Hopper flows of deformable particles.

Numerous experimental and computational studies show that continuous hopper flows of granular materials obey the Beverloo equation that relates the volume flow rate Q and the orifice width w: Q ∼ (w/σavg - k)ß, where σavg is the average particle diameter, kσavg is an offset where Q ∼ 0, the power-law scaling exponent ß = d - 1/2, and d is the spatial dimension. Recent studies of hopper flows of deformable particles in different background fluids suggest that the particle stiffness and dissipation mechanism can also strongly affect the power-law scaling exponent ß. We carry out computational studies of hopper flows of deformable particles with both kinetic friction and background fluid dissipation in two and three dimensions. We show that the exponent ß varies continuously with the ratio of the viscous drag to the kinetic friction coefficient, λ = ζ/µ. ß = d - 1/2 in the λ → 0 limit and d - 3/2 in the λ → ∞ limit, with a midpoint λc that depends on the hopper opening angle θw. We also characterize the spatial structure of the flows and associate changes in spatial structure of the hopper flows to changes in the exponent ß. The offset k increases with particle stiffness until k ∼ kmax in the hard-particle limit, where kmax ∼ 3.5 is larger for λ → ∞ compared to that for λ → 0. Finally, we show that the simulations of hopper flows of deformable particles in the λ → ∞ limit recapitulate the experimental results for quasi-2D hopper flows of oil droplets in water.

Excess entropy and long-time diffusion in colloidal fluids with short-range interparticle attraction.

Liquid structure and dynamics are experimentally investigated in colloidal suspensions with short-range depletion attraction. The colloidal fluid samples consist of hard-sphere colloidal particles suspended along with rodlike depletants based on surfactant micelles. The spheres have a range of surface chemistries, diameters, and packing fractions, and the rodlike micelle length depends on the temperature. Thus, the combination of hard-spheres and depletants generates a sample wherein short-range interparticle attraction can be temperature-tuned in situ. Video optical microscopy and particle tracking techniques are employed to measure particle trajectories from which structural and dynamical quantities are derived, including the particle pair correlation function [g(r)], mean square displacement, long-time diffusion coefficient, and the sample two-body excess entropy (S2). The samples with stronger short-range attractions exhibit more order, as characterized by g(r) and S2. The stronger short-range attractions are also observed to lead to slower long-time diffusion and more heterogeneous dynamics at intermediate time scales. Finally, the excess entropy scaling law prediction, i.e., the exponential relationship between two-body excess entropy and long-time diffusivity, is observed across the full range of samples.

Thermal vestige of the zero-temperature jamming transition.

When the packing fraction is increased sufficiently, loose particulates jam to form a rigid solid in which the constituents are no longer free to move. In typical granular materials and foams, the thermal energy is too small to produce structural rearrangements. In this zero-temperature (T = 0) limit, multiple diverging and vanishing length scales characterize the approach to a sharp jamming transition. However, because thermal motion becomes relevant when the particles are small enough, it is imperative to understand how these length scales evolve as the temperature is increased. Here we used both colloidal experiments and computer simulations to progress beyond the zero-temperature limit to track one of the key parameters-the overlap distance between neighbouring particles-which vanishes at the T = 0 jamming transition. We find that this structural feature retains a vestige of its T = 0 behaviour and evolves in an unusual manner, which has masked its appearance until now. It is evident as a function of packing fraction at fixed temperature, but not as a function of temperature at fixed packing fraction or pressure. Our results conclusively demonstrate that length scales associated with the T = 0 jamming transition persist in thermal systems, not only in simulations but also in laboratory experiments.

Local elastic response measured near the colloidal glass transition.

We examine the response of a dense colloidal suspension to a local force applied by a small magnetic bead. For small forces, we find a linear relationship between the force and the displacement, suggesting the medium is elastic, even though our colloidal samples macroscopically behave as fluids. We interpret this as a measure of the strength of colloidal caging, reflecting the proximity of the samples' volume fractions to the colloidal glass transition. The strain field of the colloidal particles surrounding the magnetic probe appears similar to that of an isotropic homogeneous elastic medium. When the applied force is removed, the strain relaxes as a stretched exponential in time. We introduce a model that suggests this behavior is due to the diffusive relaxation of strain in the colloidal sample.

Cooperative rearrangement regions and dynamical heterogeneities in colloidal glasses with attractive versus repulsive interactions.

