Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K37: Soft Matter at Interfaces (Particles)Focus
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Sponsoring Units: GSOFT Chair: Michael Rubenstein, Univ. North Carolina Room: 340 |
Wednesday, March 16, 2016 8:00AM - 8:36AM |
K37.00001: Soft particles at fluid interfaces: wetting, structure, and rheology. Invited Speaker: Lucio Isa Most of our current knowledge concerning the behavior of colloidal particles at fluid interfaces is limited to model spherical, hard and uniform objects. Introducing additional complexity, in terms of shape, composition or surface chemistry or by introducing particle softness, opens up a vast range of possibilities to address new fundamental and applied questions in soft matter systems at fluid interfaces. In this talk I will focus on the role of particle softness, taking the case of core-shell microgels as a paradigmatic example. Microgels are highly swollen and cross-linked hydrogel particles that, in parallel with their practical applications, e.g. for emulsion stabilization and surface patterning, are increasingly used as model systems to capture fundamental properties of bulk materials. Most microgel particles develop a core-shell morphology during synthesis, with a more cross-linked core surrounded by a corona of loosely linked and dangling polymer chains. I will first discuss the difference between the wetting of a hard spherical colloid and a core-shell microgel at an oil-water interface, pinpointing the interplay between adsorption at the interface and particle deformation. I will then move on to discuss the interplay between particle morphology and the microstructure and rheological properties of the interface. In particular, I will demonstrate that synchronizing the compression of a core-shell microgel-laden fluid interface with the deposition of the interfacial monolayer makes it possible to transfer the 2D phase diagram of the particles onto a solid substrate, where different positions correspond to different values of the surface pressure and the specific area. Using atomic force microscopy, we analyzed the microstructure of the monolayer and discovered a phase transition between two crystalline phases with the same hexagonal symmetry, but with two different lattice constants. The two phases correspond to shell-shell or core-core inter-particle contacts, respectively, where with increasing surface pressure the former mechanically fail enabling the particle cores to come into contact. In the phase-transition region, clusters of particles in core-core contacts nucleate, melting the surrounding shell-shell crystal, until the whole monolayer moves into the second phase. We furthermore extended our analysis to measure the interfacial rheology of the monolayers as a function of the surface pressure using an interfacial microdisk rheometer; the interfaces always show a strong elastic response, with a dip in the elastic modulus in correspondence of the melting of the shell-shell phase, followed by a steep increase upon formation of a percolating network of the core-core contacts. The presented results highlight the complex interplay between the wetting and deformation of individual soft particles at fluid interfaces and the overall interface microstructure and mechanics. They show strong connections to fundamental studies on phase transitions in two-dimensional systems and pave the way for novel nanoscale surface patterning routes. [Preview Abstract] |
Wednesday, March 16, 2016 8:36AM - 8:48AM |
K37.00002: Sculpting Pickering Emulsion Droplets by Arrest and Jamming Christopher Burke, Zengyi Wei, Marco Caggioni, Patrick Spicer, Tim Atherton Pickering emulsion droplets can be arrested into non-spherical shapes---useful for applications such as active delivery---through a general mechanism of deformation followed by absorption of additional colloidal particles onto the interface, relaxation of the droplet caused by surface tension and arrest at some point due to crowding of the particles. We perform simulations of the arrest process to clarify the relative importance of diffusive rearrangement of particles and collective forcing due to surface evolution. Experiment and theory are compared, giving insight into the stability of the resulting capsules and the robustness of the production process for higher-throughput production in, for example, microfluidic systems. We adapt theoretical tools from the jamming literature to better understand the arrested configurations and long timescale evolution of the system: using linear programming and a penalty function approach, we identify unjamming motions in kinetically arrested states. We propose a paradigm of “metric jamming” to describe the limiting behavior of this class of system: a structure is metric-jammed if it is stable with respect to collective motion of the particles as well as evolution of the hypersurface on which the packing is embedded. [Preview Abstract] |
