Location: MB1012 (Minerva Building).

*by A. J. Parnell, University of Sheffield.
*

**Abstract:**

The diversity and vividness of structural colour in the natural world has been known going back as far as William Hook in the 17th century; what has only relatively recently been recognised is the elegance and finesse of the physics used to create these effects.^{1,2} In this talk I will highlight some of the optical structures and effects responsible for colour in Butterfly scales, Bird feathers, and Beetle elytra (fig. 1) that have been studied to date.

We will also examine our current understanding of how these are created biologically and what control mechanisms nature has to produce such structures. In particular I will discuss the optical structure responsible for the colour of the Eurasian Jay feathers (*Garrulus glandarius*) where the nanostructure is produced by a phase-separation process that is arrested at a late stage; mastery of the colour is achieved by control over the duration of this phase-separation process.^{3}

**Figure 1.** A single beetle scale from the *Lepidiota stigma* beetle along with the internal scale structure responsible for this ultra-white optical effect.

**Figure 2.** The colour derived from the x-ray determined structure (top panel) and the image of the area mapped (bottom panel) using x-ray scattering, showing the colour correlation.

- Parker, A. R. 515 million years of structural colour.
*Journal of Optics A: Pure and Applied Optics*(2000). - Vukusic, P., Hallam, B. & Noyes, J. Brilliant Whiteness in Ultrathin Beetle Scales.
*Science***315,**348–348 (2007). - Parnell, A. J.
*et al.*Spatially modulated structural colour in bird feathers.*Sci. Rep.***5,**18317 (2015).

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Location: JBL0C05 (Joseph Bank Laboratories).

*by Prof Andrew Masters FRSC,
School of Chemical Engineering & Analytical Science, University of Manchester.
*

The virial expansion seeks to represent a system property, such as the pressure, as a series expansion in the density. This approach was introduced by Kamerlingh Onnes in 1901 and later put on a firm, theoretical foundation by Mayer and Mayer in 1940. For a classical fluid, where the particles interact via pairwise additive forces, the nth virial coefficient can be written as a sum of integrals involving the pair potential and the temperature. These expressions become extremely complicated for large values of n, but recent technical advances have allowed the numerical evaluation of these coefficients to high order. For example 16 coefficients have recently been calculated for the Lennard-Jones fluid.

The question arises, however, as to whether these virial series converge. For hard bodies, the convergence properties would appear to be excellent. The hard sphere fluid, for example, is well-described up to the freezing transition. Hard spheres, however, do not exhibit a gas-liquid transition. Attractive forces are needed for this. If one considers instead a system like the Lennard-Jones fluid, where there are both repulsions and attractions, convergence issues become more acute. Numerically it seems that above the critical temperature, where there is no gas-liquid transition upon compression, the virial series converges well, possibly up to freezing. Below the critical temperature, however, the series appears to diverge at a density in the region of the vapour density at vapour-liquid co-existence. It has been claimed that this is a fundamental property of the virial series and that it can never describe the gas-liquid transition.

In this talk, I would like to review some of the ideas above and then go on to show that for repulsive fluids, the virial series can predict liquid structure as well as liquid thermodynamic properties. I would then like to present an example for which the virial expansion does indeed predict a vapour-liquid transition, in contradiction to the claim above. Finally I would like to give some thoughts on whether it is possible to carry out a resummation of the virial expansion for the Lennard-Jones fluid so one can describe both the vapour and liquid states with a single function.

]]>Location: DCB1111 (David Chiddick Building).

*by Anton Souslov,
Lorentz Institute for Theoretical Physics, Universiteit Leiden, The Netherlands.
*

Active liquids are composed of self-driven microbots that endow the liquid with a unique set of mechanical characteristics. We design metamaterials using polar active liquids, i.e., liquids that flow spontaneously and without the need of external forcing. Specifically, we create chiral steady-state flow using periodically shaped microchannels. This induced flow gives rise to topologically protected density waves, which are robust against both disorder and backscattering. Furthermore, active liquids composed of self-spinning rotors are chiral by design, a feature reflected in their constitutive relations. In two dimensions, the viscosity of such liquids includes an extra component called odd (or Hall) viscosity. Odd viscosity provides no energy dissipation, but couples pressure to vorticity. We explore how this coupling may be exploited to design self-assembled hydraulic cranks that convert between linear and rotational motion in microscopic machines powered by active rotors.”

