We present mechanical measurements of the frequency-dependent linear viscoelastic storage and loss moduli, G′(ω) and G″(ω), and the yield stress, τy, and yield strain, γy, for calf thymus DNA (13 kbp) over a range of mitotically relevant concentrations from CDNA = 1 to 10 mg/ml. For large CDNA, we find a dominant plateau elasticity, G′p, at high ω. As ω decreases, G′ falls until it is equal to G′ at the crossover frequency, ωc, below which G″ dominates. We measure G′p ∼ CDNA2.25 and ωc ∼ CDNA−2.4, consistent with scaling exponents for classical polymer solutions. The mechanical |G*(ω)| agree well with those measured using a new microrheological technique based on video tracking microscopy of thermally-driven fluorescent colloidal spheres and a frequency-dependent Stokes-Einstein equation. We have developed this technique to probe how enzymes, typically available in small quantities, can affect the rheology of the DNA. Using it, we report preliminary measurements of a higher ωc for a DNA network in which the ATP-powered enzyme Topoisomerase II transiently cuts and rebinds the DNA, thereby relaxing entanglements.

Zhelev and Needham have recently created large, quasi-stable pores in artificial lipid bilayer vesicles. Initially created by electroporation, the pores remain open for up to several seconds before quickly snapping shut. This result is surprising in light of the large line tension for holes in bilayer membranes and the rapid time scale for closure of large pores. We show how pores can be dynamically stabilized via a new feedback mechanism. We also explain quantitatively the observed sudden pore closure as a tangent bifurcation. Finally we show how Zhelev and Need-ham's experiment can be used to measure accurately the pore line tension, an important material parameter. For their SOPC/CHOL mixture we obtain a line tension of 3.0 × 10−6 dyn.

We have studied the late stages of the “pearling instability” in lipid bilayers, which is brought on by applying laser tweezers to a cylindrical vesicle. This produces a front that propagates down the vesicle, leaving behind it bilayer-covered droplets separated by thin tubes, which appear under the microscope as pearls on a string. At later times, the “pearls” are observed to drift slowly towards the “trap” (the spot where the tweezers are applied, into which the surfactant is drawn). We model the hydrodynamics of the drifting pearls as a combination of translation of the string of pearls, and “slipping” of the bilayer skin over the pearls, to relate the speed of the pearls to the underlying flux of the surfactant into the trap.

We study the mechanism of the ‘pearling’ instability seen recently in experiments on lipid tubules under a local applied laser intensity. We argue that the correct boundary conditions are fixed chemical potentials, or surface tensions Σ, at the laser spot and the reservoir in contact with the tubule. While most qualitative conclusions of previous studies remain the same, the ‘ramped’ control parameter (surface tension) implies several new features. We also explore some consequences of front propagation into a noisy unstable medium.

We study the simplest model of a polyelectrolyte impinging upon a point, frictionless obstacle in the presence of a field. Using numerical simulation, we show that the wide range of impacts, ranging from direct impact forming a long-lived hairpin conformation, to glancing impacts where the chain slides off of the obstacle in short time, can be described universally. In strong field, the average collision time, 〈tc〉, and average distance traveled during collision, 〈zc〉, depend upon the impact and follow universal curves over a large range of molecular weights and field strengths. This result provides analytic formulas for the chain's mobility in an array of posts and yields insight into the effect of post spacing.

We have simulated the dynamical failure of three-dimensional notched solids under tension using molecular dynamics and up to 100 million atoms. We discovered a dynamical brittle-to-ductile transition in the rapid cleavage of rare-gas solids when the crack velocity approaches one-third of the Rayleigh sound speed. At this transition, the crack tip has already begun to roughen on the atomic scale. This suggests that the brittle crack undergoes a dynamic instability which immediately leads to the initiation of plastic failure by the spontaneous emission and proliferation of dislocations and crack arrest.

By studying the morphology of the fracture surfaces of both a metallic alloy and of a silicate glass for various very low crack propagation velocities, it is shown that the pin-ning/depinning scenario of a line moving through randomly distributed obstacles seems well adapted to describe fracture of heterogeneous materials. The crossover length separating the large length scales regime characterised by a roughness index ζ ≃ 0.78 and the small length scales one, for which ζc ≃ 0.5, is shown to decrease with the measured crack velocity as a power law, ξc ∞ v−φ, with ζ ≃ 1. ξc and v also vary with the pulling force as power laws, and the measured values of exponents ν and β are close to: ν ≃ 2 and β ≃ 2.

