Session T47: Focus Session: Heterogeneous Colloids I

Janus and Multiblock Colloids

This talk surveys emerging areas opened up by the directional interactions presented by the specially-designed spheres known as Janus. The ``diblock'' motif, mutually attractive on one domain and repulsive on another, makes this a prototypical system for elucidating, on a mechanistic level, how concepts of chemical reaction kinetics explain the development of stable and highly ordered nonequilibrium structures. With the ``triblock'' motif, spheres that attract one another on two polar regions but repel at the middle band, we go beyond this to demonstrate the self-assembly of a useful low-density lattice of spheres, the colloidal Kagome lattice, and visualize its aqueous assembly dynamics on the single-particle level. A newer area of opportunity is ``dynamic self-assembly,'' in which energy fed into the system as a control variable generates big surprises. The generalization of these design rules will be discussed.   

Synchronized Dancing of Magnetic Janus Particles

With Janus particle coated with magnetic material on one side, we demonstrate here first a complicated dance of single particle under external field. Then we show various quasi-one dimensional and two dimensional assembly from synchronized motion of a collection of these microspheres. With combination of experiment and simulation, we show how they are phase-locked in motion and self-organize into unexpected structures. By exploring a range of parameters, we also demonstrate fine control of the phase behavior of such dynamic self-assembled materials. We provide an vivid example here how synchronization in both time and space could lead to a new class of responsive materials.   

Hierarchical assemblies and cluster growth regimes of bipolar Janus nanoparticles: effect of particle characteristics

Numerous current and potential applications have motivated fundamental understanding of self assembly of colloidal Janus particles (JP). Experimental studies on nano and micron sized JPs have demonstrated a plethora of simple and complex structures. However, computational approaches to date have lacked the sophistication required to capture the rich free energy landscape of suspension of JPs especially for nanoscale particles and hence have been unable to elucidate the underlying principles that govern their complex self assembly. In this study, molecular dynamic simulation of a restricted primitive model, which also includes long range columbic interaction, has been performed in order to elucidate the underlying physics in the self assembly of bipolar JP at different surface charge density (0.2$\sim $1.3 e/nm2) , salt concentration(0$\sim $3 mM) and particle sizes. Our results clearly indicate formation of two distinct sub structures in very low JP concentration, namely: strings and rings. As the concentration of JP increases these sub structures joins and/or hierarchically assemble into larger clusters. Furthermore, the interconnection between the ionic cloud around a single JP and sequential cluster growth in JPS as a function of surface charge density, particle size and steric hindrance of surface ions has been elucidated.   

Stability of helical Janus clusters

Recent experimental and computational work has elucidated the importance of kinetic pathways in the formation of helical structures by hydrophobic-charged Janus particles.\footnote{Q. Chen, J.K. Whitmer, \emph{et al.}, Science \textbf{331}, 199 (2011).} Motivated by these findings, we perform free-energy calculations to investigate the equilibrium structure and relative stability of helical aggregates as a function of cluster size and Janus balance. These results simultaneously aid in the interpretation of experimental observations and in the design of building blocks for specific structures.   

Hierarchical Self-assembly of Triblock Janus Spheres

We show how monodisperse triblock Janus spheres in aqueous suspension, whose poles are attractive and middle band repulsive, self-assemble into hierarchical supracolloidal structures, on two sequential levels. Based upon the delicate dependence of interactions on ionic strength, we first activate attraction between larger patches, obtaining finite-sized 3D clusters. These clusters, now concrete objects themselves, are triggered later to be linked through topologically determined orientations. A family of unprecedented, complex structures is produced, with order over multiple length scales.   

Janus particles at fluid-fluid interfaces: the third face of Janus particles

Janus spheres are asymmetric particles with polar and apolar hemispheres. In this work, we study the interactions and assembly of Janus spheres -- bubbles and solid particles -- at fluid-fluid interfaces. Both the Janus bubbles and the Janus particles have strikingly different interfacial behaviour compared to their homogeneous counterparts. Janus spheres at a fluid-fluid interface interact with each other via long-ranged attractions. We show that the attractive interactions between interface-trapped Janus spheres are induced by the presence of diffuse boundary between the two hemispheres. Three phase contact line anchored around the rugged Janus boundary deforms that the fluid interface leading to attractive interactions between the spheres. The orientation and fluid deformation caused by Janus spheres are directly observed using a gel trapping method. We also show that the surface chemistry of Janus spheres plays a critical role in determining their interfacial behaviour. Potential implications of the observed long-range attractions will be discussed.   

