July 2009

Just last month we announced the release of our new Model 710 DAS Tensiometer. This remarkable lab-quality tool has piqued the interest of a number of our customers and we are confident that it will become a top-seller. If you missed our newsletter and would like to learn more, an archive copy can be found here: http://tinyurl.com/otrpuh.

This month we're pleased to announce another new product -- our new Automated Goniometer / Tensiometer Model 290. This powerful tool features the highest degree of automation available. The Automated Tilting Base (p/n 100-25-A), available as an option with Models 250 and 500, is included, allowing for the measurement of advancing and receding contact angles, roll-off angle, and contact angle hysteresis. The Automated Dispensing System (p/n 100-22-100), available as an option with Models 200 and up, is also included allowing for dynamic studies as well as precise and repeatable formation of pendant and sessile drops. Both the tilting base and dispenser are fully software-driven tools which can be integrated into a DROPimage methods-based experiment.

Model 290 includes DROPimage Advanced, our most powerful edition of DROPimage, which has the capacity to measure static and dynamic contact angles, surface energy, surface tension, and interfacial tension on pendant drops, captive bubbles, and inverted sessile and pendant drops. Add the optional Oscillator (p/n 100-28) to measure surface dilatational elasticity.

The Model 290 is a robust tool optimized for research environments where wetting, roughness, absorption, spreading, cleanliness, and surface characterization are not just casually observed. The highly automated operation and repeatability provide precise characterization of solid surfaces and liquid interfaces. This state-of-the-art software-driven tool represents four decades of contact angle tool refinement and adaptation. Dr. Zisman, the inventor of the original ramé-hart NRL Goniometer nearly fifty years ago, would be pleased.

Visit our freshly updated website at www.ramehart.com to learn more about the Model 290 as well as our other exciting surface science tools. Contact us if you would like an official quotation or require additional product information.

The new ramé-hart Model 290 Automated Goniometer is the ultimate tool for measuring dynamic contact angle. Dynamic measurements can be made using a variety of techniques. Below are (4) methods to consider: 

1. Volume is added and removed from a drop while the contact angle is dynamically measured. To perform this type of measurement, we use the Automated Dispensing System to gradually introduce volume via a dispensing tip. Optionally a stainless steel needle can be employed with the optional needle adapter. The right line option in DROPimage is turned on to filter out the needle or tip and the left and right contact angles are measured as the drop increases in volume. When the three-phase line increases in diameter, thus increasing wetting, the contact angle drops slightly until more volume is added. Multiple high speed measurements as volume is added to the drop are used to capture the maximum or advancing contact angle. Contrariwise, as volume is removed from the drop the minimum or receding contact angle is measured. At a point where the three-phase line retracts, the contact angle will increase slightly until additional volume is removed. With the standard 250µL syringe, steps as small as 0.08µL can be made. With the optional 50µL syringe, steps can be as small as 0.02µL. Alternately, the volume reduction can be accomplished by waiting for the drop to evaporate. This takes longer and only works with aqueous liquids.

2. The tilting method can also capture advancing and receding angles. A sessile drop is produced while the stage is at 0° and measured. Under normal conditions the left and right contact angles will measure close to the same. The Automated Tilting Base is used to tilt the instrument slowly (say 1° per second) while taking a series of measurements. As the tilt increases the contact angle on the downhill side (the advancing angle) increases while the contact angle on the uphill side (the receding angle) decreases as shown below. At some point the drop may roll off. The maximum downhill angle and the smallest uphill angle measured prior to roll off represent the advancing and receding contact angles. The difference between those two values is the contact angle hysteresis. The advantages to the tilting base method are: (1) there is no disturbance to the drop by the protrusion of a needle or tip; (2) the advancing and receding measurements can be measured simultaneously; and (3) the roll-off angle can be precisely measured. 

3. Dynamic contact angle can also be measured using the traditional Wilhelmy method which employs a plate that is immersed in the test liquid. As the plate is removed from the liquid a meniscus forms and the contact line moves downward relative to the solid. A balance measures the force required to pull the plate upward over a specified distance. The force value measured is adjusted for buoyancy and used to calculate the surface tension. The dynamic contact angle is determined by the surface tension values at the contact line. We employ a modified version of this technique using the Du Noüy-Padday Rod Pull Method¹ on our new Model 710 DAS Tensiometer.

4. Time-dependant studies are also dynamic. In addition to relaxation and, in the case of water, evaporation, absorption can be measured as drop volume decreases over time. With DROPimage Advanced the user can design a methods-based experiment employing a time file. The time editor allows the user to specify when each measurement is taken. Additionally, volumes can be precisely controlled as a function of time. The trigger option is used to start an experiment at the precise moment the drop is formed.

Dynamic studies provide another dimension of surface characterization and contribute to our understanding of adhesion, adsorption, evaporation, wetting, dewetting, spreading, and surface energetics.