Ordering of binary colloidal crystals by random potentials.

Structural defects are ubiquitous in condensed matter, and not always a nuisance. For example, they underlie phenomena such as Anderson localization and hyperuniformity, and they are now being exploited to engineer novel materials. Here, we show experimentally that the density of structural defects in a 2D binary colloidal crystal can be engineered with a random potential. We generate the random potential using an optical speckle pattern, whose induced forces act strongly on one species of particles (strong particles) and weakly on the other (weak particles). Thus, the strong particles are more attracted to the randomly distributed local minima of the optical potential, leaving a trail of defects in the crystalline structure of the colloidal crystal. While, as expected, the crystalline ordering initially decreases with an increasing fraction of strong particles, the crystalline order is surprisingly recovered for sufficiently large fractions. We confirm our experimental results with particle-based simulations, which permit us to elucidate how this non-monotonic behavior results from the competition between the particle-potential and particle-particle interactions.

Clustering of Janus particles in an optical potential driven by hydrodynamic fluxes.

Self-organisation is driven by the interactions between the individual components of a system mediated by the environment, and is one of the most important strategies used by many biological systems to develop complex and functional structures. Furthermore, biologically-inspired self-organisation offers opportunities to develop the next generation of materials and devices for electronics, photonics and nanotechnology. In this work, we demonstrate experimentally that a system of Janus particles (silica microspheres half-coated with gold) aggregates into clusters in the presence of a Gaussian optical potential and disaggregates when the optical potential is switched off. We show that the underlying mechanism is the existence of a hydrodynamic flow induced by a temperature gradient generated by the light absorption at the metallic patches on the Janus particles. We also perform simulations, which agree well with the experiments and whose results permit us to clarify the underlying mechanism. The possibility of hydrodynamic-flux-induced reversible clustering may have applications in the fields of drug delivery, cargo transport, bioremediation and biopatterning.

Active matter alters the growth dynamics of coffee rings.

How particles are deposited at the edge of evaporating droplets, i.e. the coffee ring effect, plays a crucial role in phenomena as diverse as thin-film deposition, self-assembly, and biofilm formation. Recently, microorganisms have been shown to passively exploit and alter these deposition dynamics to increase their survival chances under harshening conditions. Here, we show that, as the droplet evaporation rate slows down, bacterial mobility starts playing a major role in determining the growth dynamics of the edge of drying droplets. Such motility-induced dynamics can influence several biophysical phenomena, from the formation of biofilms to the spreading of pathogens in humid environments and on surfaces subject to periodic drying. Analogous dynamics in other active matter systems can be exploited for technological applications in printing, coating, and self-assembly, where the standard coffee-ring effect is often a nuisance.

Disorder-mediated crowd control in an active matter system.

Living active matter systems such as bacterial colonies, schools of fish and human crowds, display a wealth of emerging collective and dynamic behaviours as a result of far-from-equilibrium interactions. The dynamics of these systems are better understood and controlled considering their interaction with the environment, which for realistic systems is often highly heterogeneous and disordered. Here, we demonstrate that the presence of spatial disorder can alter the long-term dynamics in a colloidal active matter system, making it switch between gathering and dispersal of individuals. At equilibrium, colloidal particles always gather at the bottom of any attractive potential; however, under non-equilibrium driving forces in a bacterial bath, the colloids disperse if disorder is added to the potential. The depth of the local roughness in the environment regulates the transition between gathering and dispersal of individuals in the active matter system, thus inspiring novel routes for controlling emerging behaviours far from equilibrium.

Colloidal assemblies of oriented maghemite nanocrystals and their NMR relaxometric properties.

An elevated-temperature polyol-based colloidal-chemistry approach allows for the development of size-tunable (50 and 86 nm) assemblies of maghemite iso-oriented nanocrystals, with enhanced magnetization. (1)H-nuclear magnetic resonance (NMR) relaxometric experiments show that the ferrimagnetic cluster-like colloidal entities exhibit a remarkable enhancement (4-5 times) in transverse relaxivity when compared to that of the superparamagnetic contrast agent Endorem®, over an extended frequency range (1-60 MHz). The marked increase in the transverse relaxivity r2 at a clinical magnetic field strength (∼1.41 T), which is 405.1 and 508.3 mM(-1) s(-1) for small and large assemblies, respectively, makes it possible to relate the observed response to the raised intra-aggregate magnetic material volume fraction. Furthermore, cell tests with a murine fibroblast culture medium confirmed cell viability in the presence of the clusters. We discuss the NMR dispersion profiles on the basis of relaxivity models to highlight the magneto-structural characteristics of the materials for improved T2-weighted magnetic resonance images.

Metallosupramolecular thin films using a tritopic cyclam-based ligand.

We present the preparation and characterization of novel metallosupramolecular thin films on solid substrates. These films incorporate metallosupramolecular polymers based on a tritopic cyclam bis-terpyridine ligand and are formed using different deposition techniques. From layer-by-layer (LBL) method, alternate thin multilayers of this metallosupramolecule along with oppositely charged polyelectrolyte are constructed by electrostatic self-assembly. Using dip-coating method, homogenous monolayers are deposited. The monolayer thickness is controlled by withdraw velocity of substrate. In a direct-assembly approach: Langmuir-Blodgett (LB), well-ordered metallosupramolecular monolayer is achieved by using a Metallosupramolecular Polyelectrolyte Amphiphile Complex (MPAC). The structures of these metallosupramolecular thin films are characterized by X-ray reflectivity (XRR), Atomic Force Microscopy (AFM), Langmuir isotherms, and UV/Vis absorption. The results open access to fabricate novel molecular materials such as sensors and memory devices.

Characterization of strain recovery and "self-healing" in a self-assembled metallo-gel.

We report a self-assembled metallo suprapolymer gel exhibiting remarkable self-healing features. The Ni2BTC metallo suprapolymer gels result from the complexation of Ni(2+) metal ions by a tritopic ligand (bis-terpyridine cyclam) in dimethylformamide (DMF) and an annealing step at 50 °C for 24 hours. The self-healing properties are characterized by visual inspection, rheological and impedance spectroscopy measurements: the results are compared with those of a fatty acid-based molecular organogel chosen as a reference system. The creep-recovery analysis uses the Burgers model for low strains and characterizes a recovery capability of up to 72% of the deformation in Ni2BTC gels while it is only 32% for the fatty acid organogel. At very large strains, the impedance spectroscopy confirms the slow repairing process consistently with the visual observations. Rheological measurements demonstrate the restructuring of the fractured networks. The fatigue of the self-healed gel networks undergoing long sequences of strain-relaxation steps is characterized.