Session V47: Focus Session: Heterogeneous Colloids II

Self Assembly of Soft Matter Quasicrystals and Their Approximants

The discovery of soft-matter quasicrystals (QCs) and their approximants [1-4] hints at a unique thermodynamic mechanism underlying their stability. In the past, specific interaction potentials have been contrived to stabilize QCs and their approximants in computer simulations, but such interactions are difficult to achieve in colloidal systems. Here, we use molecular simulation to demonstrate an alternative approach for assembling dodecagonal QCs and their approximants based solely on particle functionalization and shape [5]. Our approach replaces complex energetic interactions with simpler-to-achieve bonded and excluded-volume interactions, encouraging the formation of structures with low surface contact area, including non-close-packed and polytetrahedral structures. We argue that this mechanism can be widely exploited to assemble QCs and approximants in colloidal systems, and may further elucidate the formation of soft matter QCs in experiment [1-4]. \\[4pt] [1] G. Ungar, et al., Science 299 (2003) \\[0pt] [2] X. Zeng, et al., Nature 428, (2004) \\[0pt] [3] S. Lee, M.J. Bluemle, F.S. Bates, Science, 330 (2010) \\[0pt] [4] S. Fischer, et al. Proc. Natl. Acad. Sci., 108, (2011) \\[0pt] [5] C.R. Iacovella, A.S. Keys, S.C. Glotzer, Proc. Natl. Acad. Sci., in press (2011) arXiv:1102.5589   

Self-assembly of anisotropic colloidal particles under confinement

We perform molecular simulations of two novel anisotropic colloidal particles under confinement. The first is an ensemble of hard-hemispheres (mimicking mushroom cap colloids [1]) confined between two parallel walls separated by a distance H. We simulated: a) Low values of H that restrict the rotation of the particles (in a monolayer) where three main dense crystal structures are found: buckled phase, square, and triangular structures. (b) Large values of H where the particles are able to fully rotate; here parallel (spheres-like) or anti-parallel (column-like) dimers are observed which form ordered structures with triangular and rectangular symmetries at high densities. The second system is a model of hard-square particles with rounded corners. Recently, Zhao et. al. have reported the phase behaviour of monolayers of polymeric squares platelets with rounded corners, assembled at the bottom of the container [2]. This system exhibits the formation of a hexagonal rotator phase and the rhombic crystal phase that were not observed in earlier simulations of squares [3] which found instead a tetratic and square crystal phases. By interpolating between hard discs and hard squares, we map out the phase diagram as a function of the roundness of the particles and resolve the discrepancies of the earlier studies. [1] Riley and Liddell, Langmuir 26, 11648 (2010). [2] Zhao, Bruinsma, and Mason, PNAS 108, 2684 (2011). [3] Wojciechowski and Frenkel, Comp. Met. Sci. Technol. 10, 235 (2004).   

Towards Structural Complexity with Colloids

Colloids rather easily assemble into simple crystal structures like the face-centered cubic lattice or the body-centered cubic lattice. More complex phases are harder to achieve, but have recently been reported using a number of approaches. Yet, assembling complex structures often results from trial-and-error and is not well understood. In this presentation, we show how novel crystals, quasicrystals, and liquid crystals can be achieved with colloidal building blocks by varying the interactions and the shapes of the building blocks. Using computer simulations, we demonstrate the formation of unusually ordered phases both with isotropic pair potentials, as well as with facetted shapes like polyhedra. We describe new tools we have developed to perform complex structural analysis on simulated systems and show how they may be used to analyze real space images from colloid experiments. We also compare the assembled structures with densest packings of the building blocks and show that good packings can often be distinct from what is observed to assemble from the disordered state. This suggests that dense packings may not be illustrative of what is achievable in colloid experiments.   

Novel Non-Close-Packed Structures Assembled by Spherical Colloids with Anisotropic Interactions under Electric Field

Spherical colloids with isotropic properties have been used as building blocks to assemble a variety of 2D and 3D structures in past, such as FCC, HCP, and BCT crystals. We recently, however, have observed new type of two-dimensional structures under the influence of electric field at the liquid-solid interface. This is primarily due to anisotropic interactions arising from electric field on both particles and aqueous solution. At low concentrations and low frequencies of the electric field, the isotropic spheres can form a series of colloidal clusters, ranging from 3 to 10. The analysis of cluster distributions shows non-trivial peaks for trimer, tetramer, hexamer, and nanomer. Those clusters can change bond angles freely while maintaining the overall structures intact. At high concentrations, those colloidal clusters with flexible bond angles can further assemble and connect themselves into a good variety of two-dimensional non-close-packed networks that have not been observed before. By precisely controlling the electric field strength, frequency, volume fraction of colloids, and ionic strength, we have made diversified non-close-packed structures that have potential applications in photonic crystal, catalysis, or filtration.   

