Synchronisation and liquid crystalline order in soft active fluids

We introduce a phenomenological theory for a new class of soft active fluids, with the ability to synchronise. Our theoretical framework describes the macroscopic behaviour of a collection of interacting anisotropic elements with cyclic internal dynamics and a periodic phase variable. This system (i) can spontaneously undergo a transition to a state with macroscopic orientational order, with the elements aligned: a liquid crystal, (ii) attain another broken symmetry state characterised by synchronisation of their phase variables or (iii) a combination of both types of order. We derive the equations describing a spatially homogeneous system and also study the hydrodynamic fluctuations of the soft modes in some of the ordered states. We find that synchronisation can promote the transition to a state with orientational order; and vice-versa. Finally, we provide an explicit microscopic realisation: a suspension of micro-swimmers driven by cyclic strokes.Comment: 5 pages, 3 figure

Dynamics and interactions of active rotors

We consider a simple model of an internally driven self-rotating object; a rotor, confined to two dimensions by a thin film of low Reynolds number fluid. We undertake a detailed study of the hydrodynamic interactions between a pair of rotors and find that their effect on the resulting dynamics is a combination of fast and slow motions. We analyse the slow dynamics using an averaging procedure to take account of the fast degrees of freedom. Analytical results are compared with numerical simulations. Hydrodynamic interactions mean that while isolated rotors do not translate, bringing together a pair of rotors leads to motion of their centres. Two rotors spinning in the same sense rotate with an approximately constant angular velocity around each other, while two rotors of opposite sense, both translate with the same constant velocity, which depends on the separation of the pair. As a result a pair of counter-rotating rotors are a promising model for controlled self-propulsion.Comment: 6 pages, 6 figure

Hydrodynamic synchronisation of non-linear oscillators at low Reynolds number

We introduce a generic model of weakly non-linear self-sustained oscillator as a simplified tool to study synchronisation in a fluid at low Reynolds number. By averaging over the fast degrees of freedom, we examine the effect of hydrodynamic interactions on the slow dynamics of two oscillators and show that they can lead to synchronisation. Furthermore, we find that synchronisation is strongly enhanced when the oscillators are non-isochronous, which on the limit cycle means the oscillations have an amplitude-dependent frequency. Non-isochronity is determined by a nonlinear coupling $\alpha$ being non-zero. We find that its ($\alpha$) sign determines if they synchronise in- or anti-phase. We then study an infinite array of oscillators in the long wavelength limit, in presence of noise. For $\alpha > 0$, hydrodynamic interactions can lead to a homogeneous synchronised state. Numerical simulations for a finite number of oscillators confirm this and, when $\alpha <0$, show the propagation of waves, reminiscent of metachronal coordination.Comment: 4 pages, 2 figure

Symmetry Breaking Phase Transitions in ABJM Theory with a Finite U(1) Chemical Potential

We consider the U(1) charged sector of ABJM theory at finite temperature, which corresponds to the Reissner-Nordstrom AdS black hole in the dual type IIA supergravity description. Including back-reaction to the bulk geometry, we show that phase transitions occur to a broken phase where SU(4) R-symmetry of the field theory is broken spontaneously by the condensation of dimension one or two operators. We show both numerically and analytically that the relevant critical exponents for the dimension one operator agree precisely with those of mean field theory in the strongly coupled regime of the large N planar limit.Comment: 22 pages, 6 figures, typos corrected, references added, improved figures, minor changes, accepted for publication in Phys. Rev.

Violation and persistence of the K-quantum number in warm rotating nuclei

The validity of the K-quantum number in rapidly rotating warm nuclei is investigated as a function of thermal excitation energy U and angular momentum I, for the rare-earth nucleus 163Er. The quantal eigenstates are described with a shell model which combines a cranked Nilsson mean-field and a residual two-body interaction, together with a term which takes into account the angular momentum carried by the K-quantum number in an approximate way. K-mixing is produced by the interplay of the Coriolis interaction and the residual interaction; it is weak in the region of the discrete rotational bands (U \lesim 1MeV), but it gradually increases until the limit of complete violation of the K-quantum number is approached around U \sim 2 - 2.5 MeV. The calculated matrix elements between bands having different K-quantum numbers decrease exponentially as a function of $\Delta K$, in qualitative agreement with recent data.Comment: 29 pages, 7 figure

Engaged but exhausted: Work-related wellbeing profiles of South African employees

Organizations and colleagues alike benefit from dedicated employees who are immersed in their work and energetically pursue their tasks. Unfortunately, this may come at a price for employees who may burn out. Organizations are, therefore, confronted with a responsibility to assist employees in striking a balance between eagerly engaging in their tasks and taking care of their wellbeing. Before designing and implementing interventions, it is valuable to identify how engagement and burnout components cluster within individuals and whether these different combinations have different implications for employees. The study aimed to explore whether burnout and work engagement combine within individuals to form different burnout-engagement profiles. The study also aimed to examine the implications of different profiles for employees’ psychological distress, affective commitment, and turnover intention. Among 1048 South African employees, latent profile analysis highlighted five distinct burnout-engagement profiles: Burned-out, Risky, Moderately balanced, Stars, and Workaholics. The Burned-out reported higher levels of psychological distress than the Risky. Still, both reported higher levels than the Moderately balanced, who also reported higher levels of psychological distress than the Stars. The Burned out and the Workaholics reported equal levels of psychological distress. The Stars reported the highest levels of affective commitment, followed by the Workaholics, the Moderately balanced, and the Risky, with the lowest levels reported by the Burned-out. The Burned-out reported the highest levels of turnover intention, followed by the Risky, the Workaholics, and the Moderately balanced, with the lowest levels reported by the Stars. Limitations, recommendations for future research and practical implications are discussed

Improving the optimization in model predictive controllers:Scheduling large groups of electric vehicles

In parking lots with large groups of electric vehicles (EVs), charging has to happen in a coordinated manner, among others, due to the high load per vehicle and the limited capacity of the electricity grid. To achieve such coordination, model predictive control can be applied, thereby repeatedly solving an optimization problem. Due to its repetitive nature and its dependency on the time granularity, optimization has to be(computationally) efficient.The work presented here focuses on that optimization sub-routine, its computational efficiency and how to speed up the optimization for large groups of EVs. In particular, we adapt FOCS, an algorithm that can solve the underlying optimization problem, to better suit the repetitive set-up of model predictive control by adding a pre-mature stop feature. Based on real-world data, we empirically show that the added feature speeds up the median computation time for 1-minute granularity by up to 44%.Furthermore, since FOCS is an algorithm that uses maximum flow methods as a subroutine, the impact of choosing various maximum flow methods on the runtime is investigated. Finally, we compare FOCS to a commercially available solver, concluding that FOCS outperforms the state-of-the-art when making a full-day schedule for large groups of EVs

Dynamic displacement disorder of cubic BaTiO3

The three-dimensional distribution of the x-ray diffuse scattering intensity of BaTiO3 has been recorded in a synchrotron experiment and simultaneously computed using molecular dynamics simulations of a shell model. Together, these have allowed the details of the disorder in paraelectric BaTiO3 to be clarified. The narrow sheets of diffuse scattering, related to the famous anisotropic longitudinal correlations of Ti ions, are shown to be caused by the overdamped anharmonic soft phonon branch. This finding demonstrates that the occurrence of narrow sheets of diffuse scattering agrees with a displacive picture of the cubic phase of this textbook ferroelectric material. The presented methodology allows one to go beyond the harmonic approximation in the analysis of phonons and phonon-related scattering