Phase Coexistence of a Stockmayer Fluid in an Applied Field

We examine two aspects of Stockmayer fluids which consists of point dipoles that additionally interact via an attractive Lennard-Jones potential. We perform Monte Carlo simulations to examine the effect of an applied field on the liquid-gas phase coexistence and show that a magnetic fluid phase does exist in the absence of an applied field. As part of the search for the magnetic fluid phase, we perform Gibbs ensemble simulations to determine phase coexistence curves at large dipole moments, $\mu$. The critical temperature is found to depend linearly on $\mu^2$ for intermediate values of $\mu$ beyond the initial nonlinear behavior near $\mu=0$ and less than the $\mu$ where no liquid-gas phase coexistence has been found. For phase coexistence in an applied field, the critical temperatures as a function of the applied field for two different $\mu$ are mapped onto a single curve. The critical densities hardly change as a function of applied field. We also verify that in an applied field the liquid droplets within the two phase coexistence region become elongated in the direction of the field.Comment: 23 pages, ReVTeX, 7 figure

Magnetorheology in an aging, yield stress matrix fluid

Field-induced static and dynamic yield stresses are explored for magnetorheological (MR) suspensions in an aging, yield stress matrix fluid composed of an aqueous dispersion of Laponite® clay. Using a custom-built magnetorheometry fixture, the MR response is studied for magnetic field strengths up to 1 T and magnetic particle concentrations up to 30 v%. The yield stress of the matrix fluid, which serves to inhibit sedimentation of dispersed carbonyl iron magnetic microparticles, is found to have a negligible effect on the field-induced static yield stress for sufficient applied fields, and good agreement is observed between field-induced static and dynamic yield stresses for all but the lowest field strengths and particle concentrations. These results, which generally imply a dominance of inter-particle dipolar interactions over the matrix fluid yield stress, are analyzed by considering a dimensionless magnetic yield parameter that quantifies the balance of stresses on particles. By characterizing the applied magnetic field in terms of the average particle magnetization, a rheological master curve is generated for the field-induced static yield stress that indicates a concentration–magnetization superposition. The results presented herein will provide guidance to formulators of MR fluids and designers of MR devices who require a field-induced static yield stress and a dispersion that is essentially indefinitely stable to sedimentation.Petroleum Research Fund (ACS-PRF Grant No. 49956-ND9)American Chemical Society (ACS-PRF Grant No. 49956-ND9

Force-Velocity Measurements of a Few Growing Actin Filaments

The authors propose a new mechanism for actin-based force generation based on results using chains of actin-grafted magnetic colloids

The inhibition of the Rayleigh-Taylor instability by rotation

It is well-established that the Coriolis force that acts on fluid in a rotating system can act to stabilise otherwise unstable flows. Chandrasekhar considered theoretically the effect of the Coriolis force on the Rayleigh-Taylor instability, which occurs at the interface between a dense fluid lying on top of a lighter fluid under gravity, concluding that rotation alone could not stabilise this system indefinitely. Recent numerical work suggests that rotation may, nevertheless, slow the growth of the instability. Experimental verification of these results using standard techniques is problematic, owing to the practical difficulty in establishing the initial conditions. Here, we present a new experimental technique for studying the Rayleigh-Taylor instability under rotation that side-steps the problems encountered with standard techniques by using a strong magnetic field to destabilize an otherwise stable system. We find that rotation about an axis normal to the interface acts to retard the growth rate of the instability and stabilise long wavelength modes; the scale of the observed structures decreases with increasing rotation rate, asymptoting to a minimum wavelength controlled by viscosity. We present a critical rotation rate, dependent on Atwood number and the aspect ratio of the system, for stabilising the most unstable mode

