Series digital camera. The 3-axis stage and optical bench are from the legacy unit but have been completely refurbished and rebuilt. We replace any bearings, knobs, shafts, gears, or other parts that are beyond the allowable limits of wear. Once the system is completely rebuilt, we calibrate and test it using the same strict standards that are required of our new units. We then sell these factory certified upgraded instruments at a deep discount over the cost of a new tool. Typically the savings are 30-40% off the list price for a comparable new unit depending on the overall age, condition, and number of new parts we install. In any case however, all rebuilt models come with the same 1-year warranty that we include with new systems, a new PC and LCD, a new dust cover, a new microsyringe and needle, and a complete set of documentation. 

If you have any interest in a ramé-hart rebuilt goniometer, please contact me at the phone or email below. We'd be happy to quote you on a new or rebuilt system with the accessories you require for your applications. Similarly, if you have a legacy Model 100-00 laying around or in storage, it has never been a better time to trade it in for a new instrument or commission us to upgrade it for you. Either way, you'll get ramé-hart quality and a tool that you drive for many years to come.