Multiple light scattering as a probe of foams and emulsions

• Foams and dense emulsions are opaque, due to multiple light scattering.
• Diffuse light can be used to probe internal structure and dynamics.
• Stationary unsteadiness can be characterized by higher-order correlation functions.
• Dynamics can be time-resolved and space-resolved by multispeckle techniques.
• We review the state of the art in probes using multiple light scattering.

Light propagating in foams or emulsions is strongly scattered by the gas–liquid or liquid–liquid interfaces. This feature makes it generally impossible to directly observe the structure and dynamics deep within the bulk of such materials. However, multiple light scattering can be used as the basis of non-invasive experimental techniques that probe the average bubble size, droplet size or the dispersed volume fraction. If the sample is illuminated with a laser, the transmitted or backscattered light forms a speckled interference pattern whose temporal fluctuations reveal the dynamics of internal structural changes. Such changes can be due to coarsening, flocculation, or applied strain. We briefly recall the fundamental principles of multiple light scattering and present an overview of the experimental techniques that have been developed in recent years.

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