Self Assembly of Hard, Space-Filling Polytopes

The thermodynamic behavior of systems of hard particles in the limit of infinite pressure is known to yield the densest possible packing [1,2]. Hard polytopes that tile or fill space in two or three spatial dimensions are guaranteed to obtain packing fractions of unity in the infinite pressure limit. Away from this limit, however, other structures may be possible [3]. We present the results of a simulation study of the thermodynamic self-assembly of hard, space-filling particles from disordered initial conditions. We show that for many polytopes, the infinite pressure structure readily assembles at intermediate pressures and packing fractions significantly less than one; in others, assembly of the infinite pressure structure is foiled by mesophases, jamming and phase separation. Common features of these latter systems are identified and strategies for enhancing assembly of the infinite pressure structure at intermediate pressures through building block modification are discussed.\\[4pt] [1] P. F. Damasceno, M. Engel, S.C. Glotzer arXiv:1109.1323v1 [cond-mat.soft]\\[0pt] [2] A. Haji-Akbari, M. Engel, S.C. Glotzer arXiv:1106.4765v2 [cond-mat.soft]\\[0pt] [3] U. Agarwal, F.A. Escobedo, {\em Nature Materials} {\bf 10}, 230--235 (2011)   

Self Assembly of Colloids

We are exploring the self assembly of colloidal matter using building blocks with complex shapes and functionalities. Our toolbox includes particles with tunable cavities and protrusions, particles with flexible ball-and-socket joints, colloidal cubes and particles with magnetic patches. Using these building blocks and a variety of interactions, including chemical, steric, magnetic and lock-and-key shape recognition, we aim to develop new assembly schemes to build structures with a reconfigurable structural arrangement.   

Surface Roughness directed Self-Assembly of Patchy Particles into Colloidal Micelles

Self-assembly of colloidal particles into larger structures bears potential for creating materials with unprecedented properties, such as full photonic band gaps in the visible spectrum. Colloidal particles with site-specific directional interactions, so called ``patchy particles,'' are promising candidates for bottom-up assembly routes towards such complex structures with rationally designed properties. Here we present an experimental realization of patchy colloidal particles based on material independent surface roughness specific depletion interactions. Smooth patches on rough colloids are shown to be exclusively attractive due to their different overlap volumes. We discuss in detail the case of colloids with one patch that serves as a model for molecular surfactants both with respect to their geometry and their interactions. These one-patch particles assemble into clusters that resemble surfactant micelles. We term these clusters ``colloidal micelles.'' Similarities as well as differences between the colloidal model system and molecular surfactants are discussed and quantified by employing computational and theoretical models.   

Entropic selection of patchy particle assemblies

We explore the statistical physics of triblock patchy colloids with degenerate valency, i.e., particles with excessively large attractive areas on two opposite sides, via analytic theory, which has the advantage of revealing fundamental laws of self-assembly more easily compared to the usual ``trial-and-error'' approach taken in experiments and simulations. From calculations of the free energy of multiple possible ground states, we conclude that the rotational entropy of individual particles favors certain bond angles, whereas vibrational entropy favors open rather than close-packed structures. Our analytic calculation is readily generalizable to other types of patchy particles and provides guidelines for new designs. We conclude that whereas the seemingly unfavorable degenerate valency of patchy colloids can lead to multiple energetic ground states and difficulties in selecting a unique ordered state, it also opens doors to surprising ordering phenomena. This shares pleasing commonalities with the ``order-from-disorder'' effect in frustrated magnets.   

Assembling Colloidal Clusters from Spherical Codes

Anisotropic building blocks assembled from colloidal particles are attractive building blocks for self-assembled materials because their complex interactions can be exploited to drive self-assembly. In this work we consider the thermodynamically driven self-assembly of terminal clusters of particles. We predict that clusters related to spherical codes, a mathematical sequence of points, can be synthesized via self-assembly. These anisotropic clusters, which derive from packing solutions of spheres around a sphere, can be tuned to different anisotropies via the ratio of sphere diameters and temperature. Structural and dynamical analysis of these tiny systems reveal rich and sometimes surprising properties.