Vicsek model by time-interlaced compression: A dynamical computable information density.

Collective behavior, both in real biological systems and in theoretical models, often displays a rich combination of different kinds of order. A clear-cut and unique definition of "phase" based on the standard concept of the order parameter may therefore be complicated, and made even trickier by the lack of thermodynamic equilibrium. Compression-based entropies have been proved useful in recent years in describing the different phases of out-of-equilibrium systems. Here, we investigate the performance of a compression-based entropy, namely, the computable information density, within the Vicsek model of collective motion. Our measure is defined through a coarse graining of the particle positions, in which the key role of velocities in the model only enters indirectly through the velocity-density coupling. We discover that such entropy is a valid tool in distinguishing the various noise regimes, including the crossover between an aligned and misaligned phase of the velocities, despite the fact that velocities are not explicitly used. Furthermore, we unveil the role of the time coordinate, through an encoding recipe, where space and time localities are both preserved on the same ground, and find that it enhances the signal, which may be particularly significant when working with partial and/or corrupted data, as is often the case in real biological experiments.

Light-activated self-propelled colloids.

Light-activated self-propelled colloids are synthesized and their active motion is studied using optical microscopy. We propose a versatile route using different photoactive materials, and demonstrate a multiwavelength activation and propulsion. Thanks to the photoelectrochemical properties of two semiconductor materials (α-Fe2O3 and TiO2), a light with an energy higher than the bandgap triggers the reaction of decomposition of hydrogen peroxide and produces a chemical cloud around the particle. It induces a phoretic attraction with neighbouring colloids as well as an osmotic self-propulsion of the particle on the substrate. We use these mechanisms to form colloidal cargos as well as self-propelled particles where the light-activated component is embedded into a dielectric sphere. The particles are self-propelled along a direction otherwise randomized by thermal fluctuations, and exhibit a persistent random walk. For sufficient surface density, the particles spontaneously form 'living crystals' which are mobile, break apart and reform. Steering the particle with an external magnetic field, we show that the formation of the dense phase results from the collisions heads-on of the particles. This effect is intrinsically non-equilibrium and a novel principle of organization for systems without detailed balance. Engineering families of particles self-propelled by different wavelength demonstrate a good understanding of both the physics and the chemistry behind the system and points to a general route for designing new families of self-propelled particles.

Subdiffusion of a sticky particle on a surface.

Conventional diffusion (ΔR2(t))=2Dt gives way to subdiffusion (ΔR2(t))∼t(µ), 0<µ<1 when the waiting time distribution φ(τ) is nonintegrable. We have studied a model system, colloidal particles functionalized with DNA "sticky ends" diffusing on a complementary coated surface. We observe a crossover from subdiffusive to conventional behavior for (ΔR2(t)) and φ(τ) as temperature is increased near the particle-surface melting temperature consistent with a simple Gaussian distribution of sticky ends. Our results suggest that any system with randomness in its binding energy should exhibit subdiffusive behavior as it unbinds. This will strongly affect the kinetics of self-assembly.

Lock and key colloids.

New functional materials can in principle be created using colloids that self-assemble into a desired structure by means of a programmable recognition and binding scheme. This idea has been explored by attaching 'programmed' DNA strands to nanometre- and micrometre- sized particles and then using DNA hybridization to direct the placement of the particles in the final assembly. Here we demonstrate an alternative recognition mechanism for directing the assembly of composite structures, based on particles with complementary shapes. Our system, which uses Fischer's lock-and-key principle, employs colloidal spheres as keys and monodisperse colloidal particles with a spherical cavity as locks that bind spontaneously and reversibly via the depletion interaction. The lock-and-key binding is specific because it is controlled by how closely the size of a spherical colloidal key particle matches the radius of the spherical cavity of the lock particle. The strength of the binding can be further tuned by adjusting the solution composition or temperature. The composite assemblies have the unique feature of having flexible bonds, allowing us to produce flexible dimeric, trimeric and tetrameric colloidal molecules as well as more complex colloidal polymers. We expect that this lock-and-key recognition mechanism will find wider use as a means of programming and directing colloidal self-assembly.

