Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F58: Colloids II |
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Sponsoring Units: GSOFT Chair: Urs Gasser, Paul Scherrer Institute Room: BCEC 257A |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F58.00001: Synthesis and Assembly of Janus Particles Meneka Banik, Rabibrata Mukherjee Over the years, a wide variety of Janus particles with anisotropy in terms of size, shape and chemical functionality have been explored. Synthesis and self-assembly of such particles has been an area of active research. Owing to their anisotropic nature Janus particles experience highly directional interaction. However, assembly of Janus particles on a surface driven by attractive interaction is an area not yet properly explored. One of the probable reasons for this is that most of the synthesized Janus particles are surface immobilized and suffer from significant damage during dislodgement. Also, the anisotropic particles synthesized by chemical route lack uniformity and monodispersity. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F58.00002: Adsorption and denaturation of structured polymeric nanoparticles at an interface Jie Feng, Chang Tian, H. Jeremy Cho, Sujit Datta, Robert K Prud'homme Nanoparticles (NPs) have been widely applied in fields as diverse as energy, environment, and human health. However, the adsorption and trapping of NPs at interfaces is still poorly understood. In many applications, such as drug delivery, understanding NP interactions at an interface is essential to determine and control adsorption onto targeted areas. Therapeutic NPs are especially interesting because their structures involve somewhat hydrophilic surface coronas, to prevent protein adsorption, and much more hydrophobic core phases. Here, we investigate the evolution of NP attachment and structural evolution at the air–liquid interface over time scales from 100 ms to 10s of seconds. We document three distinct stages in NP adsorption. In addition to an early stage of free diffusion and a later one with steric adsorption barriers, we find a hitherto unrealized region where the interfacial energy changes due to surface “denaturation” or restructuring of the NPs at the interface. We adopt a quantitative model to calculate the diffusion coefficient, adsorption rate and barrier, and extent of NP hydrophobic core exposure at different stages. Our results deepen the fundamental understanding of adsorption of structured NPs at an interface. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F58.00003: Phase Separation of Ferromagnetic Nematic and Isotropic Colloidal Suspensions Min Shuai, Xi Chen, Cheol Soo Park, Joseph E MacLennan, Matt Glaser, Noel Anthony Clark Suspensions of disk-shaped ferromagnetic barium hexaferrite nanoplates in isotropic solvent spontaneously form a ferromagnetic nematic phase for nanoplate concentrations higher than the Onsager isotropic–nematic phase transition point for hard disks [Nat Comm, 7: 10394, 2016]. At an overall concentration below this value and within the coexistence region, such suspensions phase separate into ferromagnetic nematic and isotropic domains. Under these conditions, the suspension can be driven into a uniform state by mechanical or magnetic stirring, and undergoes a dynamic process of phase separation immediately after the removal of stirring forces, which we have investigated by polarized optical microscopy. Starting from a diffuse, birefringent mesh, nematic regions rapidly coalesce into a three-dimensional network of fine, short, straight filaments. If left undisturbed, these filaments coarsen, combining with each other until the isotropic phase between them is eliminated. In the presence of a weak external magnetic field, filaments along the field direction tend to grow most quickly. We also observe ferromagnetic nematic domains with exotic topological structures, particularly under sudden changes in external field. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F58.00004: Topological Protection in Densely Packed Anisotropic Colloids William Zygmunt, Erin Teich, Greg van Anders, Sharon Glotzer It has been long known that there exist topologically protected phases in strongly coupled |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F58.00005: Pitch-dependent behavior of colloids in confined cholesteric liquid crystals Giuseppe Boniello, Francesca Serra, Kathleen Stebe Confined soft matter provides unique opportunities to impose energy landscapes to address and control colloid dynamics. In this context, the geometry of bounding surfaces can be employed to mold an energy landscape. Within this landscape, colloids assemble into reconfigurable, hierarchically organized structures, a leading challenge in material science. Example soft matter systems include liquid crystals. For instance, when nematic liquid crystals (NLCs) are confined in a vessel, bend and splay distortions can be used to position particles. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F58.00006: Microstructure and stress propagation in the nematic glass phase of colloidal cellulose nanocrystals Matthew Sartucci, Bharath Natarajan, Jefrey Gilman, Jeffrey S Urbach Suspensions of cellulose nanocrystals (CNC's) in low ionic strength aqueous solutions can undergo an isotropic-nematic phase transition due to the anisotropic nature of CNC's. At concentrations above a certain threshold, when the system is well into the nematic regime but far below the maximum stable concentration, these suspensions can form a soft solid with a modulus of approximately 500 Pa. The nematic phase is not homogeneous throughout this system but instead forms many domains, each consisting of collectively aligned CNC's with an average orientation that differs from neighboring domains. This work seeks to understand the mechanical contribution of the domain structure, as observed through cross polarization and boundary stress microscopy, to the macroscopic properties measured through bulk rheology. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F58.00007: Modulated colloidal deposition and accumulation in model porous media Gaetan Gerber, Philippe Coussot Suspended colloids flowing through porous structures are subject to various events, including size-exclusion clogging [1], surface adsorption, or interactions with previously attached bodies. Current models [2,3] qualitatively describe those local mechanisms, while observations are mostly limited to break-through analysis and 2D geometries. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F58.00008: Size Control and Monodispersity of Silica Nanospheres and Applications in Nanofiltration Ryan Vincent, Sean McBride Due to their low reactivity and high thermostability, amorphous silicon dioxide nanoparticles have drawn much attention in the field of nanotechnology. This work reports on the synthesis methods and reaction parameters used to produce monodispersed silica nanospheres of systematically varied sizes using modified Stöber methods. Employing W/O microemulsion systems assisted by amino acid monomers and more traditional Stöber systems, nanosphere diameters were well controlled from single-digit nanometers to several micrometers. Effects of chemical compositions and other reaction parameters effecting TEOS hydrolysis were optimized for each Stöber system. Additionally, the monodispersed spheres were assembled into close-packed systems where the porosity and volumetric flowrates were controlled by the nanoparticle radii. Detailed characterization of the particles and arrays was performed using atomic force microscopy and scanning electron microscopy. Developing a porous system with a tunable effective pore size is a fundamental interest in the field of filtration. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F58.00009: Non-Stokes drag of electrophoresis: a new insight into an old problem Maijia Liao, Ming-Tzo Wei, H Daniel Ou-Yang, Ping Sheng Mobility, the ratio of a charged-particle drift speed in a DC electric field to the magnitude of the field is typically used to quantify electrophoresis. Drag coefficient cannot be measured in a DC field because application of an external force to measure force alters the flow pattern, yielding an apparent drag force that is Stokes-like. The intrinsic electrophoretic drag can be determined, however, by optically trapping a charged particle and placing it in a low-frequency AC field. Using the frequency-dependent phase shift of the particle motion relative to that of the AC field and the magnitude of the particle displacement, this approach yields drag coefficient, effective charge and also a mobility that is the same as that measured using a DC field. The drag coefficient is markedly different from that of the Stokes. Using the drag coefficient and the mobility as input for numerical calculations based on the Planck-Nernst-Poisson equation and the Stokes equation reveals a well-defined transition between an inner and outer flow in the vicinity of the particle. This study provides a new insight into an old problem by providing experimentally measurable quantities, i.e., effective charge, electric force and drag coefficient a microscopic definition. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F58.00010: Gradient expansions to capture the nonlocal physics of the electrical double layer Pedro De Souza, Martin Bazant A hallmark of highly charged surfaces or concentrated electrolytes is strong, nonlocal electrostatic and density correlations. In these regimes, the classical Poisson-Boltzmann (PB) mean-field theory for dilute electrolytes breaks down, and the predictions using PB can be qualitatively and quantitatively incorrect. For example, the influence of nonlocal electrostatic correlations lead to charge reversal and like-charge attraction, even though PB theory predicts an exclusively repulsive interaction between like-charged surfaces. Size correlations can lead to oscillatory density profiles that are not captured by PB theory. Here, we explore using a nonlocal Landau-Ginzburg-like free energy functional to describe the structure of an electrical double layer at a charged surface, as well as overlapping electrical double layers, at high charge density and concentration. We show that the correct boundary conditions of such a nonlocal model are given by a force balance at the interface, where the nonlocal effects must vanish. We apply the model to calculate surface forces, interactions between biological molecules, and electrokinetic flows. