Droplet size in emulsions

Fine or coarse, narrow or broad

An emulsion is a mixture of two liquids that do not mix, such as water in oil or oil in water, kept together by emulsifiers and processing. How small and how uniform the droplets are decides whether the product stays stable, how it feels in the mouth, how it spreads, and how long it keeps. In water-in-oil spreads, fine droplets also contribute to microbiological stability.

Movement held back by the droplet wall

Molecules inside a droplet can move freely, but only as far as the wall of the droplet. A pulse sequence with a magnetic field gradient measures how far they move in a set time. The smaller the droplet, the more the movement is held back, and the more slowly the signal fades as the gradient is increased. From the shape of this decay, the droplet size distribution is calculated.

The result is not a single number but a size distribution: how much of the dispersed liquid sits in droplets of each size. A narrow curve at small sizes means a fine, uniform emulsion. A wider curve means a broader range of droplet sizes. A second peak at large sizes is an early warning that droplets are merging, often well before the product visibly separates.

The measurement needs a gradient option on the instrument and matching software. The product is measured as it is.

Points that matter in practice

• The method works within a range of droplet sizes. Droplets that are very large or very small fall outside it
• The measured liquid must give a signal that can be told apart from the surrounding phase
• Temperature matters, so measure at a defined and constant temperature
• Fill the tube gently, because strong shear can change the structure of a delicate emulsion
• Compare the results with microscopy or laser diffraction for your type of product before relying on them. Those techniques see droplets directly, but they usually need dilution or thin preparation, which can alter the structure