Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H9: Colloids: Experimental |
Hide Abstracts |
Sponsoring Units: DFD Chair: David Pine, New York University Room: D220 |
Tuesday, March 22, 2011 8:00AM - 8:12AM |
H9.00001: Universality in the delayed failure of colloidal gels Joris Sprakel, David Weitz The mechanical failure of heterogeneous solids is not always instantaneous with the application of a load, but can be significantly delayed. We use colloidal gels, a prototypic heterogeneous material, to unravel the microscopic mechanisms behind this delayed failure. A universal behavior is revealed; the delay time depends only on the magnitude of the applied stress not on its origin. Whether the gel succumbs to internal tension, gravitational compression or shear stresses, the behavior can be quantitatively explained using a generalized bond-rupture model that describes the microscopic events triggering macroscopic failure. [Preview Abstract] |
Tuesday, March 22, 2011 8:12AM - 8:24AM |
H9.00002: Glassy dynamics of 2D colloid crystals in a random pinning potential Sungcheol Kim, Alexandros Pertsinidis, Xinsheng Ling Recently, we have demonstrated that a monolayer charged colloidal crystal confined to a rough charged surface provides a realization of the Larkin-Ovchinnikov random-pinning model in two dimensions [1]. The statics of the system is found to agree with Larkin's prediction of balkanization into small ordered domains. However, the dynamics are in disagreement with the collective creep model. Detailed analysis of the particle trajectories suggest that collective creep is preempted by channel flow. We also find that the velocity response to a step-like driving force shows a stretched exponential behavior similar to that found in structural glasses. Here, we provide a detailed analysis of this process.\\[4pt] [1] A. Pertsinidis and X.S. Ling PRL {\bf 100} \normalfont 028303 (2008) [Preview Abstract] |
Tuesday, March 22, 2011 8:24AM - 8:36AM |
H9.00003: Field-driven pattern formation of charged particles in nonpolar solvent Tina Lin, Shmuel Rubinstein, David Weitz We combine microfluidics and high-speed imaging to investigate transport dynamics of charged colloidal particles in a nonpolar solvent as the polarity of an external electric field is switched periodically. Immediately following a switch, particles which were initially all packed against one electrode move towards the opposite electrode in an unstable manner; instead of remaining uniform, the particle front develops undulations. This results in a heterogeneous deposition of particles on the electrode wall. For a range of wait times between switches, we find that the particles localize at exceptionally well-defined periodic modes and we offer a simple physical model to account for this pattern formation. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H9.00004: Colloids with magnetic patches: synthesis and self-assembly Stefano Sacanna, Laura Rossi, William Irvine, David Pine We developed a new class of colloidal particles that programmably and reversibly self-assemble into well-defined clusters by virtue of ``magnetic patches'' carrying a permanent magnetic dipole moment. The resulting clusters form spontaneously in a zero external field, and their geometry is entirely determined by the interplay between magnetic, steric, and electrostatic interactions. Imposing an external magnetic field enables the clusters to unbind or change their geometry allowing, in principle, to create materials with tunable structural arrangements. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H9.00005: Melting Dynamics of Colloidal Thin Films on Patterned Substrates John Mergo, John Savage, Itai Cohen We present results of experiments on the melting dynamics of colloidal crystals formed on patterned substrates. Our system consists of micron-sized colloidal particles and a tunable short- range attractive depletion interaction that can be controlled by small temperature changes. We investigate the melting rates of crystalline islands that form on substrates with square and hexagonal symmetry. We find that crystals with square symmetry melt significantly slower than those with hexagonal symmetry despite the fact that particles at the edge of the hexagonal crystal are on average bound more strongly than those at the edge of a square crystal. We find that the symmetry of the substrate affects the ability of particles to diffuse away from a melting crystal, and these differences in single-particle diffusion rates account for the difference in melting rates. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H9.00006: Dynamics of Transient Vorticity Aligned Structures in Attractive Colloidal Suspensions Ajay Negi, Michelle Bebrin, Chinedum Osuji Shear rate jumps from high to low flow rates in an attractive colloidal suspension of carbon black particles in a non-polar solvent result in the formation of transient log-like structures aligned in the vorticity direction. Optical microscopy in situ with bulk rheology shows that the appearance of these aggregates is attended by an increase in the suspension viscosity. The viscosity shows a peak and then gradually recedes with passage of time under flow in concordance with the disappearance of the log-like structures. The time at which the viscosity reaches its maximum scales inversely with the shear rate applied to the system. This emergence of the peak in viscosity appears to be controlled by a critical strain and rescaling in these terms produces a common response across several different shear rates. Alteration of the attraction strength between particles by the addition of surfactant severely inhibits the structure formation. We present a simple model to account for these observations. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H9.00007: Two-dimensional Fibonacci spiral optical thermal ratchets Ke Xiao, David Grier A novel two-dimensional optical thermal ratchet has been implemented with holographic optical trapping arrays structured as the ``Fibonacci spiral'' for diffusing colloidal particles. Periodically rotating the optical trapping array by an angle in a three-step cycle yields a two-dimensional time-varying optical landscape that acts either as (1) a deterministic pump when traps are closely dispersed in space, whose induced radial and azimuthal fluxes can be quantitatively mapped out according to the geometry of Fibonacci spiral, or else as (2) an optical thermal ratchet when traps are widely dispersed, whose transport property depends on the competition between the temporal evolution in optical landscapes and Brownian particles' diffusivity. The Fibonacci ratchet displays independent flux reversals in both the radial and azimuthal directions as a function of the cycle frequency and the inter-trap separation. