Hard discs: Anomalous statics, anomalous dynamics and a new kind of gearbox

Confining a system in a small volume profoundly alters its behaviour. Hitherto, attention has focused on static confinement where the confining wall is fixed such as in porous media. However, adaptive confinement where the wall responds to the interior has clear relevance in biological systems. Here we investigate this phenomenon with a colloidal system of quasi hard discs confined by a ring of particles trapped in holographic optical tweezers, which form a flexible elastic wall. This elasticity leads to quasi-isobaric conditions within the confined region.

By measuring the displacement of the tweezed particles, we directly obtain the radial osmotic pressure. We find a novel bistable state of a hexagonal structure and concentrically layered fluid mimicking the shape of the confinement. The hexagonal configurations are found at lower pressure than those of the fluid, thus the bistability is driven by the higher entropy of disordered arrangements, unlike bulk hard systems. Unlike bulk systems where the relaxation dynamics are much reduced upon ordering, here we find that hexagonal ordering enables new mechanisms in relaxation, significantly accelerating the dynamics. We further explore the consequences of rotating the corral, which leads to shear on the nanoscale and an unexpected oscillatory steady state.

[1] Williams I, Oguz EC, Bartlett P, Loewen H and Royall CP “Direct measurement of osmotic pressure via adaptive confinement of quasi hard disc colloids”, Nature Communications 4 2555 (2013).

[2] Williams I, Oguz EC, Bartlett P, Loewen H and Royall CP “The effect of boundary adaptivity on hexagonal ordering and bistability in circularly confined quasi hard discs”, J. Chem. Phys. 140 104907 (2014).

[3] Williams I, Oguz EC, Bartlett P, Loewen H and Royall CP “Flexible confinement leads to multiple relaxation regimes in glassy colloidal liquids”, J. Chem. Phys. 142 024505 (2015).

[4] Williams I, Oguz EC, Speck T, Bartlett P, Loewen H and Royall CP, “Transmission of torque at the nanoscale”, Nature Physics doi:10.1038/nphys3490 (2015).