Photon Channelling in Foams
We report on the absorption of diffuse photons in aqueous foams by a dye added to the continuous liquid phase. For very wet and for dry foams, the absorption of the diffuse photons equals the absorption length of the liquid divided by the liquid volume fraction. This indicates that the diffuse photons propagate by a random walk, sampling each phase in proportion to its volume. Foams of intermediate wetness, by contrast, absorb photons more strongly than expected. A 2D computer simulation, modeling photons scattering in a foam crystal, also shows enhanced absorption. This encourages us to consider novel transport effects, such as the total internal reflection of photons inside the Plateau borders.