Polymers grafted to a fluid and flexible membrane: Extreme sensitivity to the grafting density

Equilibrium phase coexistence between two chemical species implies the equality of the chemical potentials and of the osmotic pressures. We study this problem on a deformable membrane when one type of the molecules serves as anchor for polymeric chains immersed in the surrounding medium (considered as a good solvent). We derive the general conditions for phase coexistence when both the curvature of the membrane and the density field of the anchor molecule are free to adjust themselves. We show that curvature favors phase segregation. Our model predicts that membranes decorated with polymeric chains exhibit new shape bifurcations without equivalent in fixed density systems. Received: 26 November 2002 / Accepted: 2 April 2003 / Published online: 12 May 2003 RID="a" ID="a"e-mail: nicolas@drfmc.ceng.cea.fr RID="b" ID="b"e-mail: bfourcade@cea.fr     

Rectangular to hexagonal columnar phase transition exhibited by a biforked mesogen

We present the structural study of a biforked compound exhibiting a Col_r-Col_h phase transition. This study was performed by means of DSC, optical microscopy, X-ray diffraction, dilatometry and molecular modelling. The molecular packing within the two columnar mesophases of this compound is discussed. For the first time, it is shown that the clusters of adjacent molecules which form the elementary section of a column are tilted with respect to the columnar axis in the rectangular mesophase. From the whole set of experimental data, the number of molecules in a columnar section is found to be about 5 in the rectangular mesophase, and 4 in the hexagonal one. The transition between the two columnar mesophases is discussed with respect to its slow kinetics. Received 1st October 2002 Published online: 16 April 2003 RID="a" ID="a"e-mail: ribeiro@cii.fc.ul.pt RID="b" ID="b"UMR 7504     

Oxygen is electroreduced to water on a "wired" enzyme electrode at a lesser overpotential than on platinum

The first enzyme-based catalyst that is superior to platinum in the four-electron electroreduction of oxygen to water is reported. The smooth Pt cathode reached half and 90% of the mass transport-limited current density at respective overpotentials of -0.4 and -0.58 V in 0.5 M sulfuric acid, and only at even higher overpotentials in pH 7.2 phosphate buffer. In contrast, the smooth "wired" bilirubin oxidase cathode reached half and 90% of the mass transport-limited current density at respective overpotentials as low as -0.2 and -0.25 V. The mass transport-limited current density for the smooth "wired" enzyme cathode in PBS was twice that with smooth Pt in 0.5 M sulfuric acid. Under 1 atm O2 pressure, O2 was electroreduced to water on a polished carbon cathode, coated with the "wired" BOD film, in pH 7.2 saline buffer (PBS) at an overpotential of -0.31 V at a current density of 9.5 mA cm-2. At the same overpotential, the current density of the polished platinum cathode in 0.5 M H2SO4 was 16-fold lower, only 0.6 mA cm-2.     

Are high perches in the blackcap Sylvia atricapilla song or listening posts? A sound transmission study

Birds often sing from high perches referred to as song posts. However, birds also listen and keep a lookout from these perches. We used a sound transmission experiment to investigate the changes for receiving and sending conditions that a territorial songbird may experience by moving upwards in the vegetation. Representative song elements of the blackcap Sylvia atricapilla were transmitted in a forest habitat in spring using a complete factorial design with natural transmission distances and speaker and microphone heights. Four aspects of sound degradation were quantified: signal-to-noise ratio, excess attenuation, distortion within the sounds determined as a blur ratio, and prolongation of the sounds with "tails" of echoes determined as a tail-to-signal ratio. All four measures indicated that degradation decreased with speaker and microphone height. However, the decrease was considerably higher for the microphone than for the speaker. This suggests that choosing high perches in a forest at spring results in more benefits to blackcaps in terms of improved communication conditions when they act as receivers than as senders.     

Numerical description of the edge mode at the beveled extremity of a plate

This paper deals with the reflection of a two-dimensional harmonic Lamb wave at the beveled end of a plate. The existence of a resonant edge mode is described by a numerical model. It is proved that the edge mode is the resonance of different complex modes. The behavior of this mode as a function of the bevel angle is studied. Its amplitude decreases and its resonance frequency shifts as the bevel angle decreases from 90 to 85 deg. An unexpected strong variation of the repartition of the reflected energy is linked to this phenomenon.     

Optimal bell tests do not require maximally entangled states

Any Bell test consists of a sequence of measurements on a quantum state in spacelike separated regions. Thus, a state is better than others for a Bell test when, for the optimal measurements and the same number of trials, the probability of existence of a local model for the observed outcomes is smaller. The maximization over states and measurements defines the optimal nonlocality proof. Numerical results show that the required optimal state does not have to be maximally entangled.     

Propagation of a dark soliton in a disordered Bose-Einstein condensate

We consider the propagation of a dark soliton in a quasi-1D Bose-Einstein condensate in presence of a random potential. This configuration involves nonlinear effects and disorder, and we argue that, contrarily to the study of stationary transmission coefficients through a nonlinear disordered slab, it is a well-defined problem. It is found that a dark soliton decays algebraically, over a characteristic length which is independent of its initial velocity, and much larger than both the healing length and the 1D scattering length of the system. We also determine the characteristic decay time.     

Non-markovian transport of DNA in microfluidic post arrays

We present an analytically solvable model for the transport of long DNA through microfluidic arrays of posts. The motion is a repetitive three-part cycle: (i) collision with the post and extension of the arms; (ii) rope-over-pulley post disengagement; and (iii) a random period of uniform translation before the next collision. This cycle, inspired by geometration, is a nonseparable (Scher-Lax) continuous-time random walk on a lattice defined by the posts. Upon adopting a simple model for the transition probability density on the lattice, we analytically compute the mean DNA velocity and dispersivity in the long-time limit without any adjustable parameters. The results compare favorably with the limited amount of experimental data on separations in self-assembled arrays of magnetic beads. The Scher-Lax formalism provides a template for incorporating more sophisticated microscale models.     

Relation between the high density phase and the very-high density phase of amorphous solid water

It has been suggested that high-density amorphous (HDA) ice is a structurally arrested form of high-density liquid (HDL) water, while low-density amorphous ice is a structurally arrested form of low-density liquid (LDL) water. Recent experiments and simulations have been interpreted to support the possibility of a second distinct high-density structural state, named very high-density amorphous (VHDA) ice, questioning the LDL-HDL hypothesis. We test this interpretation using extensive computer simulations and find that VHDA is a more stable form of HDA and that, in fact, VHDA should be considered as the amorphous ice of the quenched HDL.     

Joint statistics of the Lagrangian acceleration and velocity in fully developed turbulence

We report experimental results on the joint statistics of the Lagrangian acceleration and velocity in highly turbulent flows. The acceleration was measured up to a microscale Reynolds number R(lambda)=690 using high speed silicon strip detectors from high energy physics. The acceleration variance was observed to be strongly dependent on the velocity, following a Heisenberg-Yaglom-like u(9/2) increase. However, the shape of the probability density functions of the acceleration component conditioned on the same component of the velocity when normalized by the acceleration variance was observed to be independent of velocity and to coincide with the unconditional probability density function of the acceleration components. This observation imposes a strong mathematical constraint on the possible functional form of the acceleration probability distribution function.