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F58.00011: Computer Simulations of Packed Soft Colloids Tom Ridley, Daniel Read, Oliver G Harlen, Johan Mattsson Soft colloids are micron-scale structured objects such as polymer microgels, that are compressible and deformable. Even at high packing ratios, a soft colloid may still be able to undergo cage-breaking due to particle deformation. The link between the detailed elastic properties of soft colloids and the resulting dynamics are presently not well understood. Soft colloids show rich rheological behaviour we wish to understand the links between the single particle elastic properties and the resulting rheology. We use a recently developed algorithm, Fluctuating Finite Element Analysis, for simulating viscoelastic objects undergoing thermal excitation. While symmetric soft particle potentials have been much studied, our approach captures the detailed shape deformations of the colloidal particles so that the structure and anisotropic deformation of the particles are taken into account. We investigate varying effective volume fraction and find evidence of cage-breaking events even where the effective volume fraction exceeds the close packed limit. Additionally, we present results of varying modulus and steady shear rates for a constant effective volume fraction. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F58.00012: Diffusion of DNA-coated colloids on DNA coated surface Jeana(Aojie) Zheng, David J Pine In order for DNA-coated colloids to anneal and form crystals, they must roll and diffuse while attached to each other. Here we report on the diffusion of DNA-coated colloidal spheres on a flat DNA-coated substrate. Near the DNA-melting temperature, the mean square displacement is linear in time as expected for normal diffusion, but the diffusion coefficient is much smaller than for free diffusion. As the temperature is lowered, the motion becomes subdiffusive, which suggests the presence of random free energy barriers in the DNA-mediated interactions. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F58.00013: Microscopic dynamics of stress relaxation in a nanocolloidal soft glass Yihao Chen, Simon Rogers, Suresh Narayanan, James Harden, Robert Leheny Following the cessation of flow-inducing shear, soft disordered solids often display a protracted recovery during which the stress slowly decreases to a finite value known as the residual stress. While numerous rheology studies have characterized the macroscopic nature of this stress relaxation, little is known about the underlying microscopic structural dynamics. We report x-ray photon correlation spectroscopy (XPCS) experiments with in situ rheometry performed on a soft glass composed of a dense suspension of charged silica nanoparticles subject to step strains that induce yielding. The XPCS measurements characterize the particle-scale and mesoscale motions within the glass that underlie the subsequent slow decay of the stress. The XPCS correlation functions indicate these dynamics are anisotropic, with slow, convective-like particle motion along the direction of the preceding shear that persists for surprisingly large times after flow cessation and that is accompanied by highly intermittent motion in the perpendicular (vorticity) direction. The close correspondence between these dynamics and the stress relaxation is demonstrated by power-law scaling between the characteristic velocity of the convective motion and the rate of stress decay. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F58.00014: Particle anisotropy tunes emergent behavior in active colloidal systems Shannon Moran, Isaac Bruss, Sharon Glotzer Since early studies of active systems, investigators have sought to understand the role of particle interactions on a system’s emergent collective behavior. Particle anisotropy has been shown to impact the collective behavior of active systems, but studies to date have been qualified demonstrations of concept rather than systematic treatments. In this computational work, we investigate the role of particle anisotropy in shape and driving force director on the phase behavior of an active colloidal system. We find that these anisotropic interactions can combine to enable critical densities lower than those found in systems of isotropic particles, while in some cases actually elevating the critical density. Specifically, we find that tailoring particle anisotropy can enable more “effective'' inter-particle collisions to tune the critical system density for phase separation. Additionally, we observe nucleation of multiple clusters in the phase separation regime, similar to those observed in biological systems. In designing programmable active colloidal systems, steric interactions such as those described here may offer a simple route for tailoring emergent behaviors in active materials. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F58.00015: Reversible cluster formation and dynamical arrest in colloidal dispersions Ramon Castaneda Priego Combining molecular simulations, experimental characterizations and theoretical calculations: 1) we conclusively demonstrate that the cluster morphology in short-ranged attractive colloidal systems (SRACS) at equilibrium conditions can be uniquely determined by the reduced second virial coefficient; our findings link the reversible colloidal aggregation with the extended law of corresponding states, 2) we show that gelation in adhesive hard-sphere dispersions is the result of the rigidity percolation with coordination number equal to 2.4; these results connect the concept of critical gel formation in SRACS to the universal concept of the rigidity percolation and, finally, 3) we provide a unified description and a general overview of the different aspects of the glass transition in largely asymmetric binary mixtures of hard-spheres; we highlight the fundamental relevance in considering explicitly the dynamics of both large and small particles to properly account for the glassy scenario. |
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