The heat is on for concentrated emulsions

Emulsions—tiny droplets of one liquid dispersed in another liquid—show up in a multitude of industrial contexts, and it would be useful to have a quick, nondestructive way to characterize them. Because they’re often opaque to light, ultrasonic monitoring is a promising possibility: The attenuation of an acoustic wave as a function of its frequency depends on the droplets’ size and number density. But existing models for quantifying emulsions’ acoustic properties often fail badly, especially at high concentration. The problem? When a sound wave scatters off a droplet, some of its energy is converted to heat; conventionally, that energy has been considered to be lost. But as the heat diffuses away from the droplet, the distribution of thermal energy retains some of the wave’s oscillating structure for a short distance. If that so-called thermal wave encounters another droplet before it dies away entirely, some of its energy can be converted back into acoustic energy. In 2012 Francine Luppé (University of Le Havre, France) and her collaborators laid the theoretical foundations for such multimode scattering through a sea of randomly distributed scatterers; now, Valerie Pinfield (Loughborough University, UK) has built on that work to create a model explicitly relating the acoustic attenuation through an emulsion to the size and concentration of the droplets. Pinfield next plans to turn her attention to suspensions—solid particles dispersed in a liquid—in which shear waves are another important scattering mode. (V. J. Pinfield, J. Acoust. Soc. Am. 136, 3008, 2014 .)