Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A13: Focus Session: Polymer Colloids: Structure, Function, and Dynamics I |
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Sponsoring Units: DFD DPOLY Chair: Alberto Fernandez De Las Nieves, Georgia Institute of Technology Room: D225/226 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A13.00001: Osmotic pressure of microgel suspensions Juan Jose Lietor-Santos, Benjamin Sierra-Martin, Justin Frederick, Yesenia Laporte, George Markou, Alberto Fernandez-Nieves Microgels are crosslinked-polymeric networks in the colloidal domain, whose size can be change in response to external stimuli. They are soft particles by construction and can exhibit a very different behavior compared to hard sphere suspensions. In some cases, this different behavior has been understood by alluding to particle de-swelling at low volume fractions. For this to happen, the suspension osmotic pressure at such volume fraction should be comparable to the particle bulk modulus. In this work, we independently measure the bulk modulus of microgel particles and the suspension osmotic pressure and find that both magnitudes become comparable at a volume fraction corresponding to a liquid-to-solid transition, which we asses using rheology. Interestingly, in the solid region, the shear and compressional moduli of the suspension exhibit the same behavior with volume fraction, in analogy to emulsions. However, by contrast to emulsions, they are almost two orders of magnitude apart. This reflects the contributions from the internal modes of the microgel particles, which are absent for the case of an emulsion drop. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A13.00002: Particle Charging and Interaction in Nonpolar Colloidal Dispersions Mediated by Nonionic Surfactants Sven Behrens The electrostatic stabilization of colloidal dispersions is usually considered the domain of polar media only, but some surfactants are known to raise the conductivity of liquids with low electric permittivity and to mediate charge-stabilization of nonpolar dispersions. Here we report an example of the counterintuitive electrostatic effects of nonionic surfactants on colloidal particles in nonpolar solvents. PMMA particles in hexane solutions of sorbitan oleate (Span) surfactants exhibit a field-dependent electrophoretic mobility. In the zero field limit, we find large surface potentials whose decay with increasing surfactant concentration resembles the salt-induced screening in aqueous solutions. The amount of surface charge and screening ions in the nonpolar bulk is further characterized via ensemble measurements of the particles' pair interaction energy. In contrast to the behavior reported for systems with \textit{ionic} surfactants, we observe particle charging and a screened Coulomb type interaction both above and below the surfactant's critical micelle concentration. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A13.00003: Experimental Studies of the Brownian Diffusion of Boomerang Colloidal Particle in a Confined Geometry Ayan Chakrabarty, Feng Wang, Bhuwan Joshi, Qi-Huo Wei Recent studies shows that the boomerang shaped molecules can form various kinds of liquid crystalline phases. One debated topic related to boomerang molecules is the existence of biaxial nematic liquid crystalline phase. Developing and optical microscopic studies of colloidal systems of boomerang particles would allow us to gain better understanding of orientation ordering and dynamics at ``single molecule'' level. Here we report the fabrication and experimental studies of the Brownian motion of individual boomerang colloidal particles confined between two glass plates. We used dark-field optical microscopy to directly visualize the Brownian motion of the single colloidal particles in a quasi two dimensional geometry. An EMCCD was used to capture the motion in real time. An indigenously developed imaging processing algorithm based on MatLab program was used to precisely track the position and orientation of the particles with sub-pixel accuracy. The experimental finding of the Brownian diffusion of a single boomerang colloidal particle will be discussed. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A13.00004: Effect of Boundary Mobility on the Dynamics of Confined Colloidal Suspensions Gary L. Hunter, Kazem V. Edmond, Eric R. Weeks We use high-speed confocal microscopy to study the influence of boundary mobility on the dynamics of confined colloidal suspensions. Experiments in molecular super-cooled liquids show that confinement can enhance or hinder sample mobility, depending on whether the confining boundary is ``soft'' (mobile) or ``hard'' (immobile). We confine suspensions of PMMA microspheres within emulsion droplets of different sizes to examine the consequences of confinement. By changing the viscosity of the external, continuous phase, we also vary the boundary mobility of our samples. In this way, we decouple the effects of confinement and boundary mobility, and draw comparisons between colloidal suspensions and molecular liquids. [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A13.00005: Observing liquid-gas nucleation in a colloid-polymer solution Ryan McGorty, Vinothan N. Manoharan We study liquid-gas nucleation in a colloid-polymer solution. Though the colloidal particles are too small to resolve, we are able to observe nucleating droplets due to the refractive index mismatch between the two fluid phases. By using digital holographic microscopy and thermally-responsive colloids we are able to observe the micron-sized nucleating droplets and their fluctuations in three-dimensions. From the droplets' fluctuations we can back out the interfacial tension. Additionally, our three-dimensional imaging technique allows us to capture individual nucleation events and their rate of occurrence. We hope that our data will allow us to better understand nucleation kinetics. