James L. Harden

Evolution of particle-scale dynamics in an aging clay suspension.

Multispeckle x-ray photon correlation spectroscopy was employed to characterize the slow dynamics of a suspension of highly charged, nanometer-sized disks. At wave vectors q corresponding to interparticle length scales, the dynamic structure factor follows a form f(q,t) approximately exp([-(t/tau)(beta)], where beta approximately 1.5. The relaxation time tau increases with the sample age t(a) approximately as tau approximately t(1.8)(a) and decreases with q as tau approximately q(-1). Such behavior is consistent with models that describe the dynamics in disordered elastic media in terms of strain from random, local structural rearrangements. The measured amplitude of f(q,t) varies with q in a manner that implies caged particle motion. The decrease in the range of this motion and an increase in suspension conductivity with increasing t(a) indicate a growth in interparticle repulsion as the mechanism for internal stress development implied by these models.

Brushlike interactions between thermoresponsive microgel particles.

Using a simplified microstructural picture we show that interactions between thermosensitive microgel particles can be described by a polymer brushlike corona decorating the dense core. The softness of the potential is set by the relative thickness L0 of the compliant corona with respect to the overall size of the swollen particle R. The elastic modulus in quenched solid phases derived from the potential is found to be in excellent agreement with diffusing wave spectroscopy data and mechanical rheometry. Our model thus provides design rules for the microgel architecture and opens a route to tailor rheological properties of pasty materials.

Gel formation and aging in weakly attractive nanocolloid suspensions at intermediate concentrations

We report x-ray photon correlation spectroscopy (XPCS) and rheometry experiments to study the temporal evolution of gel formation and aging in suspensions of silica nanocolloids possessing a tunable short-range attraction. The colloid volume fractions, φ = 0.20 and 0.43, are below the glass regime at high concentration and above the fractal regime at low concentration. Following a sudden initiation of the interparticle attraction, the suspensions display a protracted latency period in which they remain fluid before acquiring a measurable elastic shear modulus. The duration of the latency period and the subsequent rate of increase of the modulus vary strongly with the strength of the attraction. The XPCS results indicate dynamic heterogeneity among the colloids during this gel formation in which a growing fraction of the particles become localized. The temporal evolution of this localization correlates with that of the rheology. In particular, the time scale over which the fraction of localized particles increases tracks the duration of the latency period. Also, at φ = 0.20 the localization length characterizing the motion of the localized fraction scales onto the shear modulus with no free parameters as predicted by a self-consistent theory based on mode coupling [K. S. Schweizer and G. Yatsenko, J. Chem. Phys. 127, 164505 (2007)], while deviations from the predicted scaling at φ = 0.43 are observed near the gel point. The XPCS results also reveal slow, hyperdiffusive motion of the colloids in the newly formed gels that is attributed to strain from the relaxation of internal stress. While some features of this motion correlate with the evolving rheology, others appear decoupled from the macroscopic mechanical behavior.

Budding and domain shape transformations in mixed lipid films and bilayer membranes.

We study the stability and shapes of domains with spontaneous curvature in fluid films and membranes, embedded in a surrounding membrane with zero spontaneous curvature. These domains can result from the inclusion of an impurity in a fluid membrane or from phase separation within the membrane. We show that for small but finite line and surface tensions and for finite spontaneous curvatures, an equilibrium phase of protruding circular domains is obtained at low impurity concentrations. At higher concentrations, we predict a transition from circular domains, or caplets, to stripes. In both cases, we calculate the shapes of these domains within the Monge representation for the membrane shape. With increasing line tension, we show numerically that there is a budding transformation from stable protruding circular domains to spherical buds. We calculate the full phase diagram and demonstrate two triple points of, respectively, bud-flat-caplet and flat-stripe-caplet coexistence.

Potent inhibition of ice recrystallization by low molecular weight carbohydrate-based surfactants and hydrogelators

Ice recrystallization inhibition (IRI) activity is a very desirable property for an effective cryoprotectant. This property was first observed in biological antifreezes (BAs), which cannot be utilized in cryopreservation due to their ability to bind to ice. To date, potent IRI active compounds have been limited to BAs or synthetic C-linked AFGP analogues (1 and 2), all of which are large peptide-based molecules. This paper describes the first example of low molecular weight carbohydrate-based derivatives that exhibit potent IRI activity. Non-ionic surfactant n-octyl-β-D-galactopyranoside (4) exhibited potent IRI activity at a concentration of 22 mM, whereas hydrogelator N-octyl-D-gluconamide (5) exhibited potent IRI activity at a low concentration of 0.5 mM. Thermal hysteresis measurements and solid-state NMR experiments indicated that these derivatives are not exhibiting IRI activity by binding to ice. For non-ionic surfactant derivatives (3 and 4), we demonstrated that carbohydrate hydration is important for IRI activity and that the formation of micelles in solution is not a prerequisite for IRI activity. Furthermore, using solid-state NMR and rheology we demonstrated that the ability of hydrogelators 5 and 6 to form a hydrogel is not relevant to IRI activity. Structure–function studies indicated that the amide bond in 5 is an essential structural feature required for potent IRI activity.