Wednesday, March 16, 2016 8:48AM - 9:00AM |
K37.00003: Droplets on a deformable membrane with uniform and anistropic tension Rafael Schulman, Ren\'{e} Ledesma-Alonso, Thomas Salez, Elie Rapha\"{e}l, Kari Dalnoki-Veress We examine the deformation produced by micro-droplets atop thin elastomeric free-standing films. Under the action of surface tension, the droplets deform the membrane thereby forming a bulge. For films with isotropic tension, we measure the contact angles of the droplet and bulge relative to the planar film surrounding the droplet as a function of membrane tension. We find the measured contact angles to be in excellent agreement with a model which features a force balance at the contact line. Experiments are also performed on membranes with anisotropic tension and compared to theory. In this case, droplets are non-spherical and generate significant deformation of the surrounding film which becomes non-planar. [Preview Abstract] |
Wednesday, March 16, 2016 9:00AM - 9:12AM |
K37.00004: Capillary Forces between Floating Objects: Superhydrophobic Surfaces Provide Mechanistic Insight Minchao Zhang, Thomas J. McCarthy, Alfred J. Crosby When two floating objects are close, they will either move towards or away from one another to minimize the energy caused by the overlap of the liquid/air interfacial deformations. Capillary forces cause these behaviors, but directly relating the interfacial deformations and the capillary interactions hasn't been explored experimentally. We choose a liquid marble, which has a superhydrophobic surface, as a free floating object and a fixed ``wall'' with carefully controlled contact angle as another object to generate two deformations at the interface. When the liquid marble is close to the wall, the two deformations interact, causing changes in the Laplace pressure at the interface and a reconfiguration of the interface. In response, the liquid marble moves either towards or away from the wall. Using image analysis of videos recording the liquid marble position as a function of time, we measured the liquid marble to wall distance and determine the spatio-temporal relationships. Furthermore, capillary forces were calculated from the velocities and accelerations. Based on this data, we present a new explanation for the capillary interactions from the perspective of Laplace pressure changing induced the reconfiguration of the interfacial deformations. [Preview Abstract] |
Wednesday, March 16, 2016 9:12AM - 9:24AM |
K37.00005: Adsorption-desorption kinetics of soft particles onto surfaces Brendan Osberg, Ulrich Gerland A broad range of physical, chemical, and biological systems feature processes in which particles randomly adsorb on a substrate. Theoretical models usually assume “hard” (mutually impenetrable) particles, but in soft matter physics the adsorbing particles can be effectively compressible, implying “soft” interaction potentials. We recently studied the kinetics of such soft particles adsorbing onto one-dimensional substrates, identifying three novel phenomena: (i) a gradual density increase, or "cramming", replaces the usual jamming behavior of hard particles, (ii) a density overshoot, can occur (only for soft particles) on a time scale set by the desorption rate, and (iii) relaxation rates of soft particles increase with particle size (on a lattice), while hard particles show the opposite trend. The latter occurs since unjamming requires desorption and many-bodied reorganization to equilibrate -a process that is generally very slow. Here we extend this analysis to a two-dimensional substrate, focusing on the question of whether the adsorption-desorption kinetics of particles in two dimensions is similarly enriched by the introduction of soft interactions. Application to experiments, for example the adsorption of fibrinogen on two-dimensional surfaces, will be discussed. [Preview Abstract] |
Wednesday, March 16, 2016 9:24AM - 9:36AM |
