Emulsions—dispersions of micron-sized droplets of one liquid in another, immiscible liquid—show up in a multitude of industrial and commercial products. Mayonnaise is a familiar example. An emulsion’s shelf life—how long it lasts before, for example, the droplets start to coalesce— depends, often sensitively, on the size distribution of the droplets. Quality control therefore requires that the droplet sizes be monitored during production.

Because emulsions tend to be opaque, monitoring them optically requires that they be diluted. Ultrasound offers a less invasive alternative: The attenuation of an acoustic wave as a function of its frequency depends on the droplets’ size and number density. The conventional model for quantifying emulsions’ acoustic properties and interpreting ultrasound measurements dates back decades.1 But it often fails badly, especially at high droplet concentrations, because it doesn’t correctly describe how energy is converted between ultrasound and heat.

In 2012 Francine Luppé (University of Le Havre, France)...

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