Session H54: Aspherical Particles in Soft Matter Self-Assembly and Granular Matter II

Extreme refractive index wing scale beads cause the bright colors of pierid butterflies

Common pieird butterflies are brightly colored, ranging from white to red, caused by various pterin pigments concentrated in scattering ellipsoidal beads arranged in the wing scales. Given the sparsity of the beads in the wing scales, the high brightness suggests a scattering strength of the beads that significantly surpasses that of typical cuticular chitin beads. We have analyzed the optical signature of the pierids highly saturated pigmentary colors by using Jamin-Lebedeff interference microscopy combined with Kramers-Kronig theory and light scattering modeling to show that extreme pterin pigment concentrations cause a very high refractive index of the beads with values above 2 across the visible wavelength range. The special arrangment of ellipsoids and their chemical composition results in a highly light scattering medium.   

Tunable 1D supramolecular architectures constructed via solution and chemical assembly of model coiled coil peptides

Coiled coils present a diverse toolbox for constructing robust biomaterials. Computational design of coiled coil forming peptides further enables their use as designer ‘molecular Legos’ to construct new materials with tunable nanostructure. We employ computationally designed coiled coil peptides that form stable antiparallel homotetramers to build novel supramolecular architectures. Specifically, by end-functionalizing peptides with complimentary ‘click’ reactive groups, coiled coils were covalently linked to form 1D chains of desired length and flexibility: a short linker between them yielded rigid rods while a long linker yielded flexible fibers. Small angle neutron scattering was used to characterize their nanostructure under different solution conditions. Polarized optical microscopy of concentrated 100 nm long rigid rods showed formation of liquid crystal phases. Interestingly, the phase behavior depended on salt concentration, rod length and the design of peptides used to construct them. This has given us valuable insight into the physics of their macroscopic assembly and is further informing future designs of more complex coiled-coil based nanomaterials.   

Electrical and Optical Anisotropy in Flow-aligned Silver Nanowires

We prepare semi-transparent coatings consisting of silver nanowire (AgNW) networks with various degrees of alignment and study how the alignment of nanowires affects the electrical and optical anisotropy of the coatings. Specifically, we utilize the rapid flow of AgNW suspension through a confined geometry to prepare a layer of AgNW network on glass substrates and are able to control the degree of alignment by varying the flow rate. The angle-dependent sheet resistance of the coatings is measured, and large anisotropy in surface conductivity is found to characterize the aligned AgNW networks. We also find angle-dependent intensity and spectrum for transmitted, reflected, and scattered visible light by aligned nanowire coatings, resulting in different transparencies and colors depending on the orientation relative to the polarized light source.   

Structure and Dynamics of pH-Responsive Nanoparticle Assembly at Oil-Water Interfaces

Responsive core-shell particles can vary their adsorption behavior and assembly structure at fluid interfaces in response to changes in environmental conditions, which benefits many applications including petroleum processes and drug delivery. Using a dissipative particle dynamics model with long-range electrostatic forces included, we discover the microstructure and dynamics of polyelectrolyte-grafted nanoparticle monolayers formed at a planar oil-water interface. The in-plane structure of the monolayer is analyzed by the structure factor, Voronoi diagram, and bond orientational order parameter. It shows a disorder-to-order phase change induced by the electrostatic forces as the degree of ionization of grafted polyelectrolytes increase. The dynamics of the monolayer is quantified by velocity autocorrelation function and the collective behavior is examined by mean orientation of particle velocity vectors. Due to randomly grafted chains, the particles do not experience isotropic electrostatic forces and defects are observed accordingly. We thus vary the polymer grafting density to probe the effect of particle anisotropy on the monolayer structure. Our findings can provide insight into controlling the structure and stability of PGNP monolayer by tuning the pH of the solution.   

