Colloidal analogs of molecular chain stoppers

Self-assembly of nanoparticles in polymer-like chains bears a strong similarity to polymerization reactions, in which monomer units are brought together by directional noncovalent interactions. Based on this similarity, the molecular concepts of polymer chemistry can be applied to achieve controllable nanoparticle assembly. On the other hand, the ability to visualize nanoparticle assemblies and to exploit characterization tools used in nanoscience offers a unique way to study polymerization reactions. Here we explore this twofold strategy for an exemplary system including the self-assembly of bifunctional metal nanorods in the presence of monofunctional nanoparticles (chain stoppers). The approach provided insight into the polymerization kinetics, side reactions, the distribution of species in the system, and the design rules for the synthesis of molecular chain stoppers

Step-Growth Polymerization of Inorganic Nanoparticles

Self-organization of nanoparticles is an efficient strategy for producing nanostructures with complex, hierarchical architectures. The past decade has witnessed great progress in nanoparticle self-assembly, yet the quantitative prediction of the architecture of nanoparticle ensembles and of the kinetics of their formation remains a challenge. We report on the marked similarity between the self-assembly of metal nanoparticles and reaction-controlled step-growth polymerization. The nanoparticles act as multifunctional monomer units, which form reversible, noncovalent bonds at specific bond angles and organize themselves into a colloidal polymer. We show that the kinetics and statistics of step-growth polymerization enable a quantitative prediction of the architecture of linear, branched, and cyclic self-assembled nanostructures; their aggregation numbers and size distribution; and the formation of structural isomers

Time-Resolved Studies of Stick-Slip Friction in Sheared Granular Layers

Sensitive and fast force measurements are performed on sheared granular layers undergoing stick-slip motion, along with simultaneous imaging. A full study has been done for spherical particles with a +-20% size distribution. Stick-slip motion due to repetitive fluidization of the layer occurs for low driving velocities. Between major slip events, slight creep occurs that is variable from one event to the next. The effects of changing the stiffness k and velocity V of the driving system are studied in detail. The stick-slip motion is almost periodic for spherical particles over a wide range of parameters, but becomes irregular when k is large and V is relatively small. At larger V, the motion becomes smoother and is affected by the inertia of the upper plate bounding the layer. Measurements of the period T and amplitude A of the relative motion are presented as a function of V. At a critical value Vc, a transition to continuous sliding motion occurs that is discontinuous for k not too large. The time dependence of the instantaneous velocity of the upper plate and the frictional force produced by the granular layer are determined within individual slipping events. The force is a multi-valued function of the instantaneous velocity, with pronounced hysteresis and a sudden drop prior to resticking. Measurements of vertical displacement reveal a small dilation of the material (about one tenth of the mean particle size in a layer 20 particles deep) associated with each slip event. Finally, optical imaging reveals that localized microscopic rearrangements precede (and follow) each slip event. The behavior of smooth particles is contrasted with that of rough particles.Comment: 20, pages, 17 figures, to appear in Phys. Rev.

Boundary Lubrication: Squeeze-out Dynamics of a Compressible 2D Liquid

The expulsion dynamics of the last liquid monolayer of molecules confined between two surfaces has been analyzed by solving the two-dimensional (2D) Navier-Stokes equation for a compressible liquid. We find that the squeeze-out is characterized by the parameter g0 ~ P0/(rho c^2), where P0 is the average perpendicular (squeezing) pressure, rho the liquid (3D) density and c the longitudinal sound velocity in the monolayer film. When g0 << 1 the result of the earlier incompressible treatment is recovered. Numerical results for the squeeze-out time, and for the time-dependence of the radius of the squeezed-out region, indicate that compressibility effects may be non-negligible both in time and in space. In space, they dominate at the edge of the squeeze-out region. In time, they are strongest right at the onset of the squeeze-out process, and just before its completion.Comment: revtex4, 6 pages, 4 figures. Published on PRB on December 31, 200

Manipulation of Signaling Thresholds in “Engineered Stem Cell Niches” Identifies Design Criteria for Pluripotent Stem Cell Screens

In vivo, stem cell fate is regulated by local microenvironmental parameters. Governing parameters in this stem cell niche include soluble factors, extra-cellular matrix, and cell-cell interactions. The complexity of this in vivo niche limits analyses into how individual niche parameters regulate stem cell fate. Herein we use mouse embryonic stem cells (mESC) and micro-contact printing (µCP) to investigate how niche size controls endogenous signaling thresholds. µCP is used to restrict colony diameter, separation, and degree of clustering. We show, for the first time, spatial control over the activation of the Janus kinase/signal transducer and activator of transcription pathway (Jak-Stat). The functional consequences of this niche-size-dependent signaling control are confirmed by demonstrating that direct and indirect transcriptional targets of Stat3, including members of the Jak-Stat pathway and pluripotency-associated genes, are regulated by colony size. Modeling results and empirical observations demonstrate that colonies less than 100 µm in diameter are too small to maximize endogenous Stat3 activation and that colonies separated by more than 400 µm can be considered independent from each other. These results define parameter boundaries for the use of ESCs in screening studies, demonstrate the importance of context in stem cell responsiveness to exogenous cues, and suggest that niche size is an important parameter in stem cell fate control

Photonic pseudo-gap-based modification of photoluminescence from CdS nanocrystal satellites around polymer microspheres in a photonic crystal

We report the combination of microsphere self-organization to form a photonic crystal, providing spectrally and angularly dependent electromagneticstructural resonances; with nanocrystal growth in situ on microsphere surfaces, providing optical functionalization with spectral control achieved through the quantum size effect. We demonstrate this material system using CdSnanocrystals coating the surfaces of poly(methyl methacrylate)–poly(methacrylic acid) (PMMA–PMAA) micrometer spheres. The in situ synthesis of the CdSnanocrystals on the surface of the PMMA/PMAA microspheres preserves the propensity of the hybrid microspheres to form ordered colloid arrays. Luminescence from surface states ensures that light is emitted at energies significantly below the absorption edge of the emitting species. Transmission and photoluminescence measurements reveal the interaction of the photonic stop band with photoluminescence from the nanocrystals

Nonlinear optical figures of merit of processible composite of poly(2-methoxy,5-(2′-(ethyl)hexyloxy)-p-phenylene vinylene) and poly(methyl methacrylate)

We report ultrafast nonlinear optical figures of merit for a highly processible guest–host blend of poly(2-methoxy,5-(2′-(ethyl)hexyloxy)-p-phenylene vinylene) with poly(methyl methacrylate). Our experiments employ 120 fs pulses at 840 nm and are designed to eliminate slow thermal nonlinearity and focus exclusively on ultrafast electronic nonlinearity. We report a two-photon absorption coefficient β of 1.5±0.21.5±0.2 cm/GW, a nonlinear refraction coefficient n2n2 of −(2.1±0.2)×10−13−(2.1±0.2)×10−13 cm2/Wcm2/W and a two-photon figure of merit T of 0.6. The blend hybridizes the desirable features of nonlinearity and processibility of its two constituents to provide (1) ultrafast response one to two orders of magnitude faster than achievable in electronic switching devices; (2) a two-photon figure of merit compatible with harnessing this nonlinearity in a practical optical device geometry; and (3) a materials system and processing methodology compatible with spin-coating and photopatterning in an ambient environment