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A. C. Maggs

Publications and source records attributed to A. C. Maggs.

67 records · Page 4Linked to original sources
arXiv2003

Local Simulation Algorithms for Coulomb Interaction

Long ranged electrostatic interactions are time consuming to calculate in molecular dynamics and Monte-Carlo simulations. We introduce an algorithmic framework for simulating charged particles which modifies the dynamics so as to allow equilibration using a local Hamiltonian. The method introduces an auxiliary field with constrained dynamics so that the equilibrium distribution is determined by the Coulomb interaction. We demonstrate the efficiency of the method by simulating a simple, charged lattice gas.

cond-mat.stat-mech↗
arXiv2001

Writhing Geometry of Stiff Polymers and Scattered Light

The geometry of a smooth line is characterized locally by its curvature and torsion, or globally by its writhe. In many situations of physical interest the line is, however, not smooth so that the classical Frenet description of the geometry breaks down everywhere. One example is a thermalized stiff polymer such as DNA, where the shape of the molecule is the integral of a Brownian process. In such systems a natural frame is defined by parallel transport. In order to calculate the writhe of such non-smooth lines we study the area distributions of random walks on a sphere. A novel transposition of these results occurs in multiple light scattering where the writhe of the light path gives rise to a Berry phase recently observed in scattering experiments in colloidal suspensions .

cond-mat.soft↗
arXiv2001

Geometric phases and polarization patterns in multiple light scattering

Multiple light scattering is widely used to characterize dense colloidal systems as well as in deep tissue imaging; experiments are often interpreted via a theory of diffusion of the light intensity within a sample, neglecting the vector nature of the electromagnetic wave. Recent experiments on diffuse backscattering with linearly polarized light from colloidal suspensions of micron size particles were found to display strong intensity variations with fourfold rotational symmetry when observed through an arbitrarily oriented linear analyzer. We show that these polarization patterns are manifestations of a Berry phase of the multiple scattered beam.

cond-mat.soft↗
arXiv2001

Writhing Photons and Berry Phases in Diffusive Wave Scattering

We study theoretically the polarization state of light in multiple scattering media in the limit of weak gradients in refractive index. Linearly polarized photons are randomly rotated due to the Berry phase associated with the scattering path. For circularly polarized light independent speckle patterns are found for the two helical states. The statistics of the geometric phase is related to the writhe distribution of semiflexible polymers such as DNA.

cond-mat.soft↗
arXiv2001

Dynamic concentration of motors in microtubule arrays

We present experimental and theoretical studies of the dynamics of molecular motors in microtubule arrays and asters. By solving a convection-diffusion equation we find that the density profile of motors in a two-dimensional aster is characterized by continuously varying exponents. Simulations are used to verify the assumptions of the continuum model. We observe the concentration profiles of kinesin moving in quasi two-dimensional artificial asters by fluorescent microscopy and compare with our theoretical results.

cond-mat.soft↗
arXiv2000

Writhing Geometry at Finite Temperature: Random Walks and Geometric phases for Stiff Polymers

We study the geometry of a semiflexible polymer at finite temperatures. The writhe can be calculated from the properties of Gaussian random walks on the sphere. We calculate static and dynamic writhe correlation functions. The writhe of a polymer is analogous to geometric or Berry phases studied in optics and wave mechanics. Our results can be applied to confocal microscopy studies of stiff filaments and to simulations of short DNA loops

cond-mat.soft↗
arXiv1999

Twist and writhe dynamics of stiff filaments

This letter considers the dynamics of a stiff filament, in particular the coupling of twist and bend via writhe. The time dependence of the writhe of a filament is $W_r^2\sim L t^{1/4}$ for a linear filament and $W_r^2\sim t^{1/2} / L $ for a curved filament. Simulations are used to study the relative importance of crankshaft motion and tube like motion in twist dynamics. Fuller's theorem, and its relation with the Berry phase, is reconsidered for open filaments

cond-mat.soft↗
arXiv1998

Dynamic Fluctuations of Semiflexible Polymers

We develop a scaling theory to describe dynamic fluctuations of a semiflexible polymer and find several distinct regimes. We performed simulations to characterize the longitudinal and transverse dynamics; using ensemble averaging for a range of different degrees of coarse-graining we avoid the problems of slow equilibration often encountered in simulations. We find that the longitudinal fluctuations of a semiflexible object scales as t^7/8. These fluctuations are correlated over a length which varies as t^1/8. Our results are pertinent to the interpretation of high frequency microrheology experiments in actin solutions.

cond-mat.soft↗
arXiv1997

Twist dynamics of semiflexible polymer

A number of strange results have been reported for the twist elasticity of a semiflexible filament, actin. In particular dynamic and static methods for determining the torsional twist modulus give very different results. I show here that internal losses (friction) in a semiflexible filament could be an important cause of dissipation and that the interpretation of dynamic experiments should be made in terms of a storage and loss modulus for actin proteins.

cond-mat.soft↗
arXiv1997

On micro-bead mechanics with actin filaments

Experiments have been performed using microscopic beads to probe the small scale mechanics of actin solutions. We show that that there are a number of regimes possible as a function of the size of the probing particle. In certain cases we argue that the quasi-static response resembles a smectic crystal rather than an isotropic solid, implying an anomalous scaling of the mechanical response of actin solutions as a function of the size of the probing particles.

cond-mat.soft↗
arXiv1997

Two plateau moduli for actin gels

Conflicting experimental results have been reported for the plateau modulus in actin solutions: Analogies are often made with the viscoelastic behaviour of flexible polymers making use of the idea of entanglement as the source of the macroscopic storage modulus. We resolve apparent experimental and theoretical contradictions by pointing out the possibility of two distinct plateau regimes as a function of frequency in semidilute solutions of semiflexible polymers. We make the point that longitudinal and transverse hindrance can have very different effects at the macroscopic scale.

cond-mat.soft↗
arXiv1997

Actin, a model semi-flexible polymer

Actin is a filamentary protein which has many remarkable properties making it an ideal system for the study of the dynamics and mechanics of semi-flexible polymer solutions and gels; actin has a persistence length of over 10 microns and can polymerize to lengths of several tens of microns, this permits the use of video microscopy and other optical methods in the study polymer dynamics. Many associated proteins exist which can be used to regulate length or crosslink filaments. We discuss the dynamics and rheology of these solutions.

cond-mat.soft↗