Slow dynamics, aging, and glassy rheology in soft and living matter

Abstract We explore the origins of slow dynamics, aging and glassy rheology in soft and living matter. Non-diffusive slow dynamics and aging in materials characterised by crowding of the constituents can be explained in terms of structural rearrangement or remodelling events that occur within the jammed state. In this context, we introduce the jamming phase diagram proposed by Liu and Nagel to understand the ergodic–nonergodic transition in these systems, and discuss recent theoretical attempts to explain the unusual, faster-than-exponential dynamical structure factors observed in jammed soft materials. We next focus on the anomalous rheology (flow and deformation behavior) ubiquitous in soft matter characterised by metastability and structural disorder, and refer to the Soft Glassy Rheology (SGR) model that quantifies the mechanical response of these systems and predicts aging under suitable conditions. As part of a survey of experimental work related to these issues, we present x-ray photon correlation spectroscopy (XPCS) results of the aging of laponite clay suspensions following rejuvenation. We conclude by exploring the scientific literature for recent theoretical advances in the understanding of these models and for experimental investigations aimed at testing their predictions.

Simulating the Entropic Collapse of Coarse-Grained Chromosomes

Depletion forces play a role in the compaction and decompaction of chromosomal material in simple cells, but it has remained debatable whether they are sufficient to account for chromosomal collapse. We present coarse-grained molecular dynamics simulations, which reveal that depletion-induced attraction is sufficient to cause the collapse of a flexible chain of large structural monomers immersed in a bath of smaller depletants. These simulations use an explicit coarse-grained computational model that treats both the supercoiled DNA structural monomers and the smaller protein crowding agents as combinatorial, truncated Lennard-Jones spheres. By presenting a simple theoretical model, we quantitatively cast the action of depletants on supercoiled bacterial DNA as an effective solvent quality. The rapid collapse of the simulated flexible chromosome at the predicted volume fraction of depletants is a continuous phase transition. Additional physical effects to such simple chromosome models, such as enthalpic interactions between structural monomers or chain rigidity, are required if the collapse is to be a first-order phase transition.

Biofunctional coatings via targeted covalent cross-linking of associating triblock proteins.

A method for creating tailorable bioactive surface coatings by targeted cross-linking of network-forming CRC protein polymers is presented. The proteins are triblock constructs composed of two self-associating leucine zipper end domains (C) separated by a soluble, disordered central block (R) containing a cell or molecular binding sequence. The end domains preferentially form trimeric bundles, leading to the formation of a regular, reversible hydrogel network in a wide range of solution conditions. These hydrogel-forming proteins are useful for creating bioactive surface coatings because they self-assemble into networks, physically adsorb to a variety of substrate materials, and can be tailored to display numerous extracellular matrix (ECM)-derived peptides that interact with cells and biological macromolecules. Moreover, due to the close proximity of complementary Glu and Lys residues in the trimeric C bundles, these protein coatings can be stabilized in a targeted manner by covalent cross-linking with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). Here, we demonstrate that such EDC-cross-linked protein coatings are stable in cell culture media and maintain a significant level of biofunctionality when various ECM-derived peptides are embedded in the central soluble block of the proteins. First, we show that EDC cross-linking enables bioinert CRC protein coatings (those without embedded cell binding domains) to resist the adhesion of human foreskin fibroblasts in normal serum medium, but does not impair the ability of cross-linked coatings of CRC-RGDS (proteins with an embedded RGDS integrin binding domain) to promote cellular attachment, focal adhesion formation, and proliferation of these cells. Next, we show that the ability of cross-linked coatings of several new CRC-based proteins containing embedded heparin-binding sequences to bind biotinylated heparin is not significantly impacted over a range of EDC concentrations. The ability to target specific functional groups for covalent cross-linking is made possible by the specificity of protein-protein interactions and represents an important advantage of protein-based materials.

Extracellular Forces Cause the Nucleus to Deform in a Highly Controlled Anisotropic Manner.

Physical forces arising in the extra-cellular environment have a profound impact on cell fate and gene regulation; however the underlying biophysical mechanisms that control this sensitivity remain elusive. It is hypothesized that gene expression may be influenced by the physical deformation of the nucleus in response to force. Here, using 3T3s as a model, we demonstrate that extra-cellular forces cause cell nuclei to rapidly deform (<1 s) preferentially along their shorter nuclear axis, in an anisotropic manner. Nuclear anisotropy is shown to be regulated by the cytoskeleton within intact cells, with actin and microtubules resistant to orthonormal strains. Importantly, nuclear anisotropy is intrinsic, and observed in isolated nuclei. The sensitivity of this behaviour is influenced by chromatin organization and lamin-A expression. An anisotropic response to force was also highly conserved amongst an array of examined nuclei from differentiated and undifferentiated cell types. Although the functional purpose of this conserved material property remains elusive, it may provide a mechanism through which mechanical cues in the microenvironment are rapidly transmitted to the genome.

Phase behavior and local dynamics of concentrated triblock copolymer micelles

We report a neutron-scattering study to characterize the ordering and local dynamics of spherical micelles formed by the triblock copolymer polyethylene oxide (PEO)--polypropylene oxide (PPO)--polyethylene oxide (Pluronic) in aqueous solution. The study focuses on two Pluronic species, F68 and F108, that have the same weight fraction of PEO but that differ in chain length by approximately a factor of 2. At sufficiently high concentration, both species undergo a sequence of phase changes with increasing temperature from dissolved chains to micelles with liquid-like order to a cubic crystal phase and finally back to a micelle liquid phase. A comparison of the phase diagrams constructed from small-angle neutron scattering indicates that crystallization is suppressed for shorter chain micelles due to fluctuation effects. The intermediate scattering function I(Q,t)I(Q,0) determined by neutron spin echo displays a line shape with two distinct relaxations. Comparisons between I(Q,t)I(Q,0) for fully hydrogenated F68 chains in D2O and for F68 with deuterated PEO blocks reveal that the slower relaxation corresponds to Rouse modes of the PPO segments in the concentrated micelle cores. The faster relaxation is identified with longitudinal diffusive modes in the PEO corona characteristic of a polymer brush.