Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T18: Colloids: Charged, Clustered, and/or Sticky |
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Sponsoring Units: DCMP GSNP Chair: Carlos Orellana, Emory University Room: 403 |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T18.00001: Self-assembly of three-dimensional open structures using patchy colloidal particles D. Zeb Rocklin, Xiaoming Mao Open structures can display a number of unusual properties, including a negative Poisson's ratio, negative thermal expansion, and holographic elasticity, and have many interesting applications in engineering. However, it is a grand challenge to self-assemble open structures at the colloidal scale, where low coordination number can leave them mechanically unstable. In this talk we discuss the self-assembly of open structures using triblock Janus particles, which have two large attractive patches that can form multiple bonds, separated by a band with purely hard-sphere repulsion. Such surface patterning leads to open structures that are stabilized by orientational entropy (in an ``order-by-disorder'' effect) and selected over close-packed structures by vibrational entropy. For different patch sizes the particles can form into either tetrahedral or octahedral structural motifs that then compose open lattices. Using an analytic theory, we examine the phase diagrams of these possible open and close-packed structures for triblock Janus particles and characterize the mechanical properties of these structures. Our theory leads to rational designs of particles for the self-assembly of three-dimensional colloidal structures that are possible using current experimental techniques. [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T18.00002: Reentrant phase transitions from depletion: colloidal crystals to flocculation Lang Feng, Bezia Laderman, Stefano Sacanna, Paul Chaikin Conventional depletion is supposed to be temperature independent. However, we find that many typical colloid-depletion systems show remarkable phenomena as temperature is varied. $1\mu m$ polystyrene spheres in water are known to form colloidal crystals when PEO is added as a depletant. When this system is heated the crystal melts at a first critical temperature $T_{1} \sim 60C$, and then at higher temperature $T_{2} \sim 70C$ the colloids flocculate. We argue that a weak temperature-dependent interaction between polymer and colloid is responsible for the observed phenomena: crystals form when the colloid-polymer interaction is repulsive, flocculation occurs when the interaction is attractive, and melting occurs in between when both phases are frustrated. The melted phase occurs due to an unexpected cancelation when combining both entropic and enthalpic attractions. We propose a simple statistical model to map out the observed transitions and fill the theoretical gap between the two established scenarios for colloid-polymer systems, namely depletion and flocculation. We have seen the same temperature dependent phenomena for TPM, PS and silica spheres with PEO and dextran as depletants. Our discovery provides a fundamental understanding of the polymer-colloid system and opens new possibilities for colloidal self-assembly and temperature-controlled viscoelastic materials. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T18.00003: Transitions of a hard-sphere colloidal crystal to a colloidal crystal with attractive interactions Matthew Gratale, Matthew Lohr, Ye Xu, Arjun Yodh Recently, colloid experiments have probed and found interesting differences in the properties of disordered glassy media as a function of the sign of the interparticle interaction [1-3]. Here, we report experiments on colloidal crystals whose constituent particles have interactions that can be rapidly varied from repulsive hard-sphere-like to attractive. Micron-size colloidal particles are suspended in a binary fluid mixture of water and 2,6-lutidine near the critical temperature of 307 K [4]; by changing temperature, the interparticle interactions can be rapidly switched from repulsive to attractive, and the accompanying variations in structure and dynamics can be tracked. Preliminary results show that when the interparticle attraction turns on, the lattice constant decreases and the system transitions from a ``repulsive'' crystal into a fluid-crystal coexistence phase. This fluid-crystal coexistence phase consists of small, dense ``attractive'' crystalline domains separated by ``voids'' filled with a very dilute colloidal fluid. These voids appear to originate at the grain boundaries and the lattice defects of the original repulsive crystal. [1] Eckert \textit{et al}., PRL \textbf{89}, 125701 (2002). [2] Kaufman \textit{et al}., J. Chem. Phys. \textbf{125}, 074716 (2006). [3] Zhang \textit{et al}., PRL \textbf{107}, 208303 (2011). [4] Hertlein \textit{et al.} Nature \textbf{451}, 172-175 (2008). [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T18.00004: Dissipative-Particle-Dynamics Simulation of Charged Colloid under Alternating Electric Fields Jiajia Zhou, Friederike Schmid We study the response of spherical charged colloids under alternating electric fields (AC-fields) by mesoscopic simulation methods, accounting in full for hydrodynamic and electrostatic interactions. A coarse-grained molecular dynamics approach is taken to model the fluids, in which the solvent particles are simulated using Dissipative Particle Dynamics, while the electrostatic interaction between all charges are computed using Particle-Particle-Particle Mesh method. Due to the interplay of the electrostatic and hydrodynamic interactions, the mobility and the polarizability exhibit a dependency on the frequency of the external fields. The effect of the ionic strength of the solution and the bare charge of the colloids are also investigated systematically. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T18.00005: Determination of colloidal osmotic equation of state by dielectrophoresis Hao Huang, Jacob Mazza, H. Daniel Ou-Yang Osmotic equation of state P(N,T) describes both the mechanical properties and phase behavior of a colloid suspension. Traditionally, it is measured by sedimentation or scattering methods. However, these methods are tedious and time consuming. Here, we propose an alternative approach to determine P(N,T) by dielectrophoresis (DEP). Confocal imaging is used to measure the particle density profile, from which we can determine the DEP force field when the particle concentration is low and the inter-particle interactions are negligible. Once the force field is known, using a generalized sedimentation equilibrium equation, we can calculate P(N,T) from the particle density profile of interacting colloids. We will report our results for charge-stabilized polystyrene latex particles under different salt concentrations, salt types, as well as added neutral polymers. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T18.00006: Multivalent Ion Screening of Charged Glass Surface Studied by Streaming Potential Measurements Ran Li, Brian Todd Ions present in solution strongly modify local electrical properties of charged surfaces. While the effects of monovalent ions are accurately described by the Poisson-Boltzmann equation, the mechanism by which multivalent ions screen charged surfaces remains unclear. A recent theory by dos Santos et. al [A. P. dos Santos, A. Diehl, and Y. Levin, J. Chem. Phys. 132, 104105 (2010)] treats the electrolyte solution as consisting of two sub-systems: a strongly coupled liquid of multivalent ions adjacent to the charged surface and a gaslike phase further into the bulk. The theory makes quantitative predictions of the electric potential in solutions containing both multivalent and monovalent ions. We used the streaming potential technique to measure electric potentials over a range of multivalent and monovalent ion concentrations and used the data to evaluate dos Santos et. al's theory. We found that SCL predictions agree quantitatively with our experimental data. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T18.00007: Effect of image charges on double layer structure and forces Rui Wang, Zhen-Gang Wang The study of the electrical double layer lies at the heart of colloid and interface sciences. Here, we examine the electrical double layer structure and forces between two neutral or like-charged plates by accounting for the image charge effects under weak-coupling conditions. By treating the fluctuation effect on the ion distribution and free energy self-consistently and nonperturbatively, we show that the image charge interaction appears as part of the self-energy in the Boltzmann factor: there is no limiting condition for which Poisson-Boltzmann (PB) theory is valid, contrary to the general consensus in the community that PB theory is the exact theory in the weak coupling limit. For electrolyte solutions between two neutral plates, we show that depletion of the salt ions by the image charge repulsion results in short-range attractive and long-range repulsive forces. If cations and anions are of different valency, the asymmetric depletion leads to the formation of an induced electrical double layer. In comparison to a 1:1 electrolyte solution, both the attractive and the repulsive parts of the interaction are stronger for the 2:1 electrolyte solution. For two charged plates, the competition between the surface charge and the image charge effect can give rise to like-charge attraction and charge inversion. These results are in stark contrast with predictions from the PB theory. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T18.00008: Self-assembly of dielectric Janus particles Huanxin Wu, Erik Luijten We consider dielectrically heterogeneous Janus particles, spherical colloids with a dielectric mismatch between their two hemispheres. When such particles are suspended in solution, this mismatch leads to different polarization charges induced on the two hemispheres. Until now, the role of these polarization charges has not been considered in the context of colloidal self-assembly. Here, we address this challenge by means of a new and efficient computational approach that dynamically and spatially resolves the polarization charge distribution within molecular dynamics simulations. Employing this approach, we explore the effect of dielectric many-body effects on the ion distribution around dielectric Janus colloids. We also investigate ways to exploit these effects for controlling aggregation and self-assembly by tuning the dielectric mismatch. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T18.00009: A sphere packing slideshow Miranda Holmes-Cerfon, Steven Gortler, Michael Brenner We have enumerated all the ways to arrange n <= 13+ spheres as a cluster that is nonlinearly rigid. We have discovered many packings that are hypostatic, namely they have fewer than the 3n-6 contacts required to be linearly rigid. Simple scaling arguments explain why these are thermodynamically important when the spheres are colloids interacting with a short-range potential. We discuss these clusters, as well as other surprises that came up along the way. ("+" means we have enumerated only a particular kind of cluster for n=14, 15, and beyond.) [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T18.00010: Dynamic of Faceted Colloidal Clusters Melinda Sindoro, Ah-Young Jee, Changqian Yu, Steve Granick We study the emulsion induced clustering of faceted metal organic frameworks (MOFs) and their dynamics. Our approach to anisotropic building block is through the rational synthesis of water stable and highly uniform MOFs. This generates colloidal-sized MOFs of defined polyhedral shape with tunable size in micrometer range that are suitable for in situ imaging. The 3D clusters formations are promoted by hydrophilic MOFs particles confined in aqueous droplets of binary water-lutidine mixture at transition temperature. Below this temperature, the water droplet decreases in volume due to one phase mixing with lutidine which forces the $N$-mers of faceted particles to aggregate in close contact. We compare the faceted clusters formed to those made of spherical particles in term of the building block sphericity. Other focus of our study involves the dynamic of the clusters. We found that, unlike spherical clusters, these faceted $N$-mers are highly stable on large scale of temperature due to their dominant capillary force on their facet-to-facet contact. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T18.00011: Generalized sedimentation equilibrium: Measuring colloidal osmotic pressure of nanoparticle suspension by optical trapping Jinxin Fu, H. Daniel Ou-Yang Generalized sedimentation equilibrium is achieved through the force balance between the osmotic pressure of colloidal nanoparticles and the trapping pressure by a focused IR laser beam. According to Einstein's diffusion theory for suspended particles at equilibrium state, the osmotic pressure of the colloidal particles can be obtained by the spatial integration of the product of the external force field and the particle number density. In our experiment, both the trapping force and the number density of the particles are measured by an optical bottle method. The measured osmotic pressure (P-N curve) of polystyrene nanospheres in the presence of KCl and PEG is found to decrease with increasing KCL concentration and PEG concentration, which is attributed to the screening of the surface charges of the nanoparticles by KCl ions, and the attractive depletion interaction by the polymer (PEG), respectively. Our experimental results can be used to predict the phase separation of colloidal nanoparticles. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T18.00012: Tunably Soft Colloids Synthesis and Characterization by Holographic Microscopy Chen Wang, Hagay Shpaisman, David Grier Polydimethylsiloxane(PDMS) is an industrially important, widely used silicon-based organic polymer. Previous work showed that the addition of trivalent cross-linker transforms PDMS emulsion droplets into complied spheres, whose elasticity scales with the concentration of cross-linker. We use holographic video microscopy to characterize the synthesized PDMS with varying degree of deformability. Holographic characterization yields measurements of cross-linker concentration through the influence on the particles' sizes and refractive indices. In the performed experiments, we are able to detect the transition between liquid droplets and complied particles. and monitor the polymerization progress. The particles' compliance can be gauged in their interactions with rigid surfaces that we measure with holographic optical trapping and holographic particle characterization. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T18.00013: New Techniques in Optical Trapping and Sensing Lulu Liu Micrometer-sized dielectric scatterers suspended in fluid can act as sensitive, dynamic probes of surface forces and optical near-fields. Because of their small size, they are optimally manipulated with light. We demonstrate several new techniques that expand our current capabilities of optical trapping and sensing. This includes the use of nearly diffractionless beams for particle confinement in only two dimensions, effective optical traps for particles with lower index than surrounding fluid, and high precision tweezing and sensing near reflective/metallic surfaces. Finally, we demonstrate the application of a combination of these techniques in the successful measurement of near-field optical and double-layer forces. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T18.00014: Digital holographic microscopy of weakly scattering nanoparticles in solution Aaron M. Goldfain, Yoav Lahini, Vinothan N. Manoharan Many important biological processes, such as virus capsid self-assembly, protein transcription, and lipid vesicle formation involve biological molecules in solution that are $\sim$10 nanometers in size. Such particles are difficult to detect using visible light because they diffuse rapidly and have small scattering cross-sections. Digital holographic microscopy (DHM) has been used to image micrometer-sized particles in solution at fast time scales in 3 dimensions, but conventional in-line DHM techniques used on nanoparticles yield low signal to background ratios. We explore methods to increase the signal to background ratio of holograms by both increasing the amount of light that is coherently scattered from objects, and by decreasing the intensity of the reference beam used to form a hologram. We record holograms of gold and polystyrene nanoparticles in solution and track them in three-dimensions with high precision at frame rates of 1 kHz. The goal is DHM of single proteins. [Preview Abstract] |
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