Diffusion through colloidal shells under stress.

The permeability of solids has long been associated with a diffusive process involving activated mechanism as originally envisioned by Eyring. Tensile stress can affect the activation energy but definitive experiments of the diffusion rate of species through a stressed solid are lacking. Here we use core-shell (liquid core-solid shell) colloidal particles that are sensitive to osmotic pressure to follow the permeation of encapsulated probes at various stresses. We unambiguously show that the tensile stress applied on colloidal shells linearly reduces the local energy barrier for diffusion.

Chiral colloidal clusters.

Chirality is an important element of biology, chemistry and physics. Once symmetry is broken and a handedness is established, biochemical pathways are set. In DNA, the double helix arises from the existence of two competing length scales, one set by the distance between monomers in the sugar backbone, and the other set by the stacking of the base pairs. Here we use a colloidal system to explore a simple forcing route to chiral structures. To do so we have designed magnetic colloids that, depending on both their shape and induced magnetization, self-assemble with controlled helicity. We model the two length scales with asymmetric colloidal dumbbells linked by a magnetic belt at their waist. In the presence of a magnetic field the belts assemble into a chain and the steric constraints imposed by the asymmetric spheres force the chain to coil. We show that if the size ratio between the spheres is large enough, a single helicity is adopted, right or left. The realization of chiral colloidal clusters opens up a new link between colloidal science and chemistry. These colloidal clusters may also find use as mesopolymers, as optical and light-activated structures, and as models for enantiomeric separation.

A 12-week, placebo-controlled study (6002-US-006) of istradefylline in Parkinson disease.

BACKGROUND: The safety and efficacy of istradefylline, a selective adenosine A(2A) receptor antagonist, was evaluated in a 12-week, double-blind study in levodopa-treated Parkinson disease (PD) subjects with motor complications. METHODS: Levodopa-treated PD subjects (n = 395) received istradefylline 20 mg/day (n = 163), istradefylline 60 mg/day (n = 155), or placebo (n = 77) at 40 sites. The primary efficacy variable was the change in the percentage of time per day spent in the OFF state. Secondary measurements assessed change in ON time, Unified Parkinson's Disease Rating Scale, and Clinical Global Impression. Safety monitoring included clinical laboratory, electrocardiograms, vital signs, physical/neurologic examinations, and adverse events (AEs). RESULTS: Changes from baseline to endpoint in the percentage OFF time in the active groups compared with placebo were -4.35% (95% CI -8.16 to -0.54; p = 0.026) for istradefylline 20 mg/day and -4.49% (95% CI -8.35 to -0.62; p = 0.024) for 60 mg/day; these changes were significant (analysis of covariance). For total hours, istradefylline demonstrated mean differences from placebo of -0.64 hours (95% CI -1.30 to 0.01) for 20 mg/day and -0.77 hours (95% CI -1.44 to -0.11) for 60 mg/day (p = 0.065; overall treatment effect). Clinical response occurred by the second week and was maintained throughout the study. Istradefylline was well tolerated. The common AEs were dyskinesia, nausea, dizziness, and hallucinations. CONCLUSIONS: Istradefylline demonstrated a significant reduction in the percentage of awake time per day spent in the OFF state, which resulted in a clinically meaningful reduction in OFF time, without an increase in ON time with troublesome dyskinesia, and was well tolerated as adjunctive treatment to levodopa in Parkinson disease.

A study of the pharmacokinetic interaction of istradefylline, a novel therapeutic for Parkinson's disease, and atorvastatin.