The Role of Repulsion in Colloidal Crystal Engineering with DNA

By systematically using the co-assembly of DNA-conjugated proteins and spherical gold nanoparticles (AuNPs) as a model system, we explore how steric repulsion between non-complementary, neighboring DNA-NPs due to overlapping DNA shells can influence their ligand-directed behavior. Specifically, our experimental data coupled with coarse-grained molecular dynamics simulations reveal that by changing factors related to NP repulsion, two structurally distinct outcomes can be achieved. When steric repulsion between DNA-AuNPs is significantly greater than that between DNA-proteins, a lower packing density crystal lattice is favored over the structure that is predicted by design rules based on DNA-hybridization considerations alone. This is enabled by the large difference in DNA density on AuNPs versus proteins and can be tuned by modulating the flexibility, and thus conformational entropy, of the DNA on the constituent particles. Such lattices are shown to undergo dynamic reorganization by changing salt concentration. This work help to elucidate the structural considerations necessary for understanding repulsive forces in DNA-assembly and lay the groundwork to increase architectural diversity in engineering colloidal crystals.   

Geometric Cohesive Granular Hexapods

A heap of dry sand collapses after being released from a bucket, because cohesion is necessary to maintain stability for convex particles. However, non-cohesive, elongated or non-convex granular `building blocks’ can form stable structures. Particles such as rods and staples are known to be geometrically cohesive, demonstrating that microscopic cohesive at contacts is not a necessary condition for effective macroscopic cohesion. Here, we show that non-cohesive and inherently non-convex hexapods have macroscopic cohesion. The hexapods are plastic particles that consist of three orthogonal sphero-cylinders. The cross sectional diameter of the sphero-cylinders is 3 mm and the length varies from 10 to 60 mm. We pack particles inside a pair of tubes and perform direct shear tests by displacing the cylinders. Data for the shear strength vs. pressure following the classical Coulomb relation yields the internal friction from the slope and the cohesion from the intercept. We determine the internal friction and cohesion as a function of hexapod geometry. We also perform shear tests inside an X-ray CT scanner. These studies yield particle centers and orientations as well as contacts.   

Diffusion of Anisotropic Proteins with Patchy Interactions – Insight From Combining Scattering, Rheology and Computer Simulations

The static and dynamic properties of concentrated protein solutions are essential ingredients for our understanding of the cellular machinery or formulating biopharmaceuticals. This is particularly demanding as many proteins have anisotropic shapes and patchy interactions. We show how we can use a combination of advanced characterization techniques such as light and x-ray scattering, neutron spin echo measurements and microrheology experiments, combined with the theoretical toolbox from colloid physics and state-of-the-art computer simulations, to assess and predict protein diffusion in concentrated solutions. We will in particular address the influence of shape anisotropy and weak attractive patches on the structural and dynamic properties of concentrated protein solutions, and discuss how we can combine interparticle interaction effects and the formation of (transient) equilibrium aggregates in an attempt to understand and predict properties such as the concentration dependence of the osmotic compressibility, the diffusion coefficient, and the zero shear viscosity of dense protein solutions.   

Isotropic-Nematic Transition of Active Brownian Particles

Using overdamped Brownian dynamics simulations we investigate the isotropic-nematic (IN) tran-
sition of anisotropic, active particles. Using two well-studied model systems (the Gay-Berne potential
and hard spherocylinders) we consistently find that the IN transition moves to higher densities with
increasing activity. The active IN phase boundary separates well-defined nonequilibrium phases and
is not dependent upon the size of the simulation, in contrast to the isotropic-polar boundary, which
occurs at higher activity.   

Isodistance in Multi-Filament Packings: Parallel but not Straight

Assemblies of one-dimensional filaments appear in a wide range of physical systems, from biopolymer bundles, organogel fibers, columnar liquid crystals, vortex arrays, and nanotube yarns, to everyday macroscopic structures, like ropes and textiles. Interactions between the different chains or filaments in these diverse examples is dominated by the shortest distance between them, leading to common geometric constraints. When the bundle is twisted—which is often the case for self assembled chiral molecules—or has an otherwise nontrivial geometry, the packings of the filaments are frustrated. Frustration in filamentous assemblies is particularly insidious since there is in general neither uniform spacing between filaments at different points in the bundle or at different positions on the same fibers. In this talk I will describe geometric constraints on packing N >>1 curves such that the distance along their lengths remains constant, a property I call isodistance, and demonstrate that all such packings fall into two classes: the arbitrarily bent but untwisted "developable domains"; and "straight", helical bundles with constant twist. We also construct examples of twisted toroidal bundles to analyze the consequences for packings that are simultaneous twisted and bent.   

