Scaling of Island Growth in Pb Overlayers on Cu(001)

The growth and ordering of a Pb layer deposited on Cu(001) at 150 K has been studied using atom beam scattering. At low coverage, ordered Pb islands with a large square unit cell and nearly hexagonal internal structure are formed. This is a high order commensurate phase with 30 atoms in the unit cell. From the measurement of the island diffraction peak profiles we find a power law for the mean island - size versus coverage with an exponent $n=0.54 \pm 0.03$. A scaling behavior of growth is confirmed and a simple model describing island growth is presented. Due to the high degeneracy of the monolayer phase, different islands do not diffract coherently. Therefore, when islands merge they still diffract as separate islands and coalescence effects are thus negligible. From the result for $n$ we conclude that the island density is approximately a constant in the coverage range $0.1 < \Theta < 0.5$ where the ordered islands are observed. We thus conclude that most islands nucleate at $\Theta < 0.1$ and then grow in an approximately self similar fashion as $\Theta$ increases.Comment: 23 pages, 10 Figures (available upon request). SU-PHYS-93-443-375

Unzipping Dynamics of Long DNAs

The two strands of the DNA double helix can be `unzipped' by application of 15 pN force. We analyze the dynamics of unzipping and rezipping, for the case where the molecule ends are separated and re-approached at constant velocity. For unzipping of 50 kilobase DNAs at less than about 1000 bases per second, thermal equilibrium-based theory applies. However, for higher unzipping velocities, rotational viscous drag creates a buildup of elastic torque to levels above kBT in the dsDNA region, causing the unzipping force to be well above or well below the equilibrium unzipping force during respectively unzipping and rezipping, in accord with recent experimental results of Thomen et al. [Phys. Rev. Lett. 88, 248102 (2002)]. Our analysis includes the effect of sequence on unzipping and rezipping, and the transient delay in buildup of the unzipping force due to the approach to the steady state.Comment: 15 pages Revtex file including 9 figure

Constraint release in the slip-link model and the viscoelastic properties of polymers

The Doi-Edwards slip-link model is used to treat in a self consistent manner the effect of tube renewal on the relaxation of an entangled polymeric medium. We derive new expressions for the stress in the terminal flow zone and for the viscoelastic properties. These expressions, which agree qualitatively with the model proposed earlier by Graessley, improve to a rather significant extent the agreement between theory and experiment, and remain mathematically very simple.On reconsidère le modèle viscoélastique de Doi-Edwards (modèle de la « chaîne à anneaux ») pour les polymères fondus dans la zone terminale, en prenant en compte le renouvellement de tube dû à la reptation des chaînes voisines. Ce traitement auto-cohérent, déjà proposé par Graessley dans le cadre d'un autre modèle, modifie notablement les prédictions théoriques : la viscosité devrait être plus faible d'un facteur deux à trois, et la distribution de modes plus large que prévu initialement. Ces tendances sont en accord qualitatif avec l'expérience. Toutefois, des différences quantitatives subsistent, qui pourraient être associées aux fluctuations négligées dans le traitement actuel. Le modèle proposé ici, d'un esprit assez proche de celui de Graessley, et conduisant à des résultats en bon accord avec ce dernier, devrait pourtant présenter par rapport à lui quelques avantages, comme une plus grande simplicité mathématique, et l'utilisation d'un exposant mesurable indépendamment au lieu d'un paramètre ajustable

Propriétés diélectriques de composés de la série p-alkoxyphénylazo p'-phényl esters et p-alkoxyphénylazoxy-p' phényl esters

The dielectric anisotropy of some typical nematic p-alkoxyphenylazo-p' phenyl esters and p-alkoxyphenylazoxy-p' phenyl esters has been studied and some conclusions proposed.On étudie l'anisotropie diélectrique de quelques composés nématogènes des séries d'esters p-alkoxyphénylazo-p' phényl et p-alkoxyphényl azoxy-p' phényl

A singular view of DNA transactions

The genetic information of an organism is encoded in the base pair sequence of its DNA. Many specialized proteins are involved in organizing, preserving and processing the vast amounts of information on the DNA. In order to do this swiftly and correctly these proteins have to move quickly and accurately along and/or around the DNA constantly rearranging it. In order to elucidate these processes we perform single-molecule experiments on model systems such as bacterial gene regulators and repair proteins. The data we use to extract forces, energies and mechanochemistry driving these dynamic transactions. The results obtained from these model systems are then generalized and though to be applicable to many DNA-protein interactions

Drag on a Tethered Chain Moving in a Polymer Melt

A chain of $N$ monomers is attached to a small colloïdal particle, and is pulled (at a velocity $V$) inside a polymer melt (chemically identical, with $P$ monomers per chain). The main parameter for this problem is the number $X(V)$ of $P$ chains entangled with the ($N$) chain. Earlier estimates of $X$ are criticized, and a new form is proposed: at large $N(N>N^2_{\rm e})$, we are led to a “Stokes” regime, $X=N^{1/2}$, while at smaller $N(N<N^2_{\rm e})$, we find a “Rouse” regime, $X=N/N_{\rm e}$ (where $N_{\rm e}$ is the number of monomers per entanglement)

Combined Lab-on-a-Chip and microarray approach for biomolecular interaction sensing using surface plasmon resonance imaging

Surface plasmon resonance imaging (SPR) is a well-established label-free detection technique for real-time biomolecular interaction measurements. An integrated LOC sensing system with fluidic control for sample movement to specific locations on microarray surface in combination with SPR imaging is demonstrated by the measurements of human IgG and anti-IgG interactions from 24 patterned regions