Automatic digital tensiometers are expensive - three to four times more so than a high-precision manual tensiometer. We hope to clearly depict when an automatic digital tensiometer is not merely nice to have, but essential.
duNouy Ring and Wilhelmy Plate Techniques.
The duNouy Ring method is based on a technique developed by P. Lecomite duNouy and popularized in a paper published in 1925. In this technique a platinum ring is first submerged below the surface of a liquid. The ring is then brought up through the surface. The force necessary to do this and break the meniscus formed at the surface of the fluid is measured. This force is transformed into surface tension terms, usually dynes per cm.
The duNouy method has been used for measuring a wide range of products. It can be used for very low surface tensions, and any surface tension up to a high of 90 dynes/cm. Traditional torsion balance tensiometers are still in wide use because of their inherent precision and stability.
The Wilhelmy plate method is based on the force applied by a liquid to pull on a material immersed in that liquid. The higher the surface tension the greater the force. Wilhelmy plates are well suited for high surface tension liquids and can be used to measure changes in surface tension over time. Tensiometers based upon electronic balances are often used for Wilhelmy Plates applications. These usually provide a digital readout but have limited capability for time-based surface tension analysis.
If your requirements call for measuring only surface tension, the more basic duNouy ring tensiometer is probably the best choice. When you have very thick, high viscosity requirements, a Wilhelm Plate instrument will work.
When conducting basic surface tension tests, in normal ranges, we recommend the duNouy Ring Tensiometer.
Requirements that Point to the Need for an Automatic Tensiometer
If you have requirements for measuring surface tension which change over time, such as measuring the reaction times of surfactant, the Automatic Tensiometer is a good solution.
An automatic tensiometer has unique capabilities for determining Lamella Length, which is the amount of stretch in a liquid between the development of maximum force and the total release of a duNouy ring.
Measurements of changes in apparent surface tension or wettability of different substrates are enhanced with automated techniques.
Most automatic tensiometers have attachments that will control the temperature of the sample.
Most automatic tensiometers also perform routine duNouy Ring and Wilhelmy plate tests. These instruments also record historical results, perform statistical analyses of multiple tests, and plot test readings.
When Do You Need An Automated Solution?
A number of testing requirements clearly point to the need for an automatic tensiometer.
- Time based surface tension tests
- Measurement of Lamella Length
- Wettability analysis
- Temperature controlled samples and/or
- Continuous recording, plotting and retention of all test results
If these are part of your analysis requirements, an automatic Tensiometer is the only practical answer. The ability to conduct your routine duNouy and Wilhelmy Method tests comes as a free added benefit.
We hope this has provided some guidance for the answer to When Do I Need An Automatic Tensiometer?
If you found this useful please share it with you colleagues and subscribe in the box above right.
The wide range of issues involved with selection and use of laboratory test equipment frequently baffle but always interest me.
Hope we help.
P.S. For automatic updates on our new blog post, sign up for the RSS feed at the top of the screen!
One of the ways to describe surface tension in fluids is: the property of a liquid’s surface that resists force. It serves as a barrier to foreign materials and holds the liquid together. This ever-present property is caused by unbalanced forces on surface molecules that pull toward the main part of the liquid.
What are some of the primary conditions that affect surface tension? The surface tension
characteristics of a fluidic substance stay basically stable, but can be changed by temperature variations, chemicals that modify the bonding characteristics of the molecules, oxidation and the presence of impurities. Let’s consider the effects of these four conditions on surface tension:
As temperature decreases, surface tension increases. Conversely, as surface tension decreases strong; as molecules become more active with an increase in temperature becoming zero at its boiling point and vanishing at critical temperature.
Adding chemicals to a liquid will change its surface tension characteristics. The effect of adding an unrelated chemical to a substance, and thereby changing its surface tension, is demonstrated by the example of putting soap (a surfactant) in water to reduce the surface tension, which allows the dirt on your hands to more easily mix with the water.
Oxidation directly affects surface tension. As surface tension increases, intermolecular forces increase. Oxygen in the atmosphere is known to decrease the surface tension of various substances. The Presence of ImpuritiesThe presence of impurities on the surface of, or dissolved in, a substance directly affects the surface tension of the liquid. The surface tension of water, for example, will increase when highly soluble impurities are added to it.
Now that we've considered the effects of variation in temperature, the addition of chemicals, oxidation, and the presence of impurities on surface tension, maybe we should next consider the effect of a surfactant over time and how we observe and measure this effect.
The subject of conditions affecting surface tension appears at this level to be quite easy to understand. I am sure that as soon as I feel comfortable with this idea, I'll get a question that rattles me again.
As followers of these rants know, Fisher Scientific stopped offering its Tensiomat Tensiometer about a year ago. As a tensiometer manufacturer,
we at CSC Scientific were very interested
in this and have tried to let the world know that we might be able to help with things such as replacement rings, trade-ins and the like.
In discussing alternatives with Tensiomat users, we are often asked, “if I am going to replace my tensiometer, which one should I get?”
Let's explore the alternatives and compare their respective capabilities.
The simplest tensiometer has a duNouy Ring hanging by a hook from a level arm. This is a staple for measuring surface tension in simple liquids as well as measuring interfacial tension between oil and water. As the liquid thickens, an option with the ring rigidly fixed to the lever arm is helpful. This version can also measure the interfacial tension between two liquids in a downward direction.
Beyond these classic models, which cost in the $4,000 range, are the starting electronic balance-driven digital versions that can perform the duNouy ring tests as well as the Wilhelmy Plate technique. This level of tensiometer works well for static surface tension analysis. Depending on the test procedure’s degree of automation, these digital tensiometers cost between $6,500 and $10,000
As the testing requirements expand to include as the likes of Lamella length, sedimentation rates and temperature studies, automation and computer analysis; tensiometer capabilities grow as well. Instruments that require such features range from $12,000 to $20,000. The cost levels usually depend on the number of alternative techniques included.
