arXiv subjects

Jens Winkelmann

Publications and source records attributed to Jens Winkelmann.

6 recordsLinked to original sources

arXiv2019

Equilibrium configurations of hard spheres in a cylindrical harmonic potential

A line of hard spheres confined by a transverse harmonic potential, with hard walls at its ends, exhibits a variety of buckled structures as it is compressed longitudinally. Here we show that these may be conveniently observed in a rotating liquid-filled tube (originally introduced by Lee et al. [T. Lee, K. Gizynski, and B. Grzybowski, Adv. Mater. 29, 1704274 (2017)] to assemble ordered three dimensional structures at higher compressions). The corresponding theoretical model is transparent and easily investigated numerically, as well as by analytic approximations. Hence we explore a wide range of predicted structures occurring via bifurcation, of which the stable ones are also observed in our experiments. Qualitatively similar structures have previously been found in trapped ion systems.

cond-mat.soft↗

arXiv2019

Implementation of Morse-Witten theory for a polydisperse wet 2D foam simulation

The Morse-Witten theory (D. Morse and T. Witten, EPL 22 (1993) 549-555) provides a formulation for the inter-bubble forces and corresponding deformations in a liquid foam, accurate in the limit of high liquid fraction. Here we show how the theory may be applied in practice, including allowing for polydispersity in the bubble sizes. The resulting equilibrated 2D structures are consistent with direct calculations, within the limitations of the theory. The path to developing a 3D model is outlined for future work.

cond-mat.soft↗

arXiv2019

Theory of rotational columnar structures of soft spheres

There is a growing interest in cylindrical structures of hard and soft particles. A promising new method to assemble such structures has recently been introduced by Lee et al. [T. Lee, K. Gizynski, and B. Grzybowski, Adv. Mater. 29, 1704274 (2017)]. They used rapid rotation around a central axis to drive spheres of lower density than the surrounding fluid towards this axis. This resulted in different structures as the number of spheres is varied. Here we present comprehensive analytic energy calculations for such self-assembled structures, based on a generic soft sphere model, from which we obtain a phase diagram. It displays interesting features, including peritectoid points. These analytic calculations are complemented by preliminary numerical simulations for finite sample sizes with soft spheres. A similar analytic approach could be used to study packings of spheres inside cylinders of fixed dimensions, but with a variation in the number of spheres.

cond-mat.soft↗

arXiv2018

Columnar structures of soft spheres: Metastability and hysteresis

Previously we reported on the stable (i.e. minimal enthalpy) structures of soft monodisperse spheres in a long cylindrical channel. Here, we present further simulations, which significantly extend the original phase diagram up to D/d = 2.714 (ratio of cylinder and sphere diameters), where the nature of densest sphere packing changes. However, macroscopic systems of this kind are not confined to the ideal equilibrium states of this diagram. Consequently, we explore some of the structural transitions to be expected as experimental conditions are varied; these are in general hysteretic. We represent these transitions in a stability diagram for a representative case. Illustrative videos are included in the supplemental material.

cond-mat.soft↗

arXiv2018

Simulation and observation of line-slip structures in columnar structures of soft spheres

We present the computed phase diagram of columnar structures of soft spheres under pressure, of which the main feature is the appearance and disappearance of line slips, the shearing of adjacent spirals, as pressure is increased. A comparable experimental observation is made on a column of bubbles under forced drainage, clearly exhibiting the expected line slip.

cond-mat.soft↗

arXiv2017

2D foams above the jamming transition: Deformation matters

Jammed soft matter systems are often modelled as dense packings of overlapping soft spheres, thus ignoring particle deformation. For 2D (and 3D) soft disks packings, close to the critical packing fraction $φ_c$, this results in an increase of the average contact number $Z$ with a square root in $φ-φ_c$. Using the program PLAT, we find that in the case of idealised two-dimensional foams, close to the wet limit, $Z$ increases linearly with $φ-φ_c$, where $φ$ is the gas fraction. This result is consistent with the different distributions of separations for soft disks and foams at the critical packing fraction. Thus, 2D foams close to the wet limit are not well described as random packings of soft disks, since bubbles in a foam are deformable and adjust their shape. This is not captured by overlapping circular disks.

physics.comp-ph↗