K37.00006: Adsorption dynamics of colloidal ellipsoids at oil-water interfaces Anna Wang, W. Benjamin Rogers, Vinothan N. Manoharan Nonspherical particles at immiscible fluid interfaces have strong interactions with each other and with the curvature of the host interface. However, the dynamics of nonspherical colloidal particles attaching to an interface have not yet been studied. We use digital holographic microscopy to image micron-sized polystyrene ellipsoids breaching an oil-water interface at hundreds of frames per second. We show that the particle height and polar angle have large fluctuations, but both change approximately logarithmic with time, likely due to contact line pinning on the surface of the particle. Equilibrium is reached on a timescale at least three orders of magnitude slower than that expected from Langevin dynamics simulations [1]. We also find that all the trajectories collapse into straight lines when we plot particle polar angle as a function of particle height, unlike the trajectories seen in simulation [1,2]. The differences between experiment and simulation suggest that contact line pinning and the shape of the three phase contact line may strongly influence the dynamics of particle adsorption. [1] The Journal of Chemical Physics 132 (16), 164902, (2010) [2] Soft Matter 10, 4977-4989 (2014) [Preview Abstract] |
Wednesday, March 16, 2016 9:36AM - 9:48AM |
K37.00007: Phase Behavior of 2D Charged Hydrophobic Colloids in Flat and Curved Space Colm Kelleher, Rodrigo Guerra, Paul Chaikin Charged hydrophobic colloids, when dispersed in oil with a relatively high dielectric constant, can become highly charged. In the presence of an interface with a conducting aqueous phase, particles bind strongly to the interface via image-charge attraction. At sufficiently high density, these charged interfacial particles self-organize into a 2D repulsive (Wigner) crystalline solid phase, while at lower densities, the particles form a 2D fluid. By observing samples prepared at different densities, we can probe various points in the phase diagram of this soft 2D material, and compare our results with applicable theory and simulations. In this talk, we present two sets of experiments we have performed on this system: first, we show how we can use gravity as an external force to create a controlled density gradient, and thereby directly measure the equation of state and other quantities of interest. Second, we discuss how, by observing particles which are bound to the surface of spherical droplets, we can explore how the presence of finite background curvature affects the phase behavior of the system. [Preview Abstract] |
Wednesday, March 16, 2016 9:48AM - 10:00AM |
K37.00008: Bidispersed Sphere Packing on Spherical Surfaces Timothy Atherton, Andrew Mascioli, Christopher Burke Packing problems on spherical surfaces have a long history, originating in the classic Thompson problem of finding the ground state configuration of charges on a sphere. Such packings contain a minimal number of defects needed to accommodate the curvature; this is predictable using the Gauss-Bonnet theorem from knowledge of the topology of the surface and the local symmetry of the ordering. Famously, the packing of spherical particles on a sphere contains a 'scar' transition, where additional defects over those required by topology appear above a certain critical number of particles and self-organize into chains or scars. In this work, we study the packing of bidispersed packings on a sphere, and hence determine the interaction of bidispersity and curvature. The resultant configurations are nearly crystalline for low values of bidispersity and retain scar-like structures; these rapidly become disordered for intermediate values and approach a so-called Appollonian limit at the point where smaller particles can be entirely accommodated within the voids left by the larger particles. We connect our results with studies of bidispersed packings in the bulk and on flat surfaces from the literature on glassy systems and jamming. [Preview Abstract] |
Wednesday, March 16, 2016 10:00AM - 10:12AM |
K37.00009: Dynamics of particles and defects on spherical crystals Rodrigo Guerra, Colm Kelleher, Paul Chaikin Repulsive particles confined to two dimensions can form nearly perfect crystals that melt via the well-know Kosterlitz-Thouless two-step process. By contrast, when identical particles are confined to the surface of a sphere, the curvature and topology of the surface distorts the crystal lattice and forces it to accommodate point-like disclinations and chains of dislocations. Extensive numerical and theoretical investigation has shown that these extended scars are intrinsic to the ground-state-energy configuration of these packings, as they relieve some of the stress induced by the curvature of the surface. Nevertheless, the effect of these defects on the kinetics and phase behavior of spherical crystals is not at all well understood. Here we present results of computer simulations and experiments that suggest that these scars facilitate the motion of particles close to them and fundamentally alter the nature of the mobility and liquid-to-solid transition of packings of particles confined to spherical surfaces. [Preview Abstract] |