Water-lutidine mixtures permit the interparticle potentials of colloidal particles suspended therein to be tuned, in situ, from repulsive to attractive. We employ these systems to directly elucidate the effects of interparticle potential on glass dynamics in experimental samples composed of the same particles at the same packing fractions. Cooperative rearrangement regions (CRRs) and heterogeneous dynamics are observed in both types of glasses. Compared to repulsive glasses, the attractive glass dynamics are found to be heterogeneous over a wider range of time and length scales, and its CRRs involve more particles. Additionally, the CRRs are observed to be stringlike structures in repulsive glasses and compact structures in attractive glasses. Thus, the experiments demonstrate explicitly that glassy dynamics can depend on the sign of the interparticle interaction.

Dynamic Heterogeneities in Colloidal Supercooled Liquids: Experimental Tests of Inhomogeneous Mode Coupling Theory.

The dynamics in supercooled liquids slow enormously upon approaching the glass transition, albeit without significant change of liquid structure. This empirical observation has stimulated development of many theoretical models which attempt to elucidate microscopic mechanisms in glasses and glass precursors. Here, quasi-two-dimensional colloidal supercooled liquids and glasses are employed to experimentally test predictions of widely used models: mode coupling theory (MCT) and its important extension, inhomogeneous MCT (IMCT). We measure two-point dynamic correlation functions in the glass forming liquids to determine structural relaxation times, τα, and mode coupling exponents, a, b, and γ; these parameters are then used to extract the mode coupling dynamic crossover packing area-fraction, ϕ c. This information, along with our measurements of supercooled liquid spatiotemporal dynamics, permits characterization of dynamic heterogeneities in the samples and facilitates direct experimental tests of the scaling predictions of IMCT. The time scales at which dynamic heterogeneities are largest, and their spatial sizes, exhibit power law growth on approaching ϕ c. Within experimental error, the exponents of the measured power laws are close to the predictions of IMCT.

Correlations between short- and long-time relaxation in colloidal supercooled liquids and glasses.

Spatiotemporal dynamics of short- and long-time structural relaxation are measured experimentally as a function of packing fraction, ϕ, in quasi-two-dimensional colloidal supercooled liquids and glasses. The relaxation times associated with long-time dynamic heterogeneity and short-time intracage motion are found to be strongly correlated and to grow by orders of magnitude with increasing ϕ toward dynamic arrest. We find that clusters of fast particles on the two timescales often overlap, and, interestingly, the distribution of minimum-spatial-separation between closest nonoverlapping clusters across the two timescales is revealed to be exponential with a decay length that increases with ϕ. In total, the experimental observations suggest short-time relaxation events are very often precursors to heterogeneous relaxation at longer timescales in glassy materials.

Hydraulic disruption and passive migration by a bacterial pathogen in oak tree xylem.

Xylella fastidiosa (Xf) is a xylem-limited bacterial pathogen that causes leaf scorch symptoms in numerous plant species in urban, agricultural, and natural ecosystems worldwide. The exact mechanism of hydraulic disruption and systemic colonization of xylem by Xf remains elusive across all host plants. To understand both processes better, the functional and structural characteristics of xylem in different organs of both healthy and Xf-infected trees of several Quercus species were studied. Hydraulic conductivity (K(s)) in Xf-infected petioles of Q. palustris and Q. rubra decreased significantly compared with healthy trees as the season progressed and plummeted to zero with the onset of scorch symptoms. Prior to the onset of symptoms, embolism was as much as 3.7 times higher in Xf-infected petioles compared with healthy controls and preceded significant reductions in K(s). Embolism likely resulted from pit membrane degradation during colonization of new petiole xylem and triggered the process that eventually led to vessel occlusion. Pit membrane porosity was studied using the following four methods to determine if a pathway exists in the xylem network of woody stems that allows for passive Xf migration: (i) calculations based on vulnerability to cavitation data, (ii) scanning electron micrographs, (iii) microsphere injections, and (iv) air seeding thresholds on individual vessels. All four methods consistently demonstrated that large pit membrane pores (i.e. greater than the diameter of individual Xf) occur frequently throughout the secondary stem xylem in several Quercus species. These large pores probably facilitate systemic colonization of the secondary xylem network and contribute to the high susceptibility to bacterial leaf scorch exhibited among these species.

Spreading and fingering in spin coating.