Dispersion-free control of hydroelastic waves down to sub-wavelength scale

International audienceHydroelastic surface waves propagate at the surface of water covered by a thin elastic sheet and can be directly measured with accurate space and time resolution. We present an experimental approach using hydroelastic waves that allows us to control waves down to the sub-wavelength scale. We tune the wave dispersion relation by varying locally the properties of the elastic cover and we introduce a local index contrast. This index contrast is independent of the frequency leading to a dispersion-free Snell-Descartes law for hydroelastic waves. We then show experimental evidence of broadband focusing, reflection and refraction of the waves. We also investigate the limits of diffraction through the example of a macroscopic analog to optical nanojets, revealing that any sub-wavelength configuration gives access to new features for surface waves. Gravity-capillary waves have been extensively used as model waves to tackle the issue of wave control at macro-scopic scale. Contrary to optics and acoustics, their temporal and spatial typical scales allow for direct and accurate observation of wave propagation inside the medium. The design and fabrication of media with given properties is generally obtained by tuning the local bathymetry, which modifies the wave phase velocity [1]. Immersed structures have been used to obtain Anderson local-ization [2] or to create macroscopic metamaterials for wave focusing [3-6] and cloaking [7, 8]. Wave control is achieved for waves with wavelength larger or comparable to the liquid depth (shallow water approximation). In this regime, damping becomes a major issue in laboratory experiments as viscous friction at the bottom dissipates most of the mechanical wave energy. These two limitations narrow the effective bandwidth of the devices created with gravity-capillary waves. Here, we propose a novel approach based on hydroe-lastic waves i.e. waves that propagate at the surface of water covered with an elastic sheet. These waves were initially introduced to describe motion in ice sheets located in the marginal ice zone [9-12] and later to study floating structures [13], wakes in the lubrication approximation [14] or cloaking [15]. In the limit of thin membranes their dispersion relation writes ω 2 = gk + T ρ k 3 + D ρ k 5 tanh kh 0 , (1) where ω = 2πf is the pulsation, k = 2π/λ is the wavenumber, g = 9.81 m.s −2 is the acceleration of gravity , T is the mechanical tension in the elastic sheet, ρ is the fluid density, D is the flexural modulus of the elastic sheet and h 0 the fluid depth. The flexural modulus D = Ee 3 12(1−ν 2) depends on the Young's modulus E of the material, its Poisson modulus ν and its thickness e. Eq. 1 exhibits three distinct regimes depending on the material properties and the wave pulsation ω: gravity waves, tension waves and flexural waves. So far, very few experiments at the laboratory scale highlighted the flexural regime using whether thin elastic polymer sheets [16, 17] or granular rafts [18]. Here, we propose to achieve spatial control of the propagation of hydroelastic waves by modifying the dispersion relation (eq. 1) through local variations of the sheet's flexural modulus D. We first describe our experimental setup and verify quantitatively the prediction from eq. 1. We introduce a local index contrast using the local phase velocity. This index contrast is independent of the frequency, which allows us to define a dispersion-free Snell-Descartes law for hydroelastic waves. To show the versatility of the system we then implement more complex structures to focus wave energy and probe wave effects due to the finite size of the system. EXPERIMENTAL SETUP We use a glass tank (80 cm × 40 cm × 20 cm) filled to a depth h 0 = 16.5 cm of water. We cover its surface with a 75 cm × 35 cm wide elastic sheet of thickness e = 20-800 µm made of an optically transparent silicone rubber sheet with Young's modulus E = 1.47 ± 0.09 MPa, density ρ s = 970kg/m 3 and Poisson's ratio ν = 0.5. This elastic film floats freely at the surface of water so that the mechanical tension T inside reduces to the water surface tension T = σ = 50 mN/m. The waves are generated with a vibration exciter powered with an amplifier controlled with a waveform generator. We work with frequencies ranging from 2 to 200 Hz and with amplitudes ζ λ to ensure the waves are in the linear regime. In addition, to guarantee that we are in the thin membrane limit we check that ρ s eω 2 {Dk 4 , T k 2 , ρg}, so that equation 1 is valid. To analyze quantitatively the wave field we use the Free-Surface Synthetic Schlieren optical technique [19] based on the apparent displacement of a random dot pattern due to the local slope of the interface. The pattern is located underneath the tank and we observe it from the top using a CCD camera located at H 2 m from th

Dispersion-free control of hydroelastic waves down to sub-wavelength scale

Hydroelastic surface waves propagate at the surface of water covered by a thin elastic sheet and can be directly measured with accurate space and time resolution. We present an experimental approach using hydroelastic waves that allows us to control waves down to the sub-wavelength scale. We tune the wave dispersion relation by varying locally the properties of the elastic cover and we introduce a local index contrast. This index contrast is independent of the frequency leading to a dispersion-free Snell-Descartes law for hydroelastic waves. We then show experimental evidence of broadband focusing, reflection and refraction of the waves. We also investigate the limits of diffraction through the example of a macroscopic analog to optical nanojets, revealing that any sub-wavelength configuration gives access to new features for surface waves