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A13.00006: Measuring the translational and rotational diffusion of colloidal clusters with digital holographic microscopy Jerome Fung, Kristopher Eric Martin, Ryan McGorty, David M. Kaz, Rebecca W. Perry, John A. Keller, Guangnan Meng, Vinothan N. Manoharan We measure the rotational and translational diffusion coefficients of individual non-spherical colloidal clusters undergoing three-dimensional Brownian motion. We image clusters comprised of spheres approximately 1 $\mu$m in diameter using digital holographic microscopy. Fitting the measured holograms to exact electromagnetic scattering calculations allows us to determine cluster positions and orientations with millisecond temporal resolution and $\sim$10 nm spatial resolution. For dimers of polystyrene spheres in an aqueous solution, our measurements of the coefficients for rotational diffusion as well as translational diffusion parallel and perpendicular to the dimer axis are consistent with theory. We discuss the extension of this work to non-axisymmetric trimers and potential applications. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A13.00007: Density of States of a Two-Dimensional NIPA-Polystyrene Colloidal Crystal Matthew Gratale, Peter Yunker, Ke Chen, Arjun Yodh In this work we are interested in how ``dopants'' affect the vibrational properties of crystals. We study the vibrational density of states of a two-dimensional colloidal crystal consisting of a mixture of hard polystyrene particles and soft NIPA microgel particles. Thus, depending on the particles involved, multiple inter-particle potentials are present in these crystals. The number ratio of hard to soft particles is varied, creating crystals consisting primarily of soft particles doped with hard particles and vice versa. We employ video microscopy to derive the phonon density of states of corresponding ``shadow'' crystals with the same geometric configuration and interactions as the experimental colloidal system, but absent damping [1,2,3]. Preliminary data reveal low frequency plane-like waves in all crystals, regardless of composition. Participation in higher frequency modes is often enhanced in one species of particles and diminished in the other.\\[4pt] [1] Chen \textit{et al}., PRL 105, 025501 (2010). [2] Kaya \textit{et al}., Science 329, 656 (2010). [3] Ghosh \textit{et al}., PRL 104, 248305 (2010). [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A13.00008: Structure and dynamics of confined colloid-polymer mixtures Jacinta Conrad, Binh Trinh, Gildardo Ceballos Colloidal processing routes typically require attractive suspensions to be flowed through fine geometries such as microchannels, nozzles, or thin films. To elucidate the effects of confinement on attractive suspensions during processing, we use confocal microscopy to image the structure and dynamics of model colloid-polymer mixtures as a function of confinement dimensionality and thickness, colloid volume fraction, and the strength and range of the attraction. We characterize the phase behavior of the confined suspensions, and find that confinement induces non-uniform structural changes within colloidal gels. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A13.00009: Controlling the size distribution of self-assembled colloidal clusters Nicholas Schade, Jesse Collins, Jonathan Fan, Miranda Holmes-Cerfon, Vinothan Manoharan Using a combination of experiment and simulation, we investigate the structures that form when spherical colloidal particles cluster around spheres of different sizes in a binary mixture. We use either oppositely charged particles or particles coated with complementary DNA sequences to form the clusters. Using optical microscopy, we examine the effect of the stoichiometric ratio, the size ratio, and the type of interaction on the distribution of clusters. These parameters serve as useful control mechanisms for the synthesis of nanostructures with tunable properties. For example, a high density of tetrahedral clusters of metallo-dielectric spheres could be used to create a bulk, isotropic metamaterial. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A13.00010: Dynamics of interfacial breach by colloidal spheres David M. Kaz, Ryan McGorty, Madhav Mani, Vinothan N. Manoharan We present observations of individual colloidal spheres as they approach and penetrate a flat aqueous interface. Polystyrene spheres with various surface chemistries (sulfate, carboxyl, etc) are brought to the boundary between an oil phase (decane) and an aqueous phase (water+glycerol+NaCl) using radiation pressure from a tightly focused laser. Holographic images are recorded at up to 24,000 frames per second and subsequently compared with Mie-scattering calculations to obtain positional data at a resolution of 5nm in x,y, and z. Typical trajectories consist of an approach to the interface that is dominated by hydrodynamics; a discontinuous jump at the point of penetration (POP); and a very long timescale relaxation that is logarithmic in time. We find that the concentration of salt in the aqueous phase must be above a certain threshold (depending on species) for breach to occur. Well above this threshold, trajectories just prior to the POP are characterized by short-timescale features that are non-monotonic in salt concentration. DLVO type calculations reproduce some aspects of these features, but the non-monotonicity remains mysterious. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A13.00011: Sub-diffusion of DNA Coated Particles Near a Complementary DNA Covered Surface Lang Feng, Qin Xu, Ruojie Sha, Nadrian Seeman, Paul Chaikin We have measured the diffusive behavior of micrometer sized colloids in a DNA covered particle-surface system. Near the particle-surface melting temperature of $\sim 45^{\circ}$ C we observe conventional diffusion but as temperature is lowered we see a crossover to sub-diffusion over a narrow temperature range. The sub-diffusive behavior is intimately related to the broad distribution of local trapping times. We present a theoretical model which explains the sub-diffusion exponent $\mu$ in $< R^2 (t) > \sim t^{\mu}$ , which ranges from $\mu = 1$ at $44.7^{\circ}$ C to $\mu= 0.33$ at $44.1^{\circ}$ C. From the distribution of number of DNA bonds we calculate the trapping time distribution and average trapping time. When the measurement time exceeds the average trapping time the system is in equilibrium and exhibits conventional diffusion. When the measurement time is less than the average trapping time the system is not in equilibrium and is sub-diffusive. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A13.00012: Multiple-Stage Melting and Freezing of Colloidal Crystallites with Short-range Attraction Liquan Pei, J.R. Savage, A.D. Dinsmore We study the dynamics of melting and freezing in a model colloidal system with short-range, temperature tunable attraction. In particular, we mix micron-sized, charge stabilized polystyrene spheres with salt and the surfactant micelles. The micelles induce depletion attraction whose range is less than 2{\%} of the sphere diameter and whose magnitude changes strongly with temperature. We use optical microscopy to record the dynamics of freezing and melting following temperature changes. We use particle tracking algorithms to identify the particles with sub-pixel resolution. For samples with area fraction less than 40{\%}, we have observed that melting and freezing occur in multiple stages, with a metastable liquid phase appearing in both processes. For the freezing sample at area fraction 55{\%}, we have found that the gas droplets are nucleated from high area fraction backgound. The data also show how nucleation dynamics are affected by the metastable gas-liquid binodal. We are also investigating the role of the second, metastalbe solid phase in melting and freezing. Our results are relevant to systems where non-equilibrium states may play a role in phase separation. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A13.00013: Colloidal aggregation in microgravity by critical Casimir forces Sandra Veen, Peter Schall, Marco Potenza, Matteo Alaimo, Stefano Mazzoni, Gerard Wegdam We study aggregation and crystal growth of spherical Teflon colloids in binary liquid mixtures in microgravity by the critical Casimir effect. The critical Casimir effect induces interactions between colloids due to the confinement of bulk fluctuations (density or concentration) near the critical point of liquids. The strength and range of the interaction depends on the length scale of these fluctuations which increase as one approaches the critical point. The interaction potential can thus be tuned with temperature. We follow the growth of structures in real time with Near Field Scattering. Measurements are performed in microgravity in order to study pure diffusion limited aggregation, without disturbance by sedimentation or flow. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A13.00014: Particle interactions in colloids are revealed in a nonlinear effect in light transmission Jinsuk Song, Daniel Ou-yang Studies on interactions between particles in highly concentrated suspensions are rare because the solutions are opaque and the interpretations from methods such as diffusing wave spectroscopy are often complicated. We propose a simple method of probing particle interactions in the opaque solution by measuring light transmission affected by optically induced particle concentration enhancement. The increase in the particle concentration with the input light intensity depends on the interactions between particles. We demonstrate how this method can be used to determine single particle trapping energy and the virial coefficients in aqueous suspensions of 190 nm polystyrene spheres. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A13.00015: Nano-dumbells pack densely to form birefringent photonic crystals Jason Forster, Jin-Gyu Park, Manish Mittal, Vinodkumar Saranathan, Heeso Noh, Carl Schreck, Richard Prum, Corey O'Hern, Hui Cao, Eric Furst, Eric Dufresne Monodisperse spherical colloidal particles robustly self-assemble into crystals at high concentration. We study the the self-assembly of polymer nano-dumbells and find that they crystallize only under strong confinement - in thin films less than three particles thick. On the other hand, external electric fields can readily align dumbbell- shaped particles to make a birefringent suspension. When the electric field is turned off, the dumbbells rapidly lose their orientational order and the birefringence quickly goes away. However, if the solvent is removed with the electric field on, the particles self-assemble into a novel dense crystalline packing hundreds of particles thick. We describe the essential physics of self-assembly of these structures through an interplay of the applied electric field and capillary forces. [Preview Abstract] |
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