Rolling and ageing in T-ramp soft adhesion

Immediately before adsorption to a horizontal substrate, sinking polymer-coated colloids can undergo a complex sequence of landing, jumping, crawling and rolling events. Using video tracking we studied the soft adhesion to a horizontal flat plate of micron-size colloids coated by a controlled molar fraction $f$ of the polymer PLL-g-PNIPAM which is temperature sensitive. We ramp the temperature from below to above $T_c=32\pm 1^{\circ}$C, at which the PNIPAM polymer undergoes a transition triggering attractive interaction between microparticles and surface. The adsorption rate, the effective in-plane ($x-y$) diffusion constant and the average residence time distribution over $z$ were extracted from the Brownian motion records during last seconds before immobilisation. Experimental data are understood within a rate-equations based model that includes ageing effects and includes three populations: the untethered, the rolling and the arrested colloids. We show that pre-adsorption dynamics casts analyze a characteristic scaling function $\alpha (f)$ proportional to the number of available PNIPAM patches met by soft contact during Brownian rolling. In particular, the increase of in-plane diffusivity with increasing $f$ is understood: the stickiest particles have the shortest rolling regime prior to arrest, so that their motion is dominated by untethered phase

Magnetic oscillations and field-induced spin-density waves in (TMTSF)_2ClO_4

We investigated the effects of magnetic field on a quasi-one-dimensional band of interacting electrons with a transverse dimerizing potential. One-particle problem in bond-antibond representation is solved exactly as well as the problem of magnetic breakdown through the dimerization gap. The resulting propagator is used to calculate the spin-density-wave (SDW) response of the interacting system within the matrix RPA for the SDW susceptibility. We find that the value of the anion potential fitting experiments in relaxed (TMTSF)_2ClO_4 is large, of the order of inter-chain hopping. In particular we predict the magnetic field induced transition of the first order between interband SDW0 and intraband SDW+/- phases. We reproduce the rapid oscillations with a period of 260 Tesla and the overall profile of the (TMTSF)_2ClO_4 phase diagram

U-J Synergy Effect for the High Tc Superconductors

Using renormalization group and exact diagonalization of small clusters we investigate the ground state phase diagram of a two-dimensional extended Hubbard model with nearest-neighbor exchange interaction J, in addition to the local Coulomb repulsion U. The main instabilities are antiferromagnetism close to half-filling and d-wave superconductivity in the doped system. Our results suggest that the combined action of J and U interactions provide a remarkably efficient mechanism to enhance both d-wave superconducting and antiferromagnetic correlations.Comment: Final version, to appear in PR

Tannin-controlled micelles and fibrils of κ\kappa-casein

Effects of green tea tannin epigallocatechin-gallate (EGCG) on thermal-stress-induced amyloid fibril formation of reduced carboxymethylated bovine milk protein $\kappa$-casein (RCMK) were studied by dynamical light scattering (DLS) and small angle x-rays scattering (SAXS). Two populations of aggregates, micelles and fibrils, dominated the time evolution of light scattering intensity and of effective hydrodynamic diameter. SAXS experiments allowed to resolve micelles and fibrils so that the time dependence of scattering profile revealed structural evolution of the two populations. The low-Q scattering intensity prior to an expected increase with time due to fibril growth, shows an intriguing rapid decrease which is interpreted as the release of monomers from micelles. This phenomenon, observed both in the absence and in the presence of EGCG, indicates that under thermal stress free native monomers are converted to amyloid-prone monomers that do not form micelles. The consumption of free native monomers results in a release of native monomers from micelles, because only native protein participate in micelle-monomer (quasi-)equilibrium. This release is reversible, indicating also that native-to-amyloid-prone monomers conversion is reversible as well. We show that EGCG does not bind to protein in fibrils, neither does it affect/prevent the pro-amyloid conversion of monomers. EGCG hinders the addition of monomers to growing fibrils. These facts allowed us to propose kinetics model for EGCG-controlled amyloid aggregation of micellar proteins. Therein, we introduced the growth-rate inhibition function which quantitatively accounts for the effect of EGCG on the fibril growth at any degree of thermal stress

Sign reversals of the Quantum Hall Effect in quasi-1D conductors

The sign reversals of the Quantum Hall Effect observed in quasi-one-dimensional conductors of the Bechgaard salts family are explained within the framework of the quantized nesting model. The sequence of reversals is driven by slight modifications of the geometry of the Fermi surface. It is explained why only even phases can have signign reversals and why negative phases are less stable than positive ones.Comment: 4 LaTex pages, 3 Postscript figure

Travelling Perversion as Constant Torque Actuator

Mechanical stress and conformation of helical elastic rod clamped at both ends were studied upon unwinding process. By axial rotation of one end, the winding number was progressively changed from the natural one (n = n0) to complete chirality inversion (n = −n0) while keeping the total elongation fixed and monitoring the applied torque M and tension T. Along the unwinding, the system crosses three well distinguished states : natural helix (+), mixed state (+/-) and inverted helix (-). The mixed state involves two helices with opposite chiralities spatially connected by a perversion (helicity inversion). Upon unwinding the perversion is first "injected" (nucleated) from one side, travels towards the opposite side where it finally gets "absorbed" (annihilated), leaving the system in the (-) state. In the mixed state the profile of M (n) is almost flat : the system behaves as a constant torque actuator. The three states are quantitatively well described in the framework of a biphasic model, neglecting the perversion energy and finite size effects. The latter are taken into account in a numerical simulation based on the Kirchhoff theory of elastic rods. It reproduces the clamped rod conformations and the observed profiles M (n) and T (n), including abrupt jumps at nucleation and annihilation of the perversion

Tunable and switchable soft adsorption of polymer-coated microparticles on a flat substrate

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