Stirring supercooled colloidal liquids at the particle scale.

We study the decay of tangential velocity profiles with distance from a local disturbance in hard-sphere colloidal suspensions as the colloidal glass transition is approached. The disturbance, generated by a dimer of superparamagnetic particles rotated by an external magnetic field, enables a precise characterization of the system's response through confocal microscopy and tracking of individual particle dynamics. The tangential velocity profiles exhibit nearly exponential decay with distance.

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 and the orifice width : ∼ (/ - ), where is the average particle diameter, is an offset where ∼ 0, the power-law scaling exponent = - 1/2, and 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 . We carry out computational studies of hopper flows of deformable particles with both kinetic friction and background fluid dissipation in two and three dimensions.

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.

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).

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.