At the Centre for Industrial Rheology, we provide dynamic surface tension measurements for your formulations at extremely short surface ages using bubble pressure tensiometry. Traditional surface tension measurements, such as those obtained through Drop Shape Analysis, can also capture dynamic surface tension; however, they can only do this from surface ages of about one second onwards.
In many practical applications, such as spraying, atomisation, coating, printing, cleaning, and foaming, new surfaces are continuously created, requiring surfactants to act within timescales of milliseconds.
In addition to this, our new capabilities allow us to perform temperature-controlled surface tension measurements between -10 and 130°C.
How Does Bubble Pressure Tensiometry Work?
In a bubble pressure tensiometer, a gas bubble is produced within a liquid at the tip of a small capillary. As this bubble grows, its curvature changes, and the internal pressure rises to a maximum Laplace pressure, directly linked to the surface tension at that instant.
What makes this method unique is that it provides the surface tension at a defined surface age, which is the time between bubble formation starting and the maximum Laplace pressure. By adjusting the rate at which bubbles are produced, we can measure how surface tension evolves over very short timescales.
This produces a complete profile of how surface tension evolves with surface age.
Applications of Dynamic Surface Tension at Short Surface Ages
Basic surface tension measurements tell only part of the story. In many applications, the critical factor is dynamic surface tension behaviour at short surface ages. This is especially important in processes such as:
- Spraying and atomisation
- Coating and printing
- Cleaning
- Foaming
Application: Spraying and atomisation
- At very short surface ages, surface tension should remain sufficiently high to stabilise the meniscus at the nozzle and prevent premature droplet breakup.
- Once the droplet has formed, surface tension should decrease rapidly to promote droplet breakup and enable effective wetting of the substrate.
Figure 5 highlights the differences in dynamic surface tension between two nasal sprays that contain the same amount of active ingredient. The large reduction in surface tension seen in the Boots Decongestant Nasal Spray sample may promote droplet breakup and more effective wetting compared to Sinex Micromist. This could potentially improve drug delivery and contribute to an enhanced perception of efficacy from the consumer’s perspective.
Application: Foaming
One specific area that could be interesting to explore is the correlation of short surface age data to foaming behaviour. As highlighted by Prof. Steven Abbott, “don’t formulate foams without measuring dynamic surface tension” [1].
The underlying issue is one of timescales; foaming is a rapid process where the ability of a surfactant to quickly reach a new interface is extremely important. A surfactant’s potential for creating a stable foam is irrelevant if the structure collapses before it reaches a new interface. Therefore, investigating dynamic surface tension could prove invaluable when paired with data obtained from Dynamic Foam Analysis.
Application: Surfactant Mobility
Insights into surfactant mobility are also provided by calculating adsorption and diffusion coefficients from dynamic surface tension measurements.
- Adsorption coefficient – a measure of the speed at which surfactant molecules are adsorbed at the surface once they arrive
- Diffusion coefficient – represents the movement of surfactant molecules from the bulk to the interface
Comparing these coefficients across different surfactants can provide formulators with a quantitative tool for selecting the right surfactant for the desired application.
Summary
Bubble pressure tensiometry bridges the gap between traditional surface tension measurements and the dynamic behaviour at short timescales of many processes. By quantifying this behaviour, we can provide formulators with information to develop more effective formulations and help to optimise processes.
If you want to learn more about how we can help characterise your formulations, do not hesitate to contact us.
References
[1] – Abbott ST. Surfactant science: principles and practice. Update. 2016 Jun;1:2-6.