Theta Pulsating Drop system

Topography measurements are suitable for samples with microscale roughness (analysis range of approximately 1–60 µm). Additionally, samples must be diffusive, i.e., opaque. Sample height is limited to 22 mm.

The minimum and maximum droplet size depends on the type of liquid and the needle used, as well as the substrate. The table below provides approximate values for water.

All volumes refer to drops suspended from the needle (except for the picoliter dispenser). This is because the amount of liquid transferred from the needle to the substrate depends on the surface area:

• if the substrate is highly hydrophilic, more liquid is transferred
• if it is highly hydrophobic, the amount of liquid on the surface may be less than in the needle

Please note that the values given are approximate and depend on the measuring system and environmental conditions.

When comparing the results obtained using the ring and plate methods, they may differ depending on the liquid—especially in surfactant solutions. This is due to differences in the measurement principle.

In the Wilhelmy plate method, the plate is stationary during the measurement, which means that the surfactant molecules have time to arrange themselves at the phase boundary, which lowers the surface tension value.

In the Du Noüy ring method, the interface is constantly changed as the ring moves during the measurement. Therefore, surface tension values are often slightly higher than those obtained using a plate. This effect can be observed even in water with minor impurities. For surfactant solutions, the Wilhelmy plate method is preferred.

The plate should be rinsed with pure ethanol and water, then fired with a Bunsen burner (~1000°C). Too low a temperature can leave impurities that cause measurement errors. The plate should be heated red-hot in the hottest part of the flame, then removed before turning off the burner. Clean before and after use.

The ring should be rinsed with ethanol and water and then fired with a Bunsen burner (~1000°C) as with the plate. Avoid low-temperature flames, as they can leave residue. The ring should be heated until red-hot and then removed before extinguishing the burner. Clean before and after use.

The powder particle size must be larger than the pore size of the holder.
– Glass holder: 1 µm
– Steel handle (Sigma 700): 5 µm
The powder must not be soluble or react with the liquid
The powder contact angle should not exceed 90° (to allow the liquid to capillary uptake)

There is no strict viscosity range because it also depends on the density, elasticity of the liquid, probe type and measurement parameters.

• up to approx. 1000 mPa s: usually measurements possible
• above 10,000 mPa s: mostly impossible
• intermediate range: requires compatibility testing

The probe should be rinsed with ethanol and distilled water. You can't use a Bunsen burner flame because the probe is not resistant to it.

At the beginning of the liquid-liquid measurement, the heavy phase (water) is first poured into the trough.

We immerse the Wilhelmy liquid-liquid plate approximately halfway down its surface and check the cleanliness of the surface by squeezing it.

Next, carefully pour the lighter liquid onto the surface. It can be poured onto the step that expands at the liquid-liquid interface. Be careful not to pour it directly onto the heavy phase, as this can cause the phases to mix. The light phase liquid must be sufficient to cover the entire Wilhelmy plate, and the plate should not be immersed in air.

Open the barriers, zero the balance, and inject the material into the interface. Wait the appropriate time for the sample to stabilize at the interface, then begin measurement as usual.

For detailed instructions on standard measurement, please refer to the LB user manual and the Monolayer kit manual.

The trough and barriers are made of Teflon and Delrin. The standard trough is made of Teflon, and the standard barriers are made of Delrin. If you're unsure whether you have a standard system, you can test the materials by placing a drop of water on both the trough and barriers. The drop will have a high contact angle on the Teflon and a low contact angle on the Delrin.

Always wear rubber gloves when handling these components. Remove the trough and barriers and wash them over the sink. Using a soft brush, cover the entire surface with pure ethanol, then rinse with clean, deionized water.
Delrin, from which the barriers are made, I don't tolerate Chloroform, but chloroform or other cleaning agents can be used to clean the Teflon trough. If a long time has passed since the trough was last used, it is worth first washing it with a commercially available detergent.