Self-assembly of highly ordered conjugated polymer aggregates with long-range energy transfer

Applications of conjugated polymers (CP) in organic electronic devices such as light-emitting diodes and solar cells depend critically on the nature of electronic energy transport in these materials^{1, 2, 3, 4, 5}. Single-molecule spectroscopy has revealed their fundamental properties with molecular detail^{6, 7, 8, 9, 10, 11, 12, 13, 14}, and recent reports suggest that energy transport in single CP chains can extend over extraordinarily long distances of up to 75 nm (refs 13, 15, 16). An important question arises as to whether these characteristics are sustained when CP chains agglomerate into a neat solid². Here, we demonstrate that the electronic energy transport in aggregates composed of tens of polymer chains takes place on a similar distance scale as that in single chains. A recently developed molecular-level understanding of solvent vapour annealing has allowed us to develop a technique to control the CP agglomeration process¹⁷. Aggregates with volumes of at least 45,000 nm³ (molecular weight ≈ 21 MDa) maintain a highly ordered morphology and show pronounced fluorescence blinking behaviour, indicative of substantially long-range energy transport. Our findings provide a new lens through which the ordering of single CP chains and the evolution of their morphological and optoelectronic properties can be observed, which will ultimately enable the rational design of improved CP-based devices.

Controlling Water Capture of Bioinspired Fibers with Hump Structures

Geometrically engineered thin fibers that feature introduced hump structures similar to wetted spider capture silk greatly improve the adhesive ability to drops than uniform ones, which is attributed to an unusual three-phase contact line that extends axially along the fibers. The hump structures improve the stability of the contact line through a combination of “slope” and “curvature” effects, which creates sufficient capillary adhesion to pin drops.

Multiplexing superparamagnetic beads driven by multi-frequency ratchets

Here, we explore the single particle dynamics of superparamagnetic beads exposed to multifrequency ratchets. Through a combination of theory, simulation, and experiment, we determine the important tuning parameters that can be used to implement multiplexed separation of polydisperse colloidal mixtures. In particular, our results demonstrate that the ratio of driving frequencies controls the transition between open and closed trajectories that allow particles to be transported across a substrate. We also demonstrate that the phase difference between the two frequencies controls not only the direction of motion but also which particles are allowed to move within a polydisperse mixture. These results represent a fundamentally different approach to colloidal separation than the previous methods which are based on controlling transitions between phase-locked and phase-slipping regimes, and have a higher degree of multiplexing capabilities that can benefit the fields of biological separation and sensing as well as provide crucial insights into general ratchet behavior.

Does size matter? Elasticity of compressed suspensions of colloidal- and granular-scale microgels


We investigate the mechanics of dense packing of very small, colloidal-scale, and larger, granular-scale microgel particles. At low particle concentration, thermally induced Brownian motion of the particles is important for the colloidal-scale systems; in contrast, such Brownian motion is irrelevant at particle packing fractions beyond jamming. As a consequence, colloidal and granular systems behave very similarly under these conditions. At sufficiently high compression of the microgel particles, their polymeric nature sets the scale of the osmotic pressure and shear modulus of the whole packing, in direct analogy with macroscopic, continuous polymer gels. This observation suggests that the particulate nature of microgels is inconsequential for their linear elasticity in a highly packed state. In contrast, the particulate nature of the microgels does become essential when the packed suspensions are forced to yield and flow; here, the differences between colloidal- and granular-scale particles are marked.

Rate-dependent interaction between thin films and interfaces during micro/nanoscale transfer printing




This paper describes the mechanism of the rate-dependent adhesion for a transfer printing system. A cohesive element model accounting for the viscoelastic effect of the stamp reveals the evolution of the stress and the stress distributions as the peeling velocity increases. Experiments using Si ribbons separating from a wafer verify this prediction and demonstrate the influence of the rate-dependent effect on the transfer efficiency. The simulations show the relaxation time of the stamp has little effect on the transfer efficiency, and implies that it can be improved by adjusting the viscoelastic modulus.

Stability and non-linear response of 1D microfluidic-particle streams




In this paper, we study the dynamic response of 1D microfluidic-droplet streams to finite-amplitude longitudinal perturbations and demonstrate experimentally that the excitation of localized jams results in the propagation of shock waves. The shock velocity is shown to vanish as the average particle density approaches a critical value thereby leaving long-lived disturbance in the spatial organization of the streams. Using a gradient expansion of the hydrodynamic coupling between the advected particles, we then theoretically derive the non-linear constitutive equation relating particle current to particle density, and show that it leads to the Burgers equation for the droplet stream density.

Three-Dimensional Paper Microfluidic Devices Assembled Using the Principles of Origami

We report a method, based on the principles of origami (paper folding), for fabricating three-dimensional (3-D) paper microfluidic devices. The entire 3-D device is fabricated on a single sheet of flat paper in a single photolithographic step. It is assembled by simply folding the paper by hand. Following analysis, the device can be unfolded to reveal each layer. The applicability of the device to chemical analysis is demonstrated by colorimetric and fluorescence assays using multilayer microfluidic networks.

Optical Effects of Special Relativity

I thought this was interesting.  A bit strange, but interesting.

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 pairs¹. 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 structures², and as models for enantiomeric separation.

