Abstract
It is only over the last three decades that a coherent picture of the liquid state has emerged, much later than our understanding of both gases and solids. The difficulties in describing liquids arise from two of their key characteristics. First, liquids are dense phases of matter. The molecules constantly interact with many other molecules, making it impossible to develop an equation that describes how a liquid's behaviour changes with pressure, temperature and volume. Such an "equation of state" can easily be derived for a gas, in which molecules are assumed to collide with one molecule at a time. Second, molecular positions and orientations are random in liquids, making them intrinsically disordered. Thus they do not have a clear-cut reference state like the perfect lattice associated with crystalline solids. The disorder is, moreover, dynamical. Individual molecules diffuse away from their initial positions and, for sufficiently long timescales, this gives rise to collective motions and hydrodynamic flow.