Ben O’Shaughnessy

Non-equilibrium in adsorbed polymer layers

High molecular weight polymer solutions have a powerful tendency to deposit adsorbed layers when exposed to even mildly attractive surfaces. The equilibrium properties of these dense interfacial layers have been extensively studied theoretically. A large body of experimental evidence, however, indicates that non-equilibrium effects are dominant whenever monomer–surface sticking energies are somewhat larger than kT, a common case. Polymer relaxation kinetics within the layer are then severely retarded, leading to non-equilibrium layers whose structure and dynamics depend on adsorption kinetics and layer ageing. Here we review experimental and theoretical work exploring these non-equilibrium effects, with emphasis on recent developments. The discussion addresses the structure and dynamics in non-equilibrium polymer layers adsorbed from dilute polymer solutions and from polymer melts and more concentrated solutions. Two distinct classes of behaviour arise, depending on whether physisorption or chemisorption is involved. A given adsorbed chain belonging to the layer has a certain fraction of its monomers bound to the surface, f, and the remainder belonging to loops making bulk excursions. A natural classification scheme for layers adsorbed from solution is the distribution of single-chain f values, P(f), which may hold the key to quantifying the degree of irreversibility in adsorbed polymer layers. Here we calculate P(f) for equilibrium layers; we find its form is very different to the theoretical P(f) for non-equilibrium layers which are predicted to have infinitely many statistical classes of chain. Experimental measurements of P(f) are compared to these theoretical predictions.

Mechanism of Cytokinetic Contractile Ring Constriction in Fission Yeast

Cytokinesis involves constriction of a contractile actomyosin ring. The mechanisms generating ring tension and setting the constriction rate remain unknown because the organization of the ring is poorly characterized, its tension was rarely measured, and constriction is coupled to other processes. To isolate ring mechanisms, we studied fission yeast protoplasts, in which constriction occurs without the cell wall. Exploiting the absence of cell wall and actin cortex, we measured ring tension and imaged ring organization, which was dynamic and disordered. Computer simulations based on the amounts and biochemical properties of the key proteins showed that they spontaneously self-organize into a tension-generating bundle. Together with rapid component turnover, the self-organization mechanism continuously reassembles and remodels the constricting ring. Ring constriction depended on cell shape, revealing that the ring operates close to conditions of isometric tension. Thus, the fission yeast ring sets its own tension, but other processes set the constriction rate.

Anomalous surface diffusion: A numerical study

We present a numerical study of bulk‐mediated effective surface diffusion at liquid surfaces where surface‐active molecules adsorb and desorb on experimental time scales. Adsorbed molecules execute Levy walks on the interface, each step entailing desorption followed by bulk diffusion and readsorption elsewhere. Our results confirm the predicted anomalous scaling of surface displacement r at times before particles are finally lost to the bulk. Moments grow as 〈rq〉∼tζ(q), where ζ(q)=q for q<1, ζ(q)=(q+1)/2 for q≳1. We have also confirmed that the ‘‘speed’’ c which characterizes the q<1 behavior, r≊ct, is universally related to other observables: c=D/h where D and h are, respectively, the bulk diffusivity and the slope of the equilibrium adsorption isotherm.

α-Actinin links extracellular matrix rigidity-sensing contractile units with periodic cell-edge retractions

During cell migration, the cell edge undergoes periodic protrusion–retraction cycles. Quantitative analyses of the forces at the cell edge that drive these cycles are provided. We show that α-actinin links local contractile units and the global actin flow forces at the cell edge and present a novel model based on these results.

Dynamics of living polymers.

Abstract.We study theoretically the dynamics of living polymers which can add and subtract monomer units at their live chain ends. The classic example is ionic living polymerization. In equilibrium, a delicate balance is maintained in which each initiated chain has a very small negative average growth rate (“velocity”) just sufficient to negate the effect of growth rate fluctuations. This leads to an exponential molecular weight distribution (MWD) with mean

Evolution of the Hemifused Intermediate on the Pathway to Membrane Fusion

\bar{N}

Diffusion controlled reactions at an interface

. After a small perturbation of relative amplitude

Kinetic isolation of persistent radicals and application to polymer–polymer reactions

\epsilon

Diffusion‐controlled reactions in entangled polymer systems

, e.g. a small temperature jump, this balance is destroyed: the velocity acquires a boost greatly exceeding its tiny equilibrium value. For

Polymer deformation in strong high‐frequency flows

\epsilon > \epsilon_{\mathrm c} \approx 1/\bar{N}^{1/2}