The effect of steady-state istradefylline, an agent for Parkinson's disease with P-glycoprotein and CYP3A inhibitory activity, on the pharmacokinetics of atorvastatin and its metabolites was evaluated in healthy volunteers. A single 40-mg dose of atorvastatin was administered to 20 subjects. After a 4-day washout, subjects received a single 40-mg atorvastatin dose following 40 mg istradefylline (n=16) or placebo (n=4) daily for 14 days. Plasma samples collected for 96 hours after atorvastatin administration, alone and in combination, were analyzed for atorvastatin, orthohydroxy atorvastatin, and parahydroxy atorvastatin. Istradefylline increased atorvastatin C(max) (53%), AUC(0-infinity) (54%), and t((1/2)) (27%); and increased AUC(0-infinity) for orthohydroxy atorvastatin (18%), but had no significant effect on its C(max) or t((1/2)); and had minimal effect on parahydroxy atorvastatin AUC(0-infinity). The lack of inhibition by istradefylline on metabolite systemic exposure, combined with increased atorvastatin systemic exposure, suggests a predominant P-glycoprotein inhibitory effect of istradefylline.

Nematic and almost-tetratic phases of colloidal rectangles.

Nonspherical colloids can exhibit liquid-crystalline phases with different degrees of broken orientational and translational symmetry. Here we investigate hard rectangles consisting of photolithographically prepared disks standing on edge. We observe a conventional Kosterlitz-Thouless transition from isotropic to nematic with almost smectic behavior at high density. But just on the isotropic side of the isotropic to nematic transition we observe an unusual regime where short-range tetratic correlations dominate over nematic correlations. This occurs due to the proliferation of Ising-like pi/2 grain boundaries that disrupt nematic order, but preserve tetratic correlations, at lengths shorter than the spacing between free disclinations.

An electric bottle for colloids.

Particle concentration is a dominant control parameter for colloids and other soft matter systems. We demonstrate a simple technique, "dielectrophoretic equilibrium," implemented as an "electric bottle," a planar capacitor in a larger volume. The uniform field in the capacitor traps particles in this force-free region at a higher density than in the zero field regions outside. We show how the technique measures the equation of state and we initiate and grow colloidal crystals. "Dielectrophoretic equilibria" enable the study of a complete concentration-dependent phase diagram from a single microscopic sample, obviating the previous need for preparing a large number of samples.

Coexistence of superconductivity and antiferromagnetism probed by simultaneous nuclear magnetic resonance and electrical transport in (TMTSF)2PF6 system.

We report simultaneous NMR and electrical transport experiments in the pressure range near the boundary of the antiferromagnetic spin density wave (SDW) insulator and the metallic/superconducting (SC) phase in (TMTSF)2PF6. Measurements indicate a tricritical point separating a line of second-order SDW/metal transitions from a line of first-order SDW/metal(SC) transitions with coexistence of macroscopic regions of SDW and metal(SC) order, with little mutual interaction but strong hysteretic effects. NMR results quantify the fraction of each phase.

Experiments on random packings of ellipsoids.

Recent simulations indicate that ellipsoids can pack randomly more densely than spheres and, remarkably, for axes ratios near 1.25:1:0.8 can approach the densest crystal packing (fcc) of spheres, with a packing fraction of 74%. We demonstrate that such dense packings are realizable. We introduce a novel way of determining packing density for a finite sample that minimizes surface effects. We have fabricated ellipsoids and show that, in a sphere, the radial packing fraction phi(r) can be obtained from V(h), the volume of added fluid to fill the sphere to height h. We also obtain phi(r) from a magnetic resonance imaging scan. The measurements of the overall density phi(avr), phi(r) and the core density phi(0) = 0.74 +/- 0.005 agree with simulations.

Two-dimensional melting transition observed in a block copolymer.

We report the observation of two-dimensional melting in a monolayer film of a sphere-forming diblock copolymer. By annealing in a well-controlled temperature gradient we obtain a complete record of the transition from a low-temperature hexatic phase to a high-temperature liquid in a single experiment. We investigate the temperature dependence of the orientational and translational correlation lengths, as well as of the topological defect density. All evidence suggests that the melting transition is first-order, but correlations in the liquid phase indicate the existence of an underlying second-order transition preempted by the first-order freezing.