Spatially controlled simultaneous patterning of multiple growth factors in three-dimensional hydrogels

Three-dimensional (3D) protein-patterned scaffolds provide a more biomimetic environment for cell culture than traditional two-dimensional surfaces, but simultaneous 3D protein patterning has proved difficult. We developed a method to spatially control the immobilization of different growth factors in distinct volumes in 3D hydrogels, and to specifically guide differentiation of stem/progenitor cells therein. Stem-cell differentiation factors sonic hedgehog (SHH) and ciliary neurotrophic factor (CNTF) were simultaneously immobilized using orthogonal physical binding pairs, barnase–barstar and streptavidin–biotin, respectively. Barnase and streptavidin were sequentially immobilized using two-photon chemistry for subsequent concurrent complexation with fusion proteins barstar–SHH and biotin–CNTF, resulting in bioactive 3D patterned hydrogels. The technique should be broadly applicable to the patterning of a wide range of proteins.

Multifunctional, Biocompatible Supramolecular Hydrogelators Consist Only of Nucleobase, Amino Acid, and Glycoside

The integration of nucleobase, amino acid, and glycoside into a single molecule results in a novel class of supramolecular hydrogelators, which not only exhibit biocompatibility and biostability but also facilitate the entry of nucleic acids into cytosol and nuclei of cells. This work illustrates a simple way to generate an unprecedented molecular architecture from the basic biological building blocks for the development of sophisticated soft nanomaterials, including supramolecular hydrogels.

A Photoactivated Artificial Muscle Model Unit: Reversible, Photoinduced Sliding of Nanosheets

A novel photoactivated artificial muscle model unit is reported. Here we show that organic/inorganic hybrid nanosheets reversibly slide horizontally on a giant scale and the interlayer spaces in the layered hybrid structure shrink and expand vertically by photoirradiation. The sliding movement of the system on a giant scale is the first example of an artificial muscle model unit having much similarity with that in natural muscle fibrils. In particular, our layered hybrid molecular system exhibits a macroscopic morphological change on a giant scale (1500 nm) relative to the molecular size of 1 nm by means of a reversible sliding mechanism.

A micromechanical model to predict the flow of soft particle glasses

Soft particle glasses form a broad family of materials made of deformable particles, as diverse as microgels¹, emulsion droplets², star polymers³, block copolymer micelles and proteins⁴, which are jammed at volume fractions where they are in contact and interact via soft elastic repulsions. Despite a great variety of particle elasticity, soft glasses have many generic features in common. They behave like weak elastic solids at rest but flow very much like liquids above the yield stress. This unique feature is exploited to process high-performance coatings, solid inks, ceramic pastes, textured food and personal care products. Much of the understanding of these materials at volume fractions relevant in applications is empirical, and a theory connecting macroscopic flow behaviour to microstructure and particle properties remains a formidable challenge. Here we propose a micromechanical three-dimensional model that quantitatively predicts the nonlinear rheology of soft particle glasses. The shear stress and the normal stress differences depend on both the dynamic pair distribution function and the solvent-mediated EHD interactions among the deformed particles. The predictions, which have no adjustable parameters, are successfully validated with experiments on concentrated emulsions and polyelectrolyte microgel pastes, highlighting the universality of the flow properties of soft glasses. These results provide a framework for designing new soft additives with a desired rheological response.

A Self-Quenched Defect Glass in a Colloid-Nematic Liquid Crystal Composite

Colloidal particles immersed in liquid crystals frustrate orientational order. This generates defect lines known as disclinations. At the core of these defects, the orientational order drops sharply. We have discovered a class of soft solids, with shear moduli up to 10⁴ pascals, containing high concentrations of colloidal particles (volume fraction ) directly dispersed into a nematic liquid crystal. Confocal microscopy and computer simulations show that the mechanical strength derives from a percolated network of defect lines entangled with the particles in three dimensions. Such a “self-quenched glass” of defect lines and particles can be considered a self-organized analog of the “vortex glass” state in type II superconductors.

Janus Particles



Janus particles, named after the double-faced Roman god, are compartmentalized colloids with two sides of different chemistry or polarity. This makes them a unique class of materials among micron- or nanosized particles. Due to this special non-centrosymmetric feature in the particle architecture, the synthesis of Janus particles remained challenging for a long time. However, major progress concerning their preparation in useful amounts has been achieved in recent years. The interest in these particles arises from their fascinating hierarchical superstructures in solution and from the fact that demanding problems in materials science, biomedicine and in the field of highly specific sensors can be tackled with this class of particles with advanced properties. This article highlights preparation pathways, self-assembly processes and various pursued applications.

On measuring colloidal volume fractions

Hard-sphere colloids are popular as models for testing fundamental theories in condensed matter and statistical physics, from crystal nucleation to the glass transition. A single parameter, the volume fraction (ϕ), characterizes an ideal, monodisperse hard-sphere suspension. In comparing experiments with theories and simulation, researchers to date have paid little attention to likely uncertainties in experimentally-quoted ϕ values. We critically review the experimental measurement of ϕ in hard-sphere colloids, and show that while statistical uncertainties in comparing relative values of ϕ can be as low as 10⁻⁴, systematic errors of 3% are probably unavoidable. The consequences of this are illustrated by way of a case study comparing literature data sets on hard-sphere viscosity and diffusion.

3D printing breaks out of its mold

Medical implants, injection-molding tools, and aircraft custom parts are just a few of the products being manufactured with technology once reserved for prototyping.