Self-assembly of polydisperse nanoparticles into monodisperse supraparticles: a computer simulation study

Experiments have shown that polydisperse inorganic nanoparticles such as CdSe, CdS and PbS self-assemble into highly uniform supraparticles with a core-shell morphology[1]. The self-assembly process is believed to be self-limiting as indicated by the time evolution of the measured surface potential and supraparticle size distribution. We performed molecular dynamics simulations to demonstrate that the balance between van der Waals attraction and Coulombic repulsion leads to the self-limiting growth of the supraparticles[2-4]. That the uniform supraparticles are stable over a wide range of density indicates that they are thermodynamically stable, rather than consequences of limited diffusion. Our simulation results further reveal that the broad nanoparticle polydispersity leads to the core-shell morphology of the supraparticles. The generic nature of the governing interactions suggests great versatility in the composition, size and shape of the constituent building blocks, and allows for a large family of hierarchical self-assembled structures, including colloidal crystals. References 1. Y. Xia et al, Nat. Nano. 6, 580, 2011. 2. Gonzalez-Mozuelos et al, J. Chem. Phys. 103, 3145, 1995. 3. Van Hyning et al, Langmuir 17, 3120, 2001. 4. J. Ramsden, Proc. R. Soc. Lond. A 413, 407, 1987   

Particle dynamics in colloidal glasses with short-range attraction

We study colloidal particle dynamics of a model glass system using confocal microscopy as the sample evolves from a repulsive glass towards an attractive glass. Short-range depletion forces induce the transition from a repulsive glass to the attractive glass. We identify particles which exhibit substantial motional events and characterize the transition using the properties of these motional events. It appears that number of particles that exhibit motional events doesn't change as the system is brought from a repulsive glass towards the attractive glass. Also, we investigate vibrational properties of these dense colloidal suspensions. Our preliminary results show that the boson peak for an attractive glass is lower than that for a repulsive system and it is shifted towards higher frequencies. To our knowledge, this is the first experimental investigation of the evolution of vibrational modes in colloidal glasses when particle interaction potential changes from repulsive to attractive.   

Self-assembly of colloidal surfactants

We developed colloidal dumbbells with a rough and a smooth part, based on a method reported in Ref. [1]. Specific attraction between the smooth parts occurs upon addition of non-adsorbing polymers of appropriate size. We present the first results in terms of the assemblies that emerge in these systems. \\[4pt] [1] D.J. Kraft, W.S. Vlug, C.M. van Kats, A. van Blaaderen, A. Imhof and W.K. Kegel, \textit{Self-assembly of colloids with liquid protrusions}, J. Am. Chem. Soc. \textbf{131}, 1182, (2009)   

The Structure of Dipolar Colloidal Gels in the Dilute Regime

Dipolar colloidal systems are one of the simpler material presenting anisotropic interactions and are highly relevant for designing new soft materials easy to control. Using stochastic dynamics simulations we investigate the gelation process in a high dilute system of dipolar colloidal particles immerse in an implicit dielectric solvent. This system self assembles in a rich variety of structures, from open percolated networks or gels to short chains of particles that self cross. We perform simulations using a ``continues model'' for the dipolar particles interactions and we present a phase diagram, density vs temperature. Each phase is characterized in detail as a function of the spatial correlation (clusters) and response. Interestingly at this low density regime, it is possible to find, characterize and follow the dynamics of a clear and huge variety of short strings configurations (cross loops, triple pointed chains, bundles), mainly at very low temperature. In addition, studying the dynamics of gelation in detail we observe an increasing gelation time while density is decreased in good agreement with previous simulations and experiments on colloidal systems with directional interactions.   

Two-Dimensional Crystals of Icosahedral Viruses at Liquid interfaces

Two-dimensional (2D) assembly of turnip yellow mosaic virus (TYMV) on cationic lipid monolayers is investigated at the air-water interface. TYMV, an icosahedral virus with a diameter of 28 nm, exhibits well-defined roughness, charge distribution, and hydrophilic/hydrophobic patches on its surface. The electrostatic attraction to the lipid-coated aqueous interface provides means to impose a specific virus orientation and hence reduce the number of possible inter-particle interactions. The 2D geometry is particularly advantageous in dissecting the role of anisotropy in aqueous-media assembly, which involves various types of similarly weak interactions. We show that the assembly approach used not only facilitates crystallization but also provides insights on how complex anisotropic interactions can be exploited to generate long-range order. Specifically, we report an \emph{in situ} x-ray scattering observation of novel 2D crystal forms of TYMV that reflect the virus' icosahedral symmetry. The symmetry, shape, and surface heterogeneities of TYMV suggest a mechanism by which these crystals are stabilized by a combination of hydrophobic, electrostatic, and steric interactions.   

Active colloids at liquid-liquid interfaces: dynamic self-assembly and functionality

Self-assembled materials must actively consume energy and remain out of equilibrium in order to support structural complexity and functional diversity. Colloids of interacting particles suspended at liquid-liquid interfaces and maintained out of equilibrium by external alternating electromagnetic fields develop nontrivial collective dynamics and self-assembly. We use ferromagnetic colloidal micro-particles (so the magnetic moment is fixed in each particle and interactions between colloids is highly anisotropic and directional) suspended over an interface of two immiscible liquids and energized by vertical alternating magnetic fields to demonstrate novel dynamic and active self-assembled structures (``asters'') which are not accessible through thermodynamic assembly. Structures are attributed to the interplay between surface waves, generated at the liquid/liquid interface by the collective response of magnetic microparticles to the alternating magnetic field, and hydrodynamic fields induced in the boundary layers of $\it{both}$ liquids forming the interface. Two types of magnetic order are reported. We demonstrate that asters develop self-propulsion in the presence of a small in-plane dc magnetic field. We show that asters can capture, transport, and position target microparticles.