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November 2008

Surface Dilatational Elasticity and Viscosity
The axisymmetric bubble shape method has proven useful in the measurement of surface dilatational elasticities and viscosities.1 In the referenced study dynamic surface tension as well surface dilatational viscoelastic properties are measured by way of the oscillating bubble. A ramé-hart goniometer with our software-controlled Automated Dispensing System, Environmental Chamber, Temperature Controller, and DROPimage Advanced software in conjunction with a specially-designed oscillating unit were employed to take the measurements. The schematic below illustrates the layout of equipment. Read the next section to learn more about our new Oscillator which is used in the study of surface dilatational rheology.

A surfactant (surface acting agent) is generally an agent used to promote wetting by lowering surface tension through adsorption. Surfactants can also reduce interfacial tension between two liquids -- such as oil and water. Polymer surfactants, as observed by Hansen, et al, produce higher surface elasticities than simple surfactants. In their study, cationic polysoaps were synthesized and evaluated. It was found that both the monomers and polymers studied exhibited strong surface activity. Fluorinated compounds showed lower surface tensions and much higher surface dilatational elasticity than their hydrocarbon counterparts. Adsorption is slower, too.

It's not possible to fully explain adsorption and surface tension of surfactants, emulsions, and foaming agents using static methods. When a surface is expanded and contracted, the surface tension has been traditionally explained using Gibb's formula:

where γ is is the surface tension and A is surface area. This term does not allow for time-dependant responses. Thus a more replete equation is now used to define complex surface elasticity:

where E' is the storage modulus and E'' is the loss modulus. The measurement of elasticity for many surfactants can be best accomplished through the use of periodic oscillatory deformation. The oscillating bubble was first introduced by Lunkenheimer2 who used Laplace pressure to measure the surface tension. The updated method uses a sessile or pendant drop or bubble and DROPimage software to measure the surface tension, drop volume, and surface area using axisymmetric shape analysis and by creating oscillatory drop deformations the dynamic properties can be measured. The method works best with an oscillation frequency under 2-3 Hz. At higher frequencies, super-harmonic effects can distort measurements.

The oscillation method has been shown to be an effective tool for the measurement of surface dilatational elasticities and to better understand dynamic surface and interfacial tension.

1 Fromyr, T.; Hansen, F. K.; Kotzev, A.; Laschewsky, A.; Adsorption and Surface Elastic Properties of Corresponding Fluorinated and Nonfluorinated Cationic Polymer Films Measured by Drop Shape Analysis, Langmuir 2001, 17, 5256-5264
2 K. Lunkenheimer, K. G., Thesis, Z. Phys. Chem. 256 (1975) 593.

New Product Announcement - Oscillator

This month we are pleased to announce our new Oscillator, p/n 100-28, which complements our Automated Dispensing System and is optimized for studying surface dilatational rheology as detailed in the above section.

ramé-hart  Oscillator Model 100-28

This tool is fully software-driven and supported by DROPimage Advanced v2.4 or above. It requires our software-controlled Automated Dispensing System (p/n 100-22-100) and current-generation Model 250-F1 or 500-F1 Goniometer or a legacy model which has been sufficiently upgraded. The Oscillator sits inline between the dispenser and the dispensing tip as detailed in the schematic in the section above. A serial interface provides communication between the PC and the Oscillator. DROPimage provides a dialog box (see below) allowing the user to manually control the device.

But perhaps the most powerful way to use the Oscillator is to integrate oscillation control into the experiment design using the methods editor. With this approach, the oscillation can be fully integrated into an experiment. Additional parameters can be set to control the drop volume, timing, and events. The dialog appears at the start of running a previously defined method.

After the experiment is run, the results window (shown below) reports an array of data including:

A0 The mean surface area (mm2)
A(amp) The area amplitude, ΔA (mm2)
Omega The angular velocity, ω (radians/s)
Fi1 The shift in start of the area curve from measurement start (radians)
Gamma The mean surface tension, γ (mN/m)
G(amp) The surface tension amplitude Δγ (mN/m)
Fi2 The shift in start of the gamma curve from measurement start (radians)
E’ The storage elasticity module
E’’ The loss elasticity module
tg(d) Tangent to the phase angle between A and Gamma, tan(δ)
A/G The ratio A(amp)/Gamma(amp)

Note that E' and E'' can also be plotted as a function of frequency. DROPimage has been upgraded to fully support the Oscillator. On the rear of the Oscillator, there is a control knob which allows the user to manually move through a complete cycle in order to start at the precise step in the cycle desired. On the front of the unit, there is a peep hole which allows access to the stroke adjustment. The stroke volume can be set to between 0 and 250µl, but most experiments will use stroke volumes under 10µl. The periodic oscillation frequency can be set anywhere between 0 and 25Hz (but is typically under 5Hz) and is controlled in DROPimage via the method editor or the Oscillator Control dialog box.  

For more information on the Oscillator including a copy of our PDF brochure for this product, or for a quotation, please contact us. On request, we can also send more detailed and technical information on surface dilatational rheology theory and how our DROPimage Advanced software employs it.

13th IACIS International Conference on Surface and Colloid Science and 83rd ACS Colloid and Surface Science Symposium - Call for Papers

Columbia University in New York City will host a rare convergence of two international groups of colloid and surface scientists on June 14-19, 2009 at its Morningside campus. The triennial 13th International Conference on Surface and Colloid Science originating from the International Association of Colloid and Interface Scientists will occur jointly with the annual 83rd Colloid and Surface Science Symposium originating from the American Chemical Society. This gathering will bring together top researchers and technologists spanning the wide range of topics that come under the umbrella of colloid and surface science; such as behaviors in particulate systems and at interfaces, and the application of this knowledge to a host of industrial technologies including: biomedical, material science, nano-engineering, water treatment, oil recovery, mineral processing, papermaking, and energy and environmental stewardship. For more information on submitting an abstract or registering to attend, please go to www.icscs.org. If you register or inquiry, please let them know that we sent you.



Carl Clegg
Director of Sales
Phone 973-448-0305
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