Experimental measurement of the photonic properties of icosahedral quasicrystals.

Quasicrystalline structures may have optical bandgap properties-frequency ranges in which the propagation of light is forbidden-that make them well-suited to the scientific and technological applications for which photonic crystals are normally considered. Such quasicrystals can be constructed from two or more types of dielectric material arranged in a quasiperiodic pattern whose rotational symmetry is forbidden for periodic crystals (such as five-fold symmetry in the plane and icosahedral symmetry in three dimensions). Because quasicrystals have higher point group symmetry than ordinary crystals, their gap centre frequencies are closer and the gaps widths are more uniform-optimal conditions for forming a complete bandgap that is more closely spherically symmetric. Although previous studies have focused on one-dimensional and two-dimensional quasicrystals, where exact (one-dimensional) or approximate (two-dimensional) band structures can be calculated numerically, analogous calculations for the three-dimensional case are computationally challenging and have not yet been performed. Here we circumvent the computational problem by doing an experiment. Using stereolithography, we construct a photonic quasicrystal with centimetre-scale cells and perform microwave transmission measurements. We show that three-dimensional icosahedral quasicrystals exhibit sizeable stop gaps and, despite their quasiperiodicity, yield uncomplicated spectra that allow us to experimentally determine the faces of their effective Brillouin zones. Our studies confirm that they are excellent candidates for photonic bandgap materials.

Co-administration of ketoconazole with H1-antagonists ebastine and loratadine in healthy subjects: pharmacokinetic and pharmacodynamic effects.

AIMS: Two studies were conducted to evaluate the effects of coadministration of ketoconazole with two nonsedating antihistamines, ebastine and loratadine, on the QTc interval and on the pharmacokinetics of the antihistamines. METHODS: In both studies healthy male subjects (55 in one study and 62 in the other) were assigned to receive 5 days of antihistamine (ebastine 20 mg qd in one study, and loratadine 10 mg qd in the other) or placebo alone using a predetermined randomization schedule, followed by 8 days of concomitant ketoconazole 450 mg qd/antihistamine or ketoconazole 400 mg qd/placebo. Serial ECGs and blood sampling for drug analysis were performed at baseline and on study days 5 (at the end of monotherapy) and 13 (at the end of combination therapy). QT intervals were corrected for heart rate using the formula QTc = QT/RR(alpha) with special emphasis on individualized alpha values derived from each subject's own QT/RR relationship at baseline. RESULTS: No significant changes in QTc interval from baseline were observed after 5 days administration of ebastine, loratadine or placebo. Ketoconazole/placebo increased the mean QTc (95% CI) by 6.96 (3.31-10.62) ms in the ebastine study and by 7.52 (4.15-10.89) ms in the loratadine study. Mean QTc was statistically significantly increased during both ebastine/ketoconazole administration (12.21 ms; 7.39-17.03 ms) and loratadine/ketoconazole administration (10.68 ms; 6.15-15.21 ms) but these changes were not statistically significantly different from the increases seen with placebo/ketoconazole (6.96 ms; 3.31-10.62 ms), P = 0.08 ebastine study, (7.52 ms; 4.15-10.89 ms), P = 0.26 loratadine study). After the addition of ketoconazole, the mean area under the plasma concentration-time curve (AUC) for ebastine increased by 42.5 fold, and that of its metabolite carebastine by 1.4 fold. The mean AUC for loratadine increased by 4.5 fold and that of its metabolite desloratadine by 1.9 fold following administration of ketoconazole. No subjects were withdrawn because of ECG changes or drug-related adverse events. CONCLUSIONS: Ketoconazole altered the pharmacokinetic profiles of both ebastine and loratadine although the effect was greater for the former drug. The coadministration of ebastine with ketoconazole resulted in a non significant mean increase of 5.25 ms (-0.65 to 11.15 ms) over ketoconazole with placebo (6.96 ms) while ketoconazole plus loratadine resulted in a nonsignificant mean increase of 3.16 ms (-2.73 to 8.68 ms) over ketoconazole plus placebo (7.52 ms). Changes in uncorrected QT intervals for both antihistamines were not statistically different from those observed with ketoconazole alone. The greater effect of ketoconazole on the pharmacokinetics of ebastine was not accompanied by a correspondingly greater pharmacodynamic effect on cardiac repolarization.

