Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M31: Directed Assembly 2: Field Driven |
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Sponsoring Units: DSOFT Chair: Alex Travesset, Ames Lab Room: 503 |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M31.00001: Hyperuniform structures formed by shearing colloidal suspensions Sam Wilken, Rodrigo Emigdio Guerra, David J Pine, Paul M Chaikin Time-reversibility is an essential feature of viscous, creeping flow that is lost when rigid particles suspended in the liquid are forced to collide with each other. In periodically sheared suspensions this loss manifests as a dynamical phase transition between reversible and chaotic particle trajectories characterized by a critical strain amplitude. Repulsive, non-hydrodynamic interactions between colliding particle surfaces have been proposed as the likely source of this broken symmetry, and a toy model that emphasizes particle collisions, called Random Organization1, qualitatively reproduces the dynamical features of this transition. This model also displays a concomitant structural reorganization characterized by vanishingly small density fluctuations on long length-scales, called hyperuniformity, at criticality2. Here we show that the particles in periodically sheared suspensions organize into structures with anisotropic short-range order but isotropic, long-range hyperuniform order when oscillatory shear amplitudes approach the critical strain amplitude. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M31.00002: Optimized Large Hyperuniform Binary Dipolar Colloidal Suspensions in Two Dimensions Zheng Ma, Enrique Lomba, Salvatore Torquato
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Wednesday, March 4, 2020 11:39AM - 11:51AM |
M31.00003: Assembly of gold nanoparticles directed by light polarization Panupon Samaimongkol, Erich M See, Cheryl L Peck, Webster L Santos, Hans Robinson When polarized light is incident on a small metal nanoparticle, the light intensity at the particle surface is enhanced in two opposite polar regions defined by the polarization direction. By functionalizing the nanoparticles with photosensitive ligands, this intensity distribution can be transferred into permanent surface properties, forming two opposite patches on the particle. Using a ligand that photocleaves to reveal a positively charged amine group, we assemble smaller (15 nm diameter) gold nanoparticles onto the larger (80 nm diameter) functionalized patchy particle. Since all areas of the 80 nm particle are exposed to light, binding can take place in any orientation, but is strongly preferential to the direction set by the polarization of the exposing light, resulting in arrays of oriented nanoparticle assemblies. Interestingly, we also see increased particle binding around the equator of the particle, where light exposure was at a minimum. Possibly, this is due to electrostatic bias in the random walk toward the polar regions being absent around the equator, leading to a relative accumulation of the 15 nm particles in this region. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M31.00004: Acoustokinetics: Organizing Soft Matter with Acoustic Holograms Mohammed Abdelaziz, David G Grier Factoring the pressure field of a harmonic sound wave into its amplitude and phase profiles provides the foundation for an analytical framework for studying acoustic forces that not only provides novel insights into the forces exerted by specified sound waves, but also addresses the inverse problem of designing sound waves to implement desired force landscapes. These force landscapes could be used to transport and organize matter on scales from microns to millimeters. We illustrate the benefits of this acoustokinetic framework through case studies of purely nonconservative force fields, standing waves, pseudo-standing waves, and tractor beams. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M31.00005: Landau theory of complex spherical packing phases Sarah Dawson, Duncan McClenagan, Anchang Shi The complex spherical packing phases, including the Frank-Kasper σ and A15 phases, appear in a diverse array of soft-condensed matter systems. The universality of these novel ordered structures points to some common underlying physics that can be probed by a suitable Landau theory. The stability of several complex phases was investigated in two theories that are often used to study pattern formation: The Landau-Brazovskii and Ohta-Kawasaki models. Unexpectedly, these phases were found to be stable in the former theory and not the latter. Appropriately scaled, these two free-energy functionals favour periodic modulation of the order parameter at a particular wave number, and differ only in the shape of the free-energy about this value. Motivated by this result, we studied how the shape of the free-energy about its optimal wave number can influence the relative stability of the complex phases over the BCC and FCC spherical phases. This talk is concerned with how these results can inform us of the general mechanisms guiding the formation of the complex spherical phases. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M31.00006: Templated assembly of self-propelled particles in targeted micromachines Antoine Aubret, Jeremie Palacci In this talk, I will show how we can carve non-equilibrium pathways for the controlled self-assembly of active microparticles using light as a tool. