Big drops, small drops

Emulsions are abundant in products we use daily, such as mayonnaise, margarine and cosmetics.  They are mixtures of two immiscible liquids, usually water and oil, wherein one liquid is dispersed as tiny droplets inside the other liquid. Because the droplets can largely affect the stability, viscosity and flow behaviour of an emulsion, scientists have developed a wide range of tools for assessing the various properties of droplets.

Now, Peter Edwards and his team from the KACST–Oxford Centre of Excellence for Petrochemicals at the University of Oxford, along with co-workers from both East China University of Science and Technology and Cardiff University, have developed a microwave-based technique for assessing the size distribution of droplets in emulsions. The new technique is fast, accurate and applicable to a wide range of emulsions, including heavy oil emulsions.

Microscopy, ultrasound and light scattering techniques are some of the methods currently used to assess droplet size distribution. They are ideal for probing emulsion samples of which the droplets are opaque. However, if the droplets are transparent, or if gas bubbles or suspended solids are present in the emulsion sample, these techniques typically underperform or simply do not work.

The new technique, known as microwave cavity perturbation analysis, uses 2.45 gigahertz microwaves to interrogate emulsion samples. Microwaves are a highly penetrative form of electromagnetic radiation. Unlike visible light or ultrasound, they can probe deep inside emulsion samples.

Microwaves are partially absorbed by emulsions, and the amount of absorption is dependent on the water content of the emulsion. Therefore, by studying how microwave absorption changes with model emulsions where the water content is known, and the droplet size distribution has been characterized using light scattering technique, one could generate calibration curves for correlating microwave absorption and droplet size distribution. These curves can then be used to assess other emulsion samples, regardless of whether their droplets are opaque or transparent, or whether gas bubbles or suspended solids are present.   

Using this technique researchers assessed the droplet size distribution of a wide range of emulsions, including paraffin, crude oil and tar oil emulsions. Because the technique does not require complex sample pre-treatment or the use of expensive equipment, it poses significant advantages over conventional ones.

“Our microwave-based technique is applicable to all systems and provides a much precise and instant tool for monitoring bubble and foam formation,” says Edwards.

1. Yan, Y., Gonzalez-Cortes, S., Yao, B.,Slocombe, D. R., Porch, A., et al.  (Last name, initial of first name / up to five). Rapid, non-invasive characterization of the dispersity of emulsions via microwaves. Chemical Science 9, 6975–6980 (2018).| article