Disordered granular packings, where the constituent particles are immobilized by contacts with their neighbours, are abundant in the everyday life and industry and play a fundamental role in physics of granular matter. Moreover, such packings are widely believed to mimic some of the basic aspects of glass formation. Dense random packings of simple hard spheres were studied, both experimentally and theoretically, for several decades; however, the physical understanding of these systems is still very poor. In particular, many different experimental protocols indicate that disordered systems of hard spheres reach their maximal packing density when the spheres constitute 64% of the total volume of the system. The theoretical understanding of this so-called “random close packed” (RCP) state is still missing.

We prepare dense random packings out of thermodynamically-equilibrated fluids of hard micron-sized spheres, known as colloids. We demonstrate, by simple experiments and computer simulations, that the density of these packings is determined by the thermodynamics of the initial fluid. We employ this dependence to study the structures of random packings in a systematic manner and suggest a relationship between the RCP limit and the thermodynamics of simple fluids of hard spheres.