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Location: MB1010 (Minerva Building).

*by C. P. Royall,
School of Physics and School of Chemistry, University of Bristol.
*

That fivefold symmetry should play a crucial role in the non-equilibrium behaviour of condensed matter was proposed in the 1950s [1]. Six decades later, the basic mechanism of the solidification of liquids remains unexplained, either in the case that the material crystallises, or that it forms an amorphous solid, a glass [2]. We will explore the implications of fivefold symmetry in the solidification of liquids and discuss two recent developments.

Crystallisation is among the most common everyday physical phenomena. Yet in the only material in which quantitative comparison has been made between experiment and theory — hard spheres — predictions of crystal nucleation rates are up to 20 orders of magnitude slower than measurements, the “second worst prediction in physics” [3]. This discrepancy casts doubt upon the theoretical methods concerned — importance sampling — which is important not only for crystallisation, because these methods are used to tackle a very wide range of problems, such as drug uptake in cells and chemical reaction pathways. We present results that show that fivefold symmetric arrangements of particles may hold the key to resolving this long-standing puzzle [4].

The nature of amorphous solids — glasses — is not understood: the possibility of a phase transition to a thermodynamically stable “ideal glass” is a contentious and challenging issue. Unlike everyday non-equilibrium glasses, such an ideal glass has a vanishing entropy — like a crystal — yet remains amorphous. Building on the ideas of Frank, the geometric frustration approach to the glass transition posits an avoided phase transition in a curved space inaccessible to experiment [5]. Here we show that such a “crystallisation” to a state comprised of fivefold symmetric icosahedra indeed occurs and consider the implications of this avoided transition in the Euclidean space relevant to experiments [6].

[1] Frank FC *Proc. R. Soc. A., ***215 **43 (1952)

[2] Royall CP & Williams SR *Phys. Rep., ***560** 1 (2015)

[3] Russo J *et al*, *Soft Matter, ***9** 7369 (2013)

[4] Taffs J & Royall CP, *Nature Communications*, **7** 13225 (2016)

[5] Tarjus G *et al*, J. Phys.:Condens. Matter **17** R1143 (2005)

[6] Turci F, Tarjus G & Royall CP, ArXiV:1609.03044 (2016)

Location: MB1010 (Minerva Building).

*by Adrian Baule, School of Mathematical Sciences, Queen Mary, University of London.
*

**Abstract:
**The question of how particle shape affects the dynamical and structural properties of particle aggregates is one of the outstanding problems in statistical mechanics with profound technological implications. Recently, it has become clear that also in the a-thermal regime the variation of particle shape allows the design of jammed granular materials with specific optimized properties [1–3]. However, a systematic exploration of the effect of shape variation in jammed systems typically relies on empirical studies based on extensive computer simulations. The underlying reason is that jammed systems are governed by geometry rather than energy and are thus not described a priori by the conventional statistical mechanics framework. In this talk, I present recent theoretical progress on our understanding of jamming in systems of non-spherical particles based on (i) a mean field approach in the spirit of Edwards statistical mechanics for granular systems [4], and (ii) an analytically solvable model of jamming, where the effect of shape variation can be systematically explored [5].

[1] A. Baule and H. A. Makse, Soft Matter 10, 4423 (2014)

[2] H. Jaeger, Soft Matter 11, 12 (2015)

[3] A. Baule, F. Morone, H. Herrmann, and H. A. Makse, arXiv:1602.04369 (2016)

[4] A. Baule, R. Mari, L. Bo, and H. A. Makse, Nature Communications 4, 2194 (2013)

[5] A. Baule, arXiv:1611.03034 (2016)