A conformai mapping technique is presented which allows to solve efficiently harmonic and bi-harmonic problems in semi-infinite domains limited by a rough boundary. This technique is applied to obtain the statistical distribution of flux and elastic stress in the vicinity of a self-affine boundary. This computation justifies the occurence of Weibull statistics for the strength of glass fibers, with Weibull modulus depending on the roughness amplitude. Another application concerns the determination of the local mode III stress intensity factor ahead of a rough crack. Extension of the method to surface stress-corrosion, and interfacial crack propagation are discussed.

We study the behavior of fracture in disordered systems close to the breakdown point. We simulate numerically both scalar (resistor network) and vectorial (spring network) models with threshold disorder, driven at constant current and stress rate respectively. We analyze the scaling of the susceptibility and the cluster size close to the breakdown. We observe avalanche behavior and clustering of the cracks. We find that the scaling exponents are consistent with those found close to a mean-field spinodal and present analogies between the coalescence of microfractures and the coalescence of droplets in a metastable magnetic system. Finally, we discuss different experimental conditions and some possible theoretical interpretations of the results.

A continuous numerical model of pure mode I crack propagation in a bidimensional heterogeneous material is presented. This model describes the propagation of a macrocrack into a two-phases brittle material containing a finite density of second phase precipitates. The morphology of cracks produced for various mechanical and microstructural conditions is analysed. It is shown that the simulated cracks are self-affine with a roughness index ≃ 0.6 independent of the microstructure. Relevant length scales, on the contrary, strongly depend on the microstructural parameters, and indicate an optimum density leading to a maximum fracture toughness.

A homogeneous mixturea of different types of granular material will often segregate when rotated in a drum mixer. In the traditional axial segregation effect, the mixture of two different sizes of granular media will appear from the surface to separate into bands of relatively pure single concentrations along the axis of rotation. The initial pattern is not stable, but evolves in time with continued rotation through metastable states of fewer and fewer bands. We describe two experimental methods for measuring this evolution which provide a more complete picture of the dynamics involved in the pattern progression. The use of a CCD camera in conjunction with digital analysis techniques provides a consistent and precise measure of the state of the surface as a function of time. Nuclear Magnetic Resonance Imaging (MRI) techniques are used to noninvasively measure the segregation beneath the surface. These methods indicate that the underlying mechanisms of the pattern evolution originate in the bulk of the material beneath the avalanching surface.

Electrokinetic phenomena in brine-saturated porous media, such as electroosmosis (fluid-flow induced by applied electric fields) and streaming current (the complementary process) depend on the density of ions adsorbed on the pore surface and the characteristic thickness of the diffuse space-charge layer λ. These, in turn, depend on brine chemistry, ambient temperature and possibly other parameters. We report on a series of measurements of natural rock and synthetic glass-bead samples: for one sample group, we varied the temperature over 0–50 ° C; for another, we changed the brine cation species. We find that the electrokinetic coefficients depend only weakly on temperature; this is shown to follow from the expected trends in λ, η, and σ. The chemistry dependence follows qualitatively but not quantitatively the predictions of the Debye-Hiickel approximation.

The ac and dc electrical properties of composite materials are studied using hierarchical lattices. First we show that the hierarchical model can correctly account for the main scaling properties of critical percolative structures. Then we study the effect of potential disorder by assuming that the microscopic conductances are distributed according to a power law distribution function. We find that in the limit of strong disorder, the predictions are in qualitative agreement with reported experimental measurements.

An influence of sheet electron beam irradiation (SEBI) on the wettability is investigated of the hydroxy apatite (HAP) [Ca10(PO4)6(OH)2]. The wettability is one of the important factors to control bio-compatibility. The SEBI is homogeneously performed by an electrocurtain processor. The temperature of the sample is below 323 K just after the irradiation. The wettability is evaluated by measuring the wet angle θ in a drop of water. The SEBI increases the wettability. Based on rate process, the influence of SEBI on wettability is discussed. Using the SEBI, we can precisely control the surface condition of HAP.

