A look inside particle stabilized foams—particle structure and dynamics

From a physico–chemical point of view, most food is soft matter. Usually, these systems are complex in the sense that they combine multiple ingredients with a wide range of structural length scales and dynamics on various time scales. Amongst these systems, foams belong to the well studied but less well understood systems. Particle stabilized foams are very common in food systems. As a model system, we produced aqueous foams from silica nanoparticle dispersions. The silica nanoparticles were hydrophobized by the in situ adsorption of short-chain alkyl amines of chain length C5 to C8 to render them surface active. We determined the role of the particles in stabilizing the produced foams. It is shown that the depletion of the bulk silica concentration during the foam formation can be quantified by precise density measurements. In the case of nanoparticle aggregation, more particles are trapped in the foam and will form a network in the foam channels. Diffusing wave spectroscopy was used to study the different time and length scales of the composite system. We find that it is possible to obtain the size of the particles within the foam by two different approaches. Additionally, the dynamics of the foam network is analyzed and it is confirmed that the formation of an aggregated particle network within the foam is responsible for a deceleration of the foam structure evolution.