Superlattices of Squishable, Self-Assembled Spheres: How does Lattice Cell Geometry Shape Thermodynamics?

Self-assembly of soft-molecules into spherical domains adopting Frank Kasper lattices have been observed in a variety of systems, including liquid-crystalline dendrimers, charged surfactants and block copolymers (BCPs). The formation of these complex phases has been previously attributed to optimal surface area and/or volume asymmetry (polydispersity or Voronoi partition of cells) leading to the question: What selects volume asymmetry in these assemblies and how does this impact surface area of the partitions? We will address these in the context of BCPs by drawing comparisons between Diblock Foam Model (DFM) that captures the Polyhedral Interface Limit (PIL) and SCFT for diblock melts. DFM describes thermodynamics of sphere phases in terms of competing geometric quantities: surface area and dimensionless stretching (or radius of gyration) of cellular volumes enclosing domains. DFM correctly predicts not only which of these lattices is favored in equilibrium (the sigma phase) but also their relative ranking in terms of entropy and enthalpy, and their equilibrium volume distribution among spheres. Comparison to SCFT results show that increasing conformational asymmetry between blocks drives a transition to radial-chain stretching and towards the PIL described by DFM calculations.   

Configurational entropy of hard particles in a confined cavity

We study the configurational entropy S of N hard particles confined in a 2D cavity using MC integration, considering particles and cavities with various geometries. For small N, we find S decreases monotonically as the cavity aspect ratio increases regardless of particle shape. For large N, we find crystalline order within the cavity, with alternating regions of high and low particle density. We find the highest density near the boundary for all particle and cavity shape combinations, while the spatial distribution of the density changes with cavity shape. Our findings may give clues to engineering particles in a confined environment, entropic barriers, and systems with depletion interaction.
  

Colloidal Cluster Self-Assembly Through Connectivity Landscape Analysis

Prefabricated micron-scale colloidal clusters offer a practical way for introducing anisotropic interactions and enabling the formation of interesting crystalline superstructures that are otherwise inaccessible with spherically symmetric interactions. However, it is apparent that the high-dimensional parameter space that defines the geometric and interaction properties of such systems poses an obstacle to assembly design and optimization.

Here, we introduce an analytical framework, connectivity landscape analysis (CLA), for analyzing the geometrical parameter space of superstructures that may be grown with colloidal clusters. CLA segments the overall parameter space into distinct regions in which specific contacts are made between different pairs of particle types. Combined with a small number of targeted molecular dynamics simulations of superstructure growth, we show that it is possible to quickly determine feasibility regions in which the superstructure can be grown. In this context, CLA corresponds to a type of dimensionality reduction, allowing the resulting simplified space to be efficiently scanned either computationally or experimentally.   

Recovering Dynamic Parameters of Nanoparticle Assembly from Disjointed Images of Nanoparticle-Polymer Composites

The incorporation of nanoparticles (NPs) into polymers constitutes a powerful strategy for enhancing their thermomechanical properties and for introducing new optical, electrical, and magnetic functionalities into the polymers. Here we will describe an approach for inferring dynamic parameters of NP assembly from spatially and temporally disjointed images of NP-polymer composites. The approach involves adjustment of the parameters of a kinetic model of assembly until the size statistics of NP clusters computed from the model match those obtained from high-throughput analysis of the experimental images. Application to shaped, metal NPs in polymer films reveals that NP structures grow via a cluster-cluster aggregation mechanism, where NPs and their clusters diffuse in a Stokes-Einstein manner and stick with a probability that depends strongly on the size and shape of the NPs and the molecular weight of the polymer.