Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U8: Colloidal Manipulation Techniques |
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Sponsoring Units: DFD Chair: Vinothan Manoharan, Harvard University Room: Morial Convention Center RO6 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U8.00001: Local perturbation caused by a particle driven through a 2-D colloidal suspension Cara Hageman, Vikram Prasad, Eric R. Weeks When polystyrene colloids are placed at a decane-water interface they form different phases based on their area fraction. These phases are: liquid, liquid-hexatic, hexatic and crystalline. In particular, the hexatic-crystal transition is characterized by a change in the functional form of the correlation functions and the density of defects. We study this system for area fractions near the hexatic-crystal transition. Using a laser tweezer we trap and drag a particle along the interface and observe its effect on the surrounding colloids. We observe a change in the local density of defects and a decay in the perturbed motion of colloids away from the trapped particle, revealing a length scale. We measure this length scale as a function of area fraction of the colloids and the applied velocity of the trapped particle. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U8.00002: Microrheology of a sticking transition Shobo Bhattacharya, Prerna Sharma, Shankar Ghosh The phenomenon of colloidal deposition in presence of shear is studied by using an optical tweezer to hold a colloidal particle in close proximity of a plate subject to a sinusoidal in-plane shaking. Through the measurement of the real and imaginary parts of the particle's response function, the coupling between the particle and the plate is found to evolve from a viscous regime to an elastic regime through an intermediate regime of time-dependent enhancement of viscoelasticity, reminiscent of aging in glasses. The sticking transition can be described in a scenario of hindered Stokes-Einstien diffusion and the Maxwell model of viscoelasticity. Upon changing the bead -plate interaction or the strength of the drive, three regimes of response: stick, stick-slip and slip are observed. The observed behavior has analogies to jamming in granular materials and the glass transition in viscous liquids. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U8.00003: A New Diffusion NMR Experimental Model System for Studies of Bidisperse Colloids Anand Yethiraj, Swomitra Palit A method to prepare monodisperse colloidal particles that are simultaneously NMR-visible and fluorescent is described. A systematic approach to obtain spectrally resolved diffusion coefficients for every component (colloid and solvent) in a monodisperse colloidal suspension is presented. We also prepared bidisperse colloidal suspensions where each colloid component has a distinct NMR spectral signature, and obtained the diffusion coefficient of both colloid species and solvent simultaneously, in concentrated colloidal suspensions with volume fractions between 20 and 50 $\%$. This colloidal model system enables the study of bidisperse colloids at different size ratios and number ratios. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U8.00004: Universal exponential tails in the displacement distribution observed in an attractive colloidal glass Yongxiang Gao, Maria Kilfoil Dynamical heterogeneities exist ubiquitously in glassy materials. They manifest themselves as a non-Gaussian distribution of the constituent particle displacements, that is, the self part of the van Hove correlation function. Though the shape of the tail of the distribution looks exponential or nearly exponential, not until recently has serious attention been paid to it. We observe pure exponential behavior---neither stretched nor compressed---over a wide range of volume fractions and time scales in an attractive colloidal system on the route to attractive glass transition. We observe universal behavior as all the distributions over a wide range of $\tau $ and $\phi $ can be scaled together. The tails arise from the mobile sub-component of the constituent particles. If time permits, I will also show our studies on the structure of colloidal gels and attractive glasses in terms of a translational order parameter and an orientational order parameter, under different interaction strength, volume fraction and buoyancy matching conditions. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U8.00005: Point response of a 2D packing of soft colloidal spheres near the jamming transition. Peter J. Yunker, Daniel T. N. Chen, Zexin Zhang, Arjun G. Yodh We have created a 2D jammed packing by confining a bidisperse mixture of thermoresponsive NIPA microgel spheres between two glass slides with a thickness of roughly the larger sphere diameter. The packing is subjected to a point compression created by local heating with optical tweezers. We use particle tracking microscopy to characterize the response as a function of particle volume fraction both above and below the jamming transition. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U8.00006: Local perturbations of dense colloidal suspensions Gianguido C. Cianci, Eric R. Weeks A rapid temperature quench can transform a liquid into a disordered solid: a glass. We model glassy materials using dense colloidal suspensions, where the transition is induced by increasing the number density rather than decreasing temperature. This transition has drawn significant attention because it poses numerous fundamental questions. For example, close to the glass transition temperature a small decrease in temperature can cause the viscosity of the liquid to increase by 14 orders of magnitude. Meanwhile the structure remains essentially unchanged --- there is no growing static length scale accompanying the transition. Fast laser scanning confocal microscopy allows us to directly observe and track thousands of colloidal particles in real time. We add a small number of superparamagnetic colloids in the sample and pull them with an external magnet. The motion of a magnetic probe locally perturbs the dense suspension and highlights its heterogeneous structure. We examine the dependence of the affected region's size on density and applied magnetic force. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U8.00007: Understanding the dynamics of closely packed microgel particles Melaku Muluneh, Hans Wyss, Giovanni Romeo, Johan Mattsson, Alberto Fernandez-Nieves, Jinwoong Kim, David Weitz Soft particles such as microgel suspensions have important applications in industry, which exploit their unusual structural and rhelogical properties. Despite their relevance, the fundamental physics that controls their behavior remains poorly understood. Intriguingly, microgels act as fluid even at high density. Techniques such as rheology, microscopy, and light scattering have been used to probe the macroscopic properties of these materials -- however, the underlying physical mechanisms demand further investigation. We use confocal microscopy to image the local dynamics of highly packed microgels. The gel particles are tracked over time to obtain information about the short and long range correlations of the local particle motion. We probe their response to changes in environmental factors such as temperature or pH using light scattering. The results obtained not only help us understand the origins of the observed macroscopic behavior, but also give us information on the dynamics of glassy arrest in general. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U8.00008: Colloidal Particle Geometry and Its Effect on Optical Trapping Rachael Harper, Alex Levine Recent experiments by Wilking and Mason (Europhysics Letters, in press) on the laser trapping of colloids of various shapes (the letters of the alphabet) show that for identical chemistries the trapping force varies wildly with particle shape. In fact, certain shapes do not trap at all. Motivated by these experimental results, we explore the trapping of particle of variable shape using a ray-optics simulation. This numerical tool allows us to perform Monte Carlo integrations of the total trapping forces and torques for a series of objects such as a cross (the letter ``x'') or a beam (the letter ``I''). We find that certain shapes feature bi-stable trapping positions/orientations, and some, indeed, do not allow for trapping at all. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U8.00009: Modeling pore formation in lipid membranes via Janus nanoparticles Alexander Alexeev, William E. Uspal, Anna C. Balazs Phospholipid membranes, which separate the cytoplasm from the extracellular environment in biological cells, embed a large diversity of proteins. Some proteins form pores for the free transport of small molecules and ions across the membrane. Here, we use coarse grained numerical simulations to design a synthetic membrane, where pores can be formed ``on demand.'' Specifically, we use dissipative particle dynamics to probe the interaction between bilayer membranes and nanoparticles. The particles are nanoscopic Janus beads that comprise both hydrophobic and hydrophilic portions. We demonstrate that when the membrane is ruptured due to an external stress, these nanoparticles diffuse to the free edge of the membrane and form stable pores, which persist after the stress is released. Pore size depends on the architecture and concentration of the nanoparticles. Once a pore is formed, a small increase in membrane tension readily reopens the pore allowing rapid transport through the membrane. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U8.00010: Three Dimensional Rotational Motion of Colloidal Clusters Justin Caram, Vinothan Manoharan We will present the results of a study into the three dimensional rotational and translational diffusion of colloidal clusters, especially dimers and trimers. We will have accomplished this study using both diffraction pattern and holograph analysis, as well as depolarized dynamic light scattering. We believe that trimers break into discrete rotational diffusion constants dependent on their geometries. These findings can be matched to the results for the rotational decay in the correlation function generated by DDLS. Understanding these diffusion constants is important to the understanding of protein and liquid crystal dynamics in solution. Furthermore, developing depolarized dynamic light scattering experimental strategy for non ellipsoidal systems may help to determine 3-dimensional hydrodynamic extent of such systems in solution. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U8.00011: Anomalous Rotation of a Pair of Spherical Particles in AC Electric Fields Pushkar Lele, Eric Furst Suspensions of colloidal particles are observed to form angled bands and vortices near surfaces in AC electric fields. We map the critical frequencies and field biases at which particles phase separate in to bands and the vortices gradually set in. The results from such mapping experiments are interpreted based on measurements of anomalous rotation on a pair of colloidal particles held in blinking optical tweezers. Our observations suggest that particle pair rotation is the characteristic motion in vortices and that the polarization of double layer around the particles influences the angular velocity of the vortex revolution. Increasing the electrolyte concentration of the medium or the frequency of the electric field results in reduction of the rotation. Based on these results, the suspension behavior can be ``tuned'' by changing the ionic strength, field strength, field frequency and particle size. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U8.00012: Interferometric and holographic imaging combined: correlating interface deformations with 3D tracking of interfacial particles David Kaz, Vinothan Manoharan We employ the techniques of digital holography and interference phase mapping to investigate particles on interfaces. Digital holography is used to track the positions of small (micron sized) particles to within a few nanometers in three dimensions, while optical interferometry maps interfacial deformations to within a few tens of nanometers. By utilizing both techniques simultaneously, we correlate the 3D position of particles trapped on an interface with deformations of that interface at up to 1000 frames per second. Such comprehensive data will serve to answer questions regarding the capillary interactions of particles on an interface. [Preview Abstract] |
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