We propose a Monte Carlo method to simulate the magnetization decay for NMR in porous media. In this method, the diffusive spins in the fluid phase are modeled by 3-d random walkers, whose absorption at the solid surface simulates surface-mediated decay. Our method is 20 times faster than direct relaxation of the diffusion equation when a variable-step size implementation is used. We demonstrate our method by computing the surface relaxivity ρ for a Fontainebleau sandstone, whose structure was determined by X-ray tomography.

Several random walk-algorithms are used to model relaxation processes on disordered structures. Substrates are regular lattices in d = 1,2 with disorder variables attached to each lattice site. For the simulations, we consider diffusing particles obeying different rules according to the transport process being studied. Relaxation description is done in the time domain by calculating the characteristic function F(k,t), and in the frequency domain through the study of the complex susceptibility χ(k,ω). Two types of relaxation mechanisms are seen both in simulations and in Nuclear Magnetic Resonance (NMR) experiments done on sandstones. These are expressed as stretched exponential forms of the relaxation function, and the Cole-Cole form of the imaginary part of the complex susceptibility. Longitudinal and transversal NMR measurements were done on fully saturated samples at 90 MHz. NMR relaxation data can be qualitatively understood using the random walk models proposed here.

Motivated by recent experiments on phase behavior of systems confined in porous media, we have studied the effect of quenched bond randomness on the nature of the phase transition in the two dimensional Potts model. To model the effects of the porous matrix we chose the couplings of the q state Potts Hamiltonian from the distribution P(Jij) = pδ(Jij – J) + (1 – p)δ(Jij). For a range of p values, away from the percolation threshold, the transition temperature follows the mean field prediction Tc(p) = Tc(1)p. Furthermore, we observed that the strong first order transition, that appears in the pure case for q = 10, changes two a second order transition. It is also clear from our simulations that the second order transition of the q = 3 pure case changes to a second order transition of a different universality class. A finite size scaling analysis suggests that in both cases the critical exponents, in the presence of disorder, fall into the universality class of the two dimensional pure Ising model. This result agrees with theoretical calculations recently published [1].

Preliminary results of an aggregation model that takes into account both the Brownian motion as well as the gravitational drift experienced by the colloidal particles and clusters is presented. It is shown that for high strengths of the drift the system crosses over to a regime different from diffusion-limited colloid aggregation, for which there is an increase of the fractal dimension, a speeding up of the aggregation rate and a widening of the cluster size distribution, becoming algebraically decaying with an exponent τ.

The effect of non-equilibrium charge screening in the kinetics of the one-dimensional, diffusion-controlled A + B → 0 reaction between charged reactants in solids and liquids is studied. Incorrectness of static, Debye-Hückel theory is shown. Our microscopic formalism is based on the Kirkwood superposition approximation for three-particle densities and the self-consistent treatment of the electrostatic interactions defined by the nonuniform spatial distribution of similar and dissimilar reactants treated in terms of the relevant joint correlation functions. Special attention is paid to the pattern formation due to a reaction-induced non-Poissonian fluctuation spectrum of reactant densities. This reflects a formation of loose domains containing similar reactants only. The effect of asymmetry in reactant mobilities (DA - 0, DB > 0) contrasting the traditional symmetric case, i.e. equal diffusion coefficients, (DA = DB) is studied. In the asymmetric case concentration decay is predicted to be accelerated, n(t) ∞ t−α, α = 1/3 as compared to the well-established critical exponent for fluctuation-controlled kinetics in the symmetric case, α - 1/4 and/or the prediction of the standard chemical kinetics, α = 1/2. Results for the present microscopic theory are compared with the mesoscopic theory.

A video photographic study of the kinetics of the precipitation reaction Na2HPO4 + CaCl2 → CaHPO4 in agarose gels is presented. Periodic precipitation, known as Liesegang bands, are observed when one of the reactants is incorporated in the gel and the other allowed to diffuse in. The time evolution of the first Liesegang band's profile provides a direct confirmation of the supersaturation model, with a decrease in intensity as the monomer concentration is depleted and the formation of a broad plateau behind the sharp band from large particles left after the incorporation of the smaller mobile particles into the rapidly growing aggregate front. The time dependence of leading edge of the precipitate front agrees with the predictions of the reaction-diffusion-supersaturation model of Liesegang band formation.