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M31.00007: Synchronization of colloidal oscillators Jinzi Huang, Yaocheng Desmond Li, Antoine Aubret, Jeremie Palacci Christiaan Huygens invented pendulum clocks and observed ‘an odd sympathy’ that pendulum clocks hanging on the same wall autonomously synchronize with one another. However, the phenomenon was a mysterious until coupling by the wall was shown to be the driving force for the synchronization. Indeed, interactions between dynamical systems promote synchronization, and we aim to study such phenomena on a micro scale by designing colloidal oscillators. Using self-assembly of microswimmers with passive structures, we develop colloidal clocks that oscillate. We show that pairs of microclocks are able to synchronize with each other, in spite of thermal noise, and compare well with a stochastic model. Further, assemblage of clocks are achieved to study how the clocks can achieve synchronicity in situations of spatial frustration. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M31.00008: Development of surface composition and ordering during reverse-emulsion assembly of binary colloidal particle mixtures Christian Heil, Thomas E Gartner, Arthi Jayaraman Nanoparticle assembly at a fluid-fluid interface is a proven route to precisely engineer materials with controlled optical properties. In this talk, we will present our recent work using coarse-grained Langevin dynamics simulation that mimics reverse-emulsion directed assembly of binary nanoparticle mixtures into supraballs. We model a binary mixture of silica and synthetic melanin particles in implicit solvent within a shrinking spherical confinement to replicate the shrinking reverse-emulsion droplet. The simulation protocol captures the physics of this process by reproducing the experimental observation of melanin and/or smaller nanoparticles enriching the water-octanol interface. For all mixtures, we observe enrichment of the melanin particles at the supraball surface compared to the supraball interior. This enrichment decreases with increasing melanin/silica size ratio. We observe appreciable crystalline ordering only for systems with similarly sized particles. Particle size dispersity, finite assembly timescale, and curvature of the supraball surface all serve to suppress particle ordering. These findings serve as design rules for tailoring the supraballs for structural color applications and improve our fundamental understanding of particle assembly near curved interfaces. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M31.00009: Enhanced Diffusion and Magnetophoresis of Paramagnetic Colloidal Particles in Rotating Magnetic Fields Zachary Sherman, Julia L Pallone, Randall M Erb, James Swan Dispersions of paramagnetic colloids can be manipulated with external magnetic fields to assemble structures and control transport. For fields held steady in time, the structure and dynamics are coupled, which becomes problematic for processes where aggregation competes against particle transport. Rotating the field direction in time drives dispersions out of equilibrium, allowing the structure and dynamics to be tuned independently to enhance transport. Fundamental transport properties, like the diffusivity and magnetophoretic mobility, dictate a suspension’s nonequilibrium response and are crucial to understand to design processes utilizing rotating fields. Here, we investigate the transport properties of paramagnetic colloids in rotating magnetic fields using dynamic simulations. We find that self-diffusion is enhanced in rotating fields compared to steady fields, and that the self-diffusivity in the plane of rotation reaches a maximum value at intermediate rotation frequencies that is larger than the Stokes-Einstein diffusivity of an isolated particle. While the magnetophoretic velocity through bulk fluid decreases with increasing rotation frequency, enhanced in-plane diffusion allows for faster magnetophoretic transport through porous materials in rotating fields. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M31.00010: Mesoscale simulation approach for dynamics and assembly of deformable objects Toluwanimi Bello, Tejus Shastry, Sangwoo Lee, Patrick Underhill In concentrated suspensions, surfactant micelles, emulsion droplets, and microgels often form contacts among the objects leading to nonspherical shapes. In this limit, the dynamics and assembly of the suspension depend more on the interfaces between objects than the bulk objects themselves. This type of deformation of domains also occurs in microphase separation of block copolymer systems. We have adapted a mesoscale approach (called vertex models) in order to apply it to the dynamics and assembly of small deformable objects. In this way, we are developing a unifying framework to understand micelles, emulsions, microgels, and block copolymers. In this talk, we will describe the application of vertex models to small deformable objects. In particular, we have quantified the phase diagram of assembly in thin films of particles (2D simulations) and bulk assembly of particles (3D simulations). A balance of thermal fluctuations and deformability leads to an order-disorder transition in both cases. A unique feature of 3D materials is that multiple ordered states are possible. We have quantified their meta-stability and transitions between ordered states. These transformations are well-known in metallic systems, but have only recently been found in experiments with soft materials. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M31.00011: Ordering and Interactions of Gold Nanoparticles (AuNPs) with Fractional Surface Coverage of Ligands Morgan E Reik, Chris Liepold, Sean D Griesemer, Wei Bu, Alex Smith, Stuart A Rice, Juan De Pablo, Binhua Lin We studied the properties of self-assembled AuNPs monolayers. These monolayers are formed from solutions with varying thiol concentrations. We show that equilibrium between adsorbed thiols on Au cores and thiols in the bulk solution imply fractional coverage of the Au cores. The equilibrium coverage of AuNPs is adequately described by Langmuir adsorption kinetics, and, therefore, we interpret the way in which varying thiol concentration affects the nanoparticle-nanoparticle interactions as a function of surface coverage of the Au core [1]. We also examine the structure and general shape of the ligand envelope as a function of the surface coverage using molecular dynamics simulation and demonstrate that the equilibrium structure of the envelope and the deformation of that envelope generated by interaction between the AuNPs are coverage-dependent, so that the shape, depth, and position of the minimum of the potential of mean force display a systematic dependence on the ligand coverage [2]. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M31.00012: Divine Proportions and the Fractal Dimension of DLCA Aggregates Chris Sorensen Aggregation is a non-equilibrium process of fundamental importance for all dispersed particulate systems. Here I present a restricted hierarchical model of diffusion limited cluster-cluster aggregation (DLCA). The model yields an analytical calculation of the fractal dimensions and self-preserving cluster shapes in two and three spatial dimensions in excellent agreement with those found in nature and simulations. Remarkably the shape is described by the Fibonacci series and the divine proportion in two dimensions and d-dimensional generalization of the Fibonacci series and the divine proportion in three and higher dimensions. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M31.00013: Polydisperse Gold Nanocrystals and Hard Sphere Diameters Xun Zha, Alex Travesset A fundamental property of nanocrystals is its hard sphere diameter. In this talk, I will provide a critical discussion on how to compute the diameter, why it is important and in what situations determines the nearest neighbor distance through packing considerations. I will then classify polydisperse nanocrystals of the truncated octahedral family and show that it consists of 351 different types within the size range 1-5nm. I will compute the hard sphere diameter and show how it changes with system size for all these cases. Connections with experiments with 16% polydispersity in alkylthiolated gold nanocrystals, which show crystal fractionalization, will also be discussed. |
Wednesday, March 4, 2020 1:51PM - 2:03PM |
M31.00014: Effect of Water Vapor on the Pair Potential Between Ligated Nanoparticles Michael Martinez, Miaochen Jin, Alex Smith, Kevin Slater, Linsey Nowack, Binhua Lin, Stuart A Rice We have carried out all-atom molecular dynamics simulations to illuminate the physical basis for the change in Young’s modulus of a monolayer of dodecane thiol ligated Au nanoparticles discovered by Wang et al.[1] For a water vapor concentration that would generate 80% monolayer coverage of the nanoparticle surface we find that water molecules form large fluctuating “hemispherical” clusters on the nanoparticle surface with the correct macroscopic contact angle. We have constructed AuNP-AuNP potential curves for both wet and dry systems; the difference in the shapes of these curves near their minima is consistent with the change in Young’s Modulus found by Wang et al.[1] The in vacuum all-atom NP ligand conformations and NP-NP potential agrees well with those previously obtained from united-atom simulations.[2] Additionally, both the wet and dry potential energy curves differ significantly between sampled frozen orientations, implying the possibility of variation in the local NP energy in a self-assembled ordered AuNP array. |
Wednesday, March 4, 2020 2:03PM - 2:15PM |
M31.00015: Ligand Structure and Free Energy of Nanoparticles on Substrates Matthew Pham, Alex Travesset 2D nanoparticle superlattices are commonly assembled on or transferred to solid substrate supports such as silicon wafers, silica, or graphene. In order to better understand nanoparticles in this context, molecular dynamics simulations were carried out to study the nanoparticle-substrate interactions. Simulations were run using HOODLT/HOOMD-blue to analyze the capping ligand vortices and adhesion free energies of substrate supported gold nanoparticles of various sizes grafted with hydrocarbons of various lengths. The relation of this work to nanoparticle superlattices is also discussed. |
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