77Se NMR probe of magnetic excitations of the magic angle effect in (TMTSF)2PF6.

We report 77Se spin-lattice relaxation rates for (TMTSF)2PF6, carried out in the regime where a set of spectacular transport anomalies known as the "magic angle effects" are observed. In situ resistance measurements (R(zz)) were used to verify the experimental conditions and give precise sample alignment information. We found that the 77Se T-11 exhibits no significant changes as the magnetic-field orientation is rotated through the magic angles, and conclude that there is no evidence for either a single-particle gap or a spin gap. The clearly observed field-induced spin-density wave transition temperature is also, unexpectedly, not enhanced at the magic angles.

Unusually dense crystal packings of ellipsoids.

In this Letter, we report on the densest-known packings of congruent ellipsoids. The family of new packings consists of crystal arrangements of spheroids with a wide range of aspect ratios, and with density phi always surpassing that of the densest Bravais lattice packing phi approximately equal to 0.7405. A remarkable maximum density of phi approximately equal to 0.7707 is achieved for maximal aspect ratios larger than sqrt[3], when each ellipsoid has 14 touching neighbors. Our results are directly relevant to understanding the equilibrium behavior of systems of hard ellipsoids, and, in particular, the solid and glassy phases.

Improving the density of jammed disordered packings using ellipsoids.

Packing problems, such as how densely objects can fill a volume, are among the most ancient and persistent problems in mathematics and science. For equal spheres, it has only recently been proved that the face-centered cubic lattice has the highest possible packing fraction phi=pi/18 approximately 0.74. It is also well known that certain random (amorphous) jammed packings have phi approximately 0.64. Here, we show experimentally and with a new simulation algorithm that ellipsoids can randomly pack more densely-up to phi= 0.68 to 0.71 for spheroids with an aspect ratio close to that of M&M's Candies-and even approach phi approximately 0.74 for ellipsoids with other aspect ratios. We suggest that the higher density is directly related to the higher number of degrees of freedom per particle and thus the larger number of particle contacts required to mechanically stabilize the packing. We measured the number of contacts per particle Z approximately 10 for our spheroids, as compared to Z approximately 6 for spheres. Our results have implications for a broad range of scientific disciplines, including the properties of granular media and ceramics, glass formation, and discrete geometry.

Laterally modulated 2D electron system in the extreme quantum limit.

We report on magnetotransport of a two-dimensional electron system (2DES), located 32 nm below the surface, with a surface superlattice gate structure of periodicity 39 nm imposing a periodic modulation of its potential. For low Landau level fillings nu, the diagonal resistivity displays a rich pattern of fluctuations, even though the disorder dominates over the periodic modulation. Theoretical arguments based on the combined effects of the long-wavelength, strong disorder and the short-wavelength, weak periodic modulation present in the 2DES qualitatively explain the data.

Periodic oscillation of a colloidal disk near a wall in an optical trap.

Colloidal disks can be stably trapped using optical tweezers. However, when the tweezers press the disk against an opposing wall, we observe an instability leading to periodic motion which we model using coupled nonlinear equations. The resulting "switchback" oscillation involves combined orientational and translational motion of the disk. This observation reveals a new degree of freedom in colloidal architectures, that is, the ability to drive translational motion from a static light field energy source.