Two mechanisms of interaction of particles in a fluid are proposed on the basis of forces mediated by hydrodynamic thermal fluctuations. The first one is similar to the conventional van der Waals interaction, but instead of being mediated by electromagnetic fluctuations, it is mediated by fluctuations of hydrodynamic sound waves. The second one is due to a thermal drift of particles to a region with a bigger effective mass, which is formed by the surrounding fluid involved and depends on the inter-particle distance. Both mechanisms are likely to be relevant in the interpretation of the observed long-range attraction of colloidal particles, since a set of different experiments show an attraction energy of the order of kBT and, perhaps, only a fluctuation mechanism of attraction can provide this universality.
