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Special Issue on Liquid Matter 2017

This special issue of the Journal of Physics: Condensed Matter contains a number of selected articles submitted by speakers at the 10th Liquid Matter Conference, which was held in Ljubljana in July 2017. The program of the 10th Liquid Matter Conference shows some striking differences with that of the first Liquid Matter Conference, held in Lyon in 1990. The change is partly due to the emergence of new fields (e.g. active matter) and partly due to a redistribution of topics over other international meetings. For instance, in 1990 there were two sessions devoted to macroscopic hydrodynamic phenomena (one session on ‘Instabilities and Turbulence’, and the other on ‘Selected topics in fluid mechanics’). In the Ljubljana meeting there was no session on macroscopic hydrodynamics, although there were contributions on rheology and nano-fluidics, where the focus was on mescoscale phenomena in which the nature of the constituent molecules or the confining surfaces play a key role. However, the most striking difference in the programme is that the session on ‘Simple Liquids’, which was one of the largest in 1990, had disappeared completely in 2017. The underlying reason is that our view of what constitutes a ‘simple liquid’ has changed. In 1990, argon was considered a ‘simple liquid’ by virtue of the fact that it has no (low energy) internal degrees of freedom and, importantly, because it (or more precisely, the LennardJones model representing argon) had been used as the guinea pig to test theories of liquid structure and dynamics. In 1990, there was even a contribution on water in the session on simple liquids, although water, even though consisting of small molecules, can hardly be considered simple: something that has been amply demonstrated during the past decades by a huge number of experiments and simulations. Hence, in 2017, water is still very much on the programme, but not as a simple liquid. The more interesting question is: what happened to argon? Here the story is a bit different: argon is maybe not be quite as simple as its LennardJones representation, but still it is studied easily in theory and simulation. The difference is in experiments: during the past decades, colloids have emerged as the new ‘simple liquids’ (even though they are usually called ‘complex liquids’). The reason why colloids are now considered ‘simple’ and why the colloids session was by far the largest at the Ljubljana meeting, is that experiments can now visualise the statics and dynamics of colloidal suspension, using confocal or regular video microscopy: we can see the colloids move, we can observe them undergo structural arrest and we see how they nucleate crystals or liquid crystals in the bulk or at interfaces. The work of Thorneywork et al in this issue is a shining example. The nature of the images thus obtained experimentally is such that the exper imentalists presenting them often must stress several times that what they show are not snapshots from simulations. In contrast, we have no experimental tools to visualise the structure or dynamics of argon. There, the workhorse is still (neutron or x-ray) scattering. The second reason why colloids are popular as a model system is that they can indeed be made ‘simple’. Suitably prepared suspensions of colloidal spheres really do behave (at least in their static properties) as the hard spheres studied in simulations. Hence the comparison with theory and simulation should be (and, occasionally even is) straightforward. Finally, colloids can be made in many different shapes and with different functionalizations. Hence, even beyond simple liquids, colloids often offer the easiest route to explore the experimental consequences of a new functionality for liquid-state properties. Colloids are the building blocks of designer liquids. Having said that, liquid matter science is emphatically not just colloid science. The meeting in Ljubljana was full of exciting contributions on ionic liquids, liquid crystals, polymers and supercooled and glassy materials to name but a few. In addition, interfacial phenomena continue to deliver surprises and, as I mentioned in the beginning, the field of D Frenkel