We study the spreading and fingering of drops of silicone oil on a rotating substrate for a range of rotation speeds and drop volumes. The spreading of the drop prior to the onset of fingering is found to follow the theoretically predicted time dependence, but with a large shift in time scale. For the full range of experimental parameters studied, the contact line becomes unstable and fingers develop when the radius of the drop becomes sufficiently large. We study the growth of perturbations around the perimeter of the drop and find the growth rate of the most unstable mode to agree well with the predictions of lubrication theory. The number of fingers which form around the perimeter of the drop is found to be a function of both rotation speed and drop volume, and is also in excellent agreement with theoretical predictions.

Lowering the overall charge on TMPyP4 improves its selectivity for G-quadruplex DNA.

Ligands that stabilize non-canonical DNA structures called G-quadruplexes (GQs) might have applications in medicine as anti-cancer agents, due to the involvement of GQ DNA in a variety of cancer-related biological processes. Five derivatives of 5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP4), where a N-methylpyridyl group was replaced with phenyl (4P3), 4-aminophenyl (PN3M), 4-phenylamidoproline (PL3M), or 4-carboxyphenyl (PC3M and P2C2M) were investigated for their interactions with human telomeric DNA (Tel22) using fluorescence resonance energy transfer (FRET) assay, and UV-visible and circular dichroism spectroscopies in K+ buffer. The molecules are cationic or zwitterionic with an overall charge of 3+ (4P3, PN3M, and PL3M), 2+ (PC3M) or neutral (P2C2M). All porphyrins except P2C2M stabilize human telomeric DNA in FRET assays by ∼20 °C at 5 eq CD melting experiments suggest that 4P3 is the most stabilizing ligand with a stabilization temperature of 16.8 °C at 4 eq. Importantly, 4P3, PC3M and PL3M demonstrate excellent selectivity for quadruplexes, far superior to that of TMPyP4. Binding constants, determined using UV-vis titrations, correlate with charge: triply cationic 4P3, PN3M and PL3M display Ka of 5-9 µM-1, doubly cationic PC3M displays Ka of 1 µM-1, and neutral P2C2M displays weak-to-no binding. UV-vis data suggest that binding interactions are driven by electrostatic attractions and that the binding mode may be base-stacking (or end-stacking) judging by the high values of red shift (15-20 nm) and hypochromicity (40-50%). We conclude that lowering the charge on TMPyP4 to 3+ can achieve the desired balance between stabilizing ability, affinity, and high selectivity required for an excellent quadruplex ligand.

Vibrational properties of quasi-two-dimensional colloidal glasses with varying interparticle attraction.

We measure the vibrational modes and particle dynamics of quasi-two-dimensional colloidal glasses as a function of interparticle interaction strength. The interparticle attractions are controlled via a temperature-tunable depletion interaction. Specifically, the interparticle attraction energy is increased gradually from a very small value (nearly hard-sphere) to moderate strength (∼4k_{B}T), and the variation of colloidal particle dynamics and vibrations are concurrently probed. The particle dynamics slow monotonically with increasing attraction strength, and the particle motions saturate for strengths greater than ∼2k_{B}T, i.e., as the system evolves from a nearly repulsive glass to an attractive glass. The shape of the phonon density of states is revealed to change with increasing attraction strength, and the number of low-frequency modes exhibits a crossover for glasses with weak compared to strong interparticle attraction at a threshold of ∼2k_{B}T. This variation in the properties of the low-frequency vibrational modes suggests a new means for distinguishing between repulsive and attractive glass states.

Fingering instability of a sheet of yield-stress fluid.

We study the fingering instability that occurs at the contact line of a thin sheet of a yield-stress fluid flowing down an incline. We derive an expression for the wavelength of the finger pattern as a function of inclination angle for a Herschel-Bulkley fluid. The wavelength is predicted to diverge at a finite angle which is related to the yield stress of the fluid. We also measure the wavelength of the finger pattern with suspensions of bentonite clay in water. Our experimental results agree well with the theoretical prediction.

Morphology and scaling of impact craters in granular media.

We present the results of experiments on impact craters formed by dropping a steel ball vertically into a container of small glass beads. As the energy of impact increases, we observe a progression of crater morphologies analogous to that seen in craters on the moon. We find that both the diameter and the depth of the craters are proportional to the 1/4 power of the energy. The ratio of crater diameter to rim-to-floor depth is constant for low-energy impacts, but increases at higher energy, similar to what is observed for lunar craters.