548 research outputs found
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 being non-zero.
We find that its () 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 , hydrodynamic
interactions can lead to a homogeneous synchronised state. Numerical
simulations for a finite number of oscillators confirm this and, when , show the propagation of waves, reminiscent of metachronal coordination.Comment: 4 pages, 2 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
Excitable Patterns in Active Nematics
We analyze a model of mutually-propelled filaments suspended in a
two-dimensional solvent. The system undergoes a mean-field isotropic-nematic
transition for large enough filament concentrations and the nematic order
parameter is allowed to vary in space and time. We show that the interplay
between non-uniform nematic order, activity and flow results in spatially
modulated relaxation oscillations, similar to those seen in excitable media. In
this regime the dynamics consists of nearly stationary periods separated by
"bursts" of activity in which the system is elastically distorted and solvent
is pumped throughout. At even higher activity the dynamics becomes chaotic.Comment: 4 pages, 4 figure
Rheology of Active Filament Solutions
We study the viscoelasticity of an active solution of polar biofilaments and
motor proteins. Using a molecular model, we derive the constitutive equations
for the stress tensor in the isotropic phase and in phases with liquid
crystalline order. The stress relaxation in the various phases is discussed.
Contractile activity is responsible for a spectacular difference in the
viscoelastic properties on opposite sides of the order-disorder transition.Comment: 4 pages, 1 figur
Nematic and Polar order in Active Filament Solutions
Using a microscopic model of interacting polar biofilaments and motor
proteins, we characterize the phase diagram of both homogeneous and
inhomogeneous states in terms of experimental parameters. The polarity of motor
clusters is key in determining the organization of the filaments in homogeneous
isotropic, polarized and nematic states, while motor-induced bundling yields
spatially inhomogeneous structures.Comment: 4 pages. 3 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
Bridging the microscopic and the hydrodynamic in active filament solutions
Hydrodynamic equations for an isotropic solution of active polar filaments
are derived from a microscopic mean-field model of the forces exchanged between
motors and filaments. We find that a spatial dependence of the motor stepping
rate along the filament is essential to drive bundle formation. A number of
differences arise as compared to hydrodynamics derived (earlier) from a
mesoscopic model where relative filament velocities were obtained on the basis
of symmetry considerations. Due to the anisotropy of filament diffusion, motors
are capable of generating net filament motion relative to the solvent. The
effect of this new term on the stability of the homogeneous state is
investigated.Comment: 7 pages, 2 figures, submitted to Europhys. Let
Micellar Aggregates of Gemini Surfactants: Monte Carlo Simulation of a Microscopic Model
We propose a "microscopic" model of gemini surfactants in aqueous solution.
Carrying out extensive Monte Carlo simulations, we study the variation of the
critical micellar concentration (CMC) of these model gemini surfactants with
the variation of the (a) length of the spacer connecting the two hydrophilic
heads, (b) length of the hydrophobic tail and (c) the bending rigidity of the
hydrocarbon chains forming the spacer and the tail; some of the trends of
variation are counter-intuitive but are in excellent agreement with the
available experimental results. Our simulations also elucidate the dependence
of the shapes of the micellar aggregates and the magnitude of the CMC on the
geometrical shape and size of the surfactant molecules and the electrical
charge on the hydrophilic heads
A scoping review and assessment of essential elements of shared decision-making of parent-involved interventions in child and adolescent mental health
Parents play a critical role in child and adolescent mental health care and treatment. With the increasing implementation of shared decision-making (SDM) across health settings, there is a growing need to understand the decision support interventions used to promote SDM in child and adolescent mental health services (CAMHS). The overall aim of this review is to identify and examine the existing decision support interventions available for parents. A broad search was conducted using the key concepts "shared decision-making", "parents" and "child and adolescent mental health". Five electronic databases were searched: PsycInfo, Embase, Medline, Web of Science and the Cochrane Library. In addition to these relevant databases, we searched the Ottawa's Inventory of Decision Aids, Children's Hospital of Eastern Ontario website, Google, Google Play and known CAMHS' websites. The search identified 23 interventions available for use with parents. These interventions targeted parents providing care for children with ADHD, ASD, emotional and behavioural problems including depression (EBD), self-harm or universal mental health care. Various modalities including face-to-face, digital and paper-based versions were adopted. The majority of the interventions were able to "present options" (87%) and "discuss the pros and cons" (83%) of treatment. Time, accessibility and appropriateness of the intervention emerged as factors influencing usage and implementation of interventions. Our findings suggest that SDM interventions involving parents have been implemented differently across various presenting mental health difficulties in CAMHS. This review brings awareness of existing parent-involved interventions and has implications for the development, implementation and usage of new interventions
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