Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E57: Aspherical Particles in Soft Matter Self-Assembly and Granular Matter IFocus Session
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Sponsoring Units: GSOFT GSNP Chair: Gerd Schroeder-Turk, Murdoch Univ Room: LACC 518 |
Tuesday, March 6, 2018 8:00AM - 8:36AM |
E57.00001: The effect of particle shape in self-assembly and self-organisation of soft materials Invited Speaker: Marjolein Dijkstra The ability of atomic, colloidal, and nanoparticles to self-organize into highly ordered crystalline structures makes the prediction of crystal structures in these systems an important challenge for science. The question itself is deceivingly simple: assuming that the underlying interaction between constituent particles is known, which crystal structures are stable. In this talk, I will describe a Monte Carlo simulation method [1] combined with a triangular tesselation method [2] to describe the surface of arbitrarily shaped particles that can be employed to predict close-packed crystal structures in colloidal hard-particle systems. I will show that particle shape alone can give rise to a wide variety of structures with unusual properties [3-6], e.g., photonic band gap structures by using mixtures of spheres and tetramers or highly diffusive crystals in the case of truncated cubes or binary hard-sphere mixtures. Finally, I will show that the number of possible structures can even be enlarged further by combining the choice of particle shape with external fields, like confinement [7]. |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E57.00002: Identity crises in hard polyhedral glass-formers Erin Teich, Greg Van Anders, Sharon Glotzer Colloidal systems are capable of self-assembling into a wide variety of ordered structures, ranging from the simple to the exceedingly complex. Often, however, no such assembly occurs, and the system instead displays dynamical characteristics of glass-formation. Here, we computationally investigate assembly failure in a family of monodisperse, one-component systems, composed of colloidal particles of polyhedral shapes with no interactions aside from those of excluded volume. We study the role that local structure plays in dynamical arrest in these entropic systems, and find that assembly failure arises from an ``identity crisis” experienced on a local level and manifested in shape space. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E57.00003: Predicting Self-assembly of Nonspherical Particles using Density Functional Theory Matthieu Marechal, Rene Wittmann, Simone Dussi, Marjolein Dijkstra Due to the great wealth of experimentally available particle shapes, efficient methods for predicting self-assembly are of paramount importance in soft matter. I will present recent advances in density functional theory that allow fast exploration of parameter space to be followed by more precise particle-resolved computer simulation techniques or experiments. We have been focusing on liquid crystal phases of hard particles so far, since these constitute stringent tests on our theory due to the small free energy differences between the phases involved. Comparison between the theory and particle-resolved simulations for spherocylinders and rod-like polyhedra shows that the theory predicts self-assembly of liquid crystals with the required accuracy for fast exploration. We find that the flat-topped, rod-like polyhedra exhibit a significantly more stable smectic phase than the archetypical spherocylinders. This subtle effect of the particle shape is well captured by the theory, which is promising for applications of the theory to other phases and other interaction potentials. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E57.00004: Entropically driven self-assembly of star-shaped micelles in pear-sphere-mixtures Philipp Schönhöfer, Matthieu Marechal, Douglas Cleaver, Gerd Schroeder-Turk Improvements in the synthesis of colloids mean that a great variety of complex aspherical |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E57.00005: Tunable Two-Dimensional Crystals Comprising Aspherical Colloids Zhuoqiang Jia, Jaehyun Kim, Stefano Sacanna, Stephanie Lee Aspherical colloidal particles were assembled in a reversible and tunable manner by means of dielectrophoresis (DEP). Spherical particles with single spherical cavities (referred as “dimples”) adopted lattices with non-closed packed cmm plane group symmetry and a packing fraction of 0.68 at low electric field strengths. With increasing electric field strength, the packing structure reverted to the entropically favored close-packed structure with p6m symmetry and a packing fraction of 0.90 typically observed for spherical colloids. Open packing structures were also achieved in systems comprising cubic hematite colloids with cylindrical arms protruding from each face, referred to as “hexapods”. Upon application of an electric field, perpendicular to the field direction, the arms of the hexapods prevented close packing. The presence of a magnetic field was found to anneal the hexapod crystals, decreasing the number of defects. The magnetic field could also be used to manipulate the orientation and packing structure of the crystals. These findings suggest that the interaction between external fields and aspherical colloidal geometries can be used to induced open-packed and tunable structures in two-dimensional crystals. |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E57.00006: Liquid crystal ordering transition and colloidal interactions of cellulose nanocrystals (CNCs) Emily Facchine, SooAh Jin, Richard Spontak, Orlando Rojas, Saad Khan CNCs are materials of increasing interest because of their size, aspect ratio, stiffness, renewability, and liquid crystal (LC) ordering. CNCs self-assemble into LC phases above a critical concentration, which is dictated by particle size and surface properties. In this work, we study this transition by observing the interactions of CNCs across a range of concentrations using isothermal titration calorimetry (ITC). This technique allows us to take highly precise heat measurements from the interactions of very small sample volumes. Addition of electrolytes also has a pronounced effect on the suspension behavior: at low ionic strengths (0-10 mM), the electrostatic double layer is compressed and the effective particle size is reduced. With increasing ionic strength (>20 mM), the electrolytes destabilize the suspension by neutralizing the electrostatic repulsive forces and inducing aggregation. The nature of this transition is examined for different electrolytes and CNC concentrations, with ITC