As requirements expand to include determining dynamic surface tension, powder contact angle, powder wettability and dynamic surface tension, the cost level can increase to the $40,000 plus range.
When the requirement is for just surface tension, the basic units can usually meet the needs for periodic testing. However, wth high test volumes and expanded test functions, the higher-cost units become important alternatives.
Our affiliate Scientific Gear constructed a capability comparison table to help determine the type of instrument needed to best support an application. Click on the button for a copy.
We are available to discuss your specific application and help identify the instrument that will most effectively meet your needs. We hope this short review will be helpful in understanding the tensiometer alternatives and their particular capabilities.
I remain a continually astonished,
P.S. You can subscribe to these raves and rants and be notified when a new one has been conjured up.
You may recall that I promised to offer my interpretation as to how Surface Tension is related to Viscosity.
To begin with, liquid surface tension and viscosity share a common trait: they both involve properties of fluids. After that, things start to get murky.
Let us start with surface tension. This relates to the property of a liquid’s surface that resists force; it serves as a barrier to foreign materials as well as keeping the liquid together. This ever-present property is caused by unbalanced forces on surface molecules that pull toward the main part of the liquid.
Viscosity, on the other hand, is related to a liquid’s resistance to being deformed or moved. This is caused by the friction between molecules.
Compared to viscosity, surface tension is a simpler phenomenon. It is basically stable, changed mostly by temperature and chemicals that modify the bonding characteristics of the molecules. As temperature decreases, surface tension increases. The effects of adding an unrelated substance is illustrated by the example of putting soap (a surfactant) in water to reduce the surface tension, which allows the dirt on your hands to more easily mix with the water.
Regarding viscosity, knowing the type of liquid is essential. For example, there are Newtonian fluids that react to forces (sometimes called shear rate) that move the liquid (sometimes called shear stress) in a straight-forward, linear manner.
However, non-Newtonian fluids follow different sets of rules. Shear-thinning fluids decrease in viscosity as the pressure or force increases. Thixotropic fluids change viscosity over time -- Example gels and colloids, and yes ketchup are stable at rest, but become fluid when agitated.
Thus we see that finding the true value of viscosity [which some of us may think of as simply thickness] is a complex process. Viscosity, unlike surface tension [which tends to be a static phenomenon], is all about movement. All that should concern us in regard to measuring surface tension is whether to use a Wilhelmy Plate or a duNouy Ring. (That is enough to keep me entertained.)
The last question, which perhaps should have been the first, is about the correlation between surface tension and viscosity. You would think that thick fluids would translate to a high surface tension and that thin fluids would produce lower surface tension. Not true. In fact, my research has shown that there is no conclusive correlation.
This got into a lot more theoretical entanglement than I expected when I first considered taking on what seemed like a simple comparison. The answer, however, is clear: no correlation. The reasons are not so simple. I guess a good summary would be that surface tension is about steady state and viscosity is about movement.
Until next time, I remain as confused as ever.
P.S. Did you know that you can subscribe to these exposés, rants, raves and ramblings? All you have to do is click on the RSS Feed symbol at the upper left and you will get a notice when a new one is published. Or, if you prefer, you can also subscribe for e-mail notice by jotting your address in the box just to the right of the title.
By Art Gatenby
When tempted to think I know all there is about surface tension measurement, further information brings me back to earth. I’m conversant with the principal applications: surfactant analysis, plating, detecting contaminants, development of ink and the like. I have assisted customers to set up and calibrate duNouy Ring tensiometers for most applications -- all the while taking for granted that the Ring technique was the method of choice -- with only infrequent questions arising about Wilhelmy Plate tensiometers.
My lack of concern had a least some justification. The duNouy method has been around for more than 50 years -- twice as long as the Wilhelmy -- and is the most widely used technique. Several ASTM standards call for it.
There are more Ring-style instruments used worldwide than any other type.
CSC Scientific is readying plans to introduce an automatic tensiometer, the manufacturer of which is a strong proponent of the Wilhelmy Plate method. Prudence therefore dictates gaining an education on this technique -- which I'm sharing with you here.
The two tensiometer methods differ in that the duNouy Ring is pulled though the surface to make the measurement, while the Wilhelmy Plate is stationary.
Using the Ring technique, causes a non-equilibrium state in the liquid as the ring is pulled through the surface.
The Wilhelmy Plate, by contrast, is placed at the liquid’s surface and a meniscus is formed on its perimeter, causing a downward pull. The Plate is not in motion thus the entire surface is in equilibrium. The force is constant or varies only with a change in surface tension.
There are other noteworthy differences regarding applicability of these tensiometer methods.
Measuring surface tension variations over time, (such as measuring time and end-point surface tension for formation of Critical Micelle Concentration of a surfactant) can be made with the Wilhelmy Plate, but not the duNouy Ring. Because high-viscosity liquids cause larger stress on the somewhat fragile ring, the Wilhelmy technique is preferred for these applications.
The Ring method is easier to use on a manual instrument. Although it tends to yield higher surface tension readings, there is a significant body of historical data for the duNouy technique.
Automatic tensiometer evolution has made Wilhelmy Plate use easy. These instruments also adapt to the duNouy technique. The automated techniques permit the easy evaluation of the two methods on the same sample, facilitating comparisons of test resuls.
Watch for the announcement of the New Automatic Tensiometer.
Hopefully this short story about duNouy Rings and Wilhelmy Plates has been somewhat informative and helpful.
Until the next time.
P.S. If you'd like to discuss your application, call me at 703-876-4030.