Wednesday, March 16, 2016 10:12AM - 10:24AM |
K37.00010: Phase nucleation in curved space Leopoldo G\'omez, Nicol\'as Garc\'ia, Vincenzo Vitelli, José Lorenzana, Vega Daniel Nucleation and growth is the dominant relaxation mechanism driving first-order phase transitions. In two-dimensional flat systems, nucleation has been applied to a wide range of problems in physics, chemistry and biology. Here we study nucleation and growth of two-dimensional phases lying on curved surfaces and show that curvature modifies both critical sizes of nuclei and paths towards the equilibrium phase. In curved space, nucleation and growth becomes inherently inhomogeneous and critical nuclei form faster on regions of positive Gaussian curvature. Substrates of varying shape display complex energy landscapes with several geometry-induced local minima, where initially propagating nuclei become stabilized and trapped by the underlying curvature (G\'omez, L. R. et al. Phase nucleation in curved space. Nat. Commun. 6:6856 doi: 10.1038/ncomms7856 (2015).). [Preview Abstract] |
Wednesday, March 16, 2016 10:24AM - 10:36AM |
K37.00011: Clusters of polyhedra in spherical confinement Erin Teich, Greg van Anders, Daphne Klotsa, Julia Dshemuchadse, Sharon Glotzer Dense particle packing in a confining volume is a rich, largely unexplored problem, with applications in blood clotting, plasmonics, industrial packaging and transport, colloidal molecule design, and information storage. We report simulation results for dense clusters of the Platonic solids in spherical confinement, for up to N $=$ 60 constituent particles. We discuss similarities between clusters in terms of symmetry, a connection to spherical codes, and generally the interplay between isotropic geometrical confinement and anisotropic particle shape. Our results showcase the structural diversity and experimental utility of families of solutions to the problem of packing in confinement. [Preview Abstract] |
Wednesday, March 16, 2016 10:36AM - 10:48AM |
K37.00012: Computer Simulation study of polyhedral nanoparticle self-assembly at interfaces. Vikram Thapar, Unmukt Gupta, Fernando Escobedo The self-assembly of polyhedral particles confined to a fluid-fluid interface is studied using Monte Carlo simulations. Several polyhedral shapes are studied, which are selected from a family of truncated cubes which include cubes, cuboctahedra, and octahedra. First we studied the case of hard particles pinned to the interface by restricting their movement in the direction perpendicular to it while allowing their free rotations. Our results suggest that the known solid phases and mesophases of these shapes in the 3D bulk are ``translated'' into variants in 2D space. These insights on 2D entropic self-assembly of polyhedral particles is a first step toward understanding the self-assembly of particles at fluid-fluid interfaces, which is driven by a complex interplay of entropic and enthalpic forces. As a second step we hence studied the particle-surface and particle-particle interactions associated with a fluid-fluid interface using both continuum and polybead models to assess the role of enthalpic interactions in determining the particle orientation behavior with respect to interface. We find that the thickness of the interface can introduce non-trivial effects on the preferential particle orientations. [Preview Abstract] |
Wednesday, March 16, 2016 10:48AM - 11:00AM |
K37.00013: The Relationship between Self-Assembly and Conformal Mappings Carlos Duque, Christian Santangelo The isotropic growth of a thin sheet has been used as a way to generate programmed shapes through controlled buckling. We discuss how conformal mappings, which are transformations that locally preserve angles, provide a way to quantify the area growth needed to produce a particular shape. A discrete version of the conformal map can be constructed from circle packings, which are maps between packings of circles whose contact network is preserved. This provides a link to the self-assembly of particles on curved surfaces. We performed simulations of attractive particles on a curved surface using molecular dynamics. The resulting particle configurations were used to generate the corresponding discrete conformal map, allowing us to quantify the degree of area distortion required to produce a particular shape by finding particle configurations that minimize the area distortion. [Preview Abstract] |
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