results for both liquid crystal transition and ionic strength effects being corroborated using rheology and polarized optical microscopy (POM). These techniques provide relevant microstructural information in the form of rheology, and the presence and extent of anisotropic domains. |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E57.00007: Coarse-grained multibody interactions in colloids Martin Girard, Trung Nguyen, Monica Olvera De La Cruz Coarse-graining colloidal interactions usually assumes that multi-body effects can be |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E57.00008: Chiral symmetry-breaking in C-shaped lock-and key colloids: Monte Carlo simulation studies Robin Selinger, Dong Li We model aspherical lock-and-key colloids to explore fundamental mechanisms driving chiral symmetry-breaking and chiral amplification. Recent experiments by Wang and Mason [1] demonstrated that achiral C-shaped colloidal particles form both right- and left-handed dimers, a form of chiral symmetry-breaking. We explore this phenomenon via Monte Carlo simulation of hard C-shaped particles confined to a plane. We calculate the equilibrium constant associated with dimerization and its dependence on osmotic pressure, and find qualitative agreement with experiment. We find that like-signed dimers tend to phase segregate, and that larger clusters display locally crystalline order. We examine kinetic mechanisms driving chiral phase segregation, and speculate that the mechanism observed here may be related to spontaneous chiral phase separation in other systems such as bent-core liquid crystals. These results provide new insight into mechanisms of chiral symmetry-breaking, chiral amplification, and possible origins of biological homochirality. [1] P.-Y. Wang and T.G. Mason, J. Am. Chem. Soc. 137 15308 (2015). |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E57.00009: Hypostatic Jammed Packings of Frictionless Nonspherical Particles Kyle VanderWerf, Weiwei Jin, Corey O'Hern, Mark Shattuck We perform numerical simulations to study jammed packings containing a variety of nonspherical particle shapes (e.g. dimers, circulo-lines, circulo-polygons, ellipses, and dumbbells) in two spatial dimensions. By analyzing these packings, we propose criteria that particle shapes must satisfy to give rise to hypostatic jammed packings, with fewer contacts than degrees of freedom using naive constraint counting arguments. We show the packing fraction $\phi$ and coordination number $z$ for jammed packings of the particle shapes under study. In particular, we find that $\phi$ and $z$ obey a master curve for different particle shapes when they are plotted as a function of the asphericity ${\cal A} = p^2/4\pi a$, where $p$ and $a$ are the perimeter and area of the particles. We also calculate the principal curvatures of the particle contact constraint surfaces in high-dimensional configuration space to identify specific contacts in packings of spherocylinders that allow them to be jammed, yet hypostatic. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E57.00010: Confined packing of granular rods: Bulk experiments Julian Freeman, Cong Cao, Sean Peterson, Scott Franklin, Yujie Wang, Eric Weeks We conduct experiments to observe the effects surfaces have on the internal packing structure of particles. To observe this, we run an experiment using cylindrical containers of different diameters, and rods of aspect ratios ranging from 4 to 32. We find that the rods packed into smaller cylindrical containers yielded lower volume fractions than in larger containers. Our results are extrapolated to an infinite container size, and the subsequent volume fraction decreases with increasing aspect ratios, in agreement with previous simulations. The results also suggest that the surface effect on internal packing decreases with aspect ratio as well. We also conduct a second series of experiments using vibration to more closely pack the containers. This results in denser packings as expected, and also changes the influence of the boundaries. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E57.00011: Confined Packing of Granular Rods: X-ray Tomography Experiments Cong Cao, Julian Freeman, Sean Peterson, Scott Franklin, Yujie Wang, Eric Weeks We study 3D rod packings in cylindrical containers with two sizes by a computed tomography scanner. The aspect ratio of our rods is 1:8. We observe rods pack to a higher volume fraction phi in the larger container, suggesting a strong influence of the walls on the packing. In both containers, we observe that particles pack isotropically in the bulk, but pack differently near the side and bottom boundaries. Near the bottom boundary, the rods lie flat against the container boundary and thus pack with a higher phi than the bulk. In contrast, rods are loosely packed near the side boundary. For both the bottom and side boundaries, the range of influence of the boundaries is shorter than a rod length. The differences between the bottom and side boundaries show that gravity plays an important role in the packing of rods in small containers. |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E57.00012: Confined packing of granular rods: Simulations Sean Peterson, Cong Cao, Julian Freeman, Eric Weeks, Scott Franklin We model the packing of rodlike particles in confined geometries. Simulations are optimized for parallel processing on a Graphics Processing Unit. Simulated particles are spherocylinders (cylinders with hemispherical end-caps) of aspect ratios ranging from 4 to 32. Infinitesimal particles are distributed at random in a cylindrical container and allowed to expand until they overlap. A conjugate gradient method is used to minimize the elastic potential energy that accompanies overlap, and the growth/minimization process continues until the energy can no longer be minimized to an infinitesimal value. We find the jamming packing fraction decreases as the container becomes small, and is consistent with the idea of a boundary layer of randomly packed particles. From simulations we extract the large-container asymptotic packing fraction Φ∞, the decrease in packing fraction within the boundary layer δΦ and the size of the boundary layer δL. We also characterize orientation in the boundary layer at the container wall, floor and ceiling. |
Tuesday, March 6, 2018 10:48AM - 11:00AM |
E57.00013: Rheology of Aspherical Granular Particles K. Michael Salerno, Dan Bolintineanu, Gary Grest, Jeremy Lechman, Steven Plimpton, Ishan Srivastava, Leo Silbert We investigate the flow behavior of aspherical, granular particles in |
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