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

Shear flow induced isotropic to nematic transition in a suspension of active filaments

We study the effects of externally applied shear flow on a model of suspensions of motors and filaments, via the equations of active hydrodynamics [PRL {\bf 89} (2002) 058101; {\bf 92} (2004) 118101]. In the absence of shear, the orientationally ordered phase of {\it both} polar and apolar active particles is always unstable at zero-wavenumber. An imposed steady shear large enough to overcome the active stresses stabilises both apolar and moving polar phases. Our work is relevant to {\it in vitro} studies of active filaments, the reorientation of endothelial cells subject to shear flow and shear-induced motility of attached cells.Comment: 8 pages, 4 figures submitted to Europhysics Letter

Substrate rigidity deforms and polarizes active gels

We present a continuum model of the coupling between cells and substrate that accounts for some of the observed substrate-stiffness dependence of cell properties. The cell is modeled as an elastic active gel, adapting recently developed continuum theories of active viscoelastic fluids. The coupling to the substrate enters as a boundary condition that relates the cell's deformation field to local stress gradients. In the presence of activity, the coupling to the substrate yields spatially inhomogeneous contractile stresses and deformations in the cell and can enhance polarization, breaking the cell's front-rear symmetry.Comment: 6 pages, 4 figures, EPL forma

Associations between child mental health, carer worry and help-seeking

Carers may not always express child mental health concerns to health professionals. Therefore, identifying factors delaying help-seeking is important. The aim of this study was to examine the relationship between carer affect and help-seeking. In a secondary analysis of data collected from school-aged children (N = 1,857, mean age = 9.85 years, 51% female), we used logistic regression to examine the associations between carer worry, help-seeking and child mental health. Regarding worry, higher levels of emotional problems (OR = 1.42, 95% CI = 1.33-1.52), conduct problems (OR = 1.24, 95% CI = 1.12, 1.36), peer problems (OR = 1.17, 95% CI = 1.05-1.27) or functional impairment (OR = 1.37, 95% CI = 1.2-.56) were associated with higher levels of carer worry. Regarding help-seeking, higher levels of functional impairment were associated with higher levels of help-seeking (OR = 1.51, 95% CI = 1.09-2.11). After controlling for mental health problems, carers who reported being worried about their child's mental health were less likely than other carers to seek help (OR = 0.13, 95% CI = 0.05-0.35). Knowledge of these factors may inform early interventions. Alongside implications for future research and practice, limitations of the study are discussed

A Caregiver Digital Intervention to Support Shared Decision Making in Child and Adolescent Mental Health Services: Development Process and Stakeholder Involvement Analysis

BACKGROUND: Parents and caregivers are generally recognized by literature and the law as key to child and adolescent mental health decisions. Digital interventions are increasingly being used to support care and treatment in child and adolescent mental health services (CAMHS). However, evidence of the design and development process is generally not made available. OBJECTIVE: In light of calls for more transparency, this paper aims to describe the development of an evidence-based, theoretically informed digital decision support intervention for parents and caregivers of young people accessing CAMHS. METHODS: The intervention was developed in line with the UK Medical Research Council framework for developing complex interventions. The process incorporated the steps for developing patient decision aids, as follows: assessing need, assessing feasibility; defining objectives; identifying the framework of decision support; and selecting the methods, designs, and dissemination approach. We synthesized theory, research, international guidelines, and input from relevant stakeholders using an iterative design approach. RESULTS: The development steps resulted in Power Up for Parents, a decision support intervention, with five key features (ie, decisions, goals, journey, support, and resources). The intervention aims to encourage discussion, allow parents to ask questions during sessions or seek further information between sessions, and allow service providers to tailor the shared decision-making process to accommodate the needs of the parent and child. CONCLUSIONS: We confirmed that it is possible to use input from end users-integrated with theory and evidence-to create digital interventions to be used in CAMHS. Key lessons with implications for practice, policy, and implementation science, along with preliminary findings, are presented. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): RR2-10.2196/14571

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

Anomalous fluctuations of active polar filaments

Using a simple model, we study the fluctuating dynamics of inextensible, semiflexible polar filaments interacting with active and directed force generating centres such as molecular motors. Taking into account the fact that the activity occurs on time-scales comparable to the filament relaxation time, we obtain some unexpected differences between both the steady-state and dynamical behaviour of active as compared to passive filaments. For the statics, the filaments have a {novel} length-scale dependent rigidity. Dynamically, we find strongly enhanced anomalous diffusion.Comment: 5 pages, 3 figure

Spontaneous flow states in active nematics: a unified picture

Continuum hydrodynamic models of active liquid crystals have been used to describe dynamic self-organising systems such as bacterial swarms and cytoskeletal gels. A key prediction of such models is the existence of self-stabilising kink states that spontaneously generate fluid flow in quasi-one dimensional channels. Using simple stability arguments and numerical calculations we extend previous studies to give a complete characterisation of the phase space for both contractile and extensile particles (ie pullers and pushers) moving in a narrow channel as a function of their flow alignment properties and initial orientation. This gives a framework for unifying many of the results in the literature. We describe the response of the kink states to an imposed shear, and investigate how allowing the system to be polar modifies its dynamical behaviour.Comment: 6 pages, 6 figures; submitted to Europhysics Letter

The Dynamics of a Meandering River

We present a statistical model of a meandering river on an alluvial plane which is motivated by the physical non-linear dynamics of the river channel migration and by describing heterogeneity of the terrain by noise. We study the dynamics analytically and numerically. The motion of the river channel is unstable and we show that by inclusion of the formation of ox-bow lakes, the system may be stabilised. We then calculate the steady state and show that it is in agreement with simulations and measurements of field data.Comment: Revtex, 12 pages, 2 postscript figure