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H9.00008: Using shear to assemble colloidal strings Itai Cohen, Xiang Cheng Sheared colloidal suspensions exhibit various fascinating phases under the influence of hydrodynamic, interparticle and thermal interactions. These shear-induced phases have been intensively studied for suspensions well above the crystalline threshold, but remain relatively unexplored for amorphous suspensions. Here, we report a novel string phase in less concentrated colloidal suspensions under shear, where particles assemble into long strings normal to the plane of shear. This finding contradicts previous numerical results that predict the formation of particle strings along the shear velocity direction. We systematically investigate how the phase depends on the shear rates, the confinement of shear plates, and the volume fractions of samples. We demonstrate the relation between the string phase of low volume fraction samples and the shear-induced crystallization of high volume fraction samples. A simple mechanism for the formation of this novel phase is suggested. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H9.00009: Biomembrane-mediated control of like-charge colloidal attraction Maunta Manandhar, Yupeng Kong, Raghuveer Parthasarathy The nature of attractions observed between like-charged colloidal particles near a confining wall is still mysterious, due in part to the lack of experimental systems with tunable inter-particle interactions. Biomembranes are appealing candidates for colloidal functionalization, enabling access to electrostatic and chemical properties that influence inter-particle relations. Previous optical-trap based examinations of lipid membrane functionalized particles revealed a surprising linear relationship between the magnitude of the attractive pair potential and the particle charge in presence of a wall of constant charge density. Here, using lipid membranes to also functionalize the confining wall, thereby controlling its charge density, we find a non-linear relationship between inter-particle attraction and charge. Our results highlight the role of substrate-induced fields in controlling pair interactions between colloidal microparticles. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H9.00010: Colloidal Gas-Liquid Condensation induced by the Critical Casimir Effect Duc Nguyen, Suzanne Faber, Gerard H. Wegdam, Peter Schall We explore a new temperature control over colloidal phase formation by using the Critical Casimir effect. This effect allows direct control over particle interactions via temperature-dependent solvent fluctuations: In analogy to the confinement of fluctuations of the electromagnetic field between two dielectrics (quantum mechanical Casimir effect), the confinement of fluctuations of a critical solvent leads to an attraction between surfaces that are immersed in this solvent. This allows exquisite temperature control over the interactions of colloidal particles that are suspended in this critical solvent. We show that this temperature control allows us to ``freeze'' a dilute colloidal gas into a dense colloidal liquid, and a crystalline solid. By using confocal microscopy, we follow these phase transitions directly in real space, and we measure the particle pair potential. We show that we can quantitatively account for the gas-liquid condensation by using Van der Waals theory. We study the growth of colloidal liquid droplets by following the mean droplet radius $<$R$>$ with dynamic light scattering. We find $<$R$>$ $\sim$ t1/2 and $<$R$>$ $\sim$ t1/3 indicating that the droplets form by nucleation, followed by diffusion limited growth. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H9.00011: Comparison of different analysis techniques in inline holographic video microscopy Fook Chiong Cheong Holographic video microscope can be analyzed on a frame-by-frame basis to track individual colloidal particles' three-dimensional motions with nanometer resolution. In this work, we compare the performance of two complementary analysis techniques, one based on fitting to the exact Lorenz-Mie theory and the other based on phenomenological interpretation of the scattered light field reconstructed with Rayleigh-Sommerfeld back-propagation. Although Lorenz-Mie tracking provides more information and is inherently more precise, Rayleigh-Sommerfeld reconstruction is faster and more general. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H9.00012: Dynamics of colloidal particles in ice Melissa Spannuth, S.G.J. Mochrie, S.S.L. Peppin, J.S. Wettlaufer Solidification of the solvent phase of a colloidal suspension occurs in many natural and technological settings and is becoming a popular technique for creating microporous structures and composite materials. During freezing, regions of high particle density can form as particles are rejected from the growing solid and guided into a variety of macroscopic morphologies. The particles in the high density regions form an amorphous colloidal solid that deforms in response to internal and external stresses. Using X-ray Photon Correlation Spectroscopy, we studied this deformation for silica particles in polycrystalline ice. We found that the particles in the high density regions underwent ballistic motion coupled with a non-exponential decay of the intensity autocorrelation function (ACF) that transitions from a stretched to a compressed exponential with increasing scattering vector q. While ballistic motion and a compressed exponential decay of the ACF is common, the coupling with a stretched exponential decay is very rare and a transition with increasing q has not previously been reported. We explain this behavior in terms of ice grain boundary migration. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H9.00013: Imaging the microscopic structure of shear thinning and thickening colloidal suspensions Xiang Cheng, Jonathan McCoy, Jacob Israelachvili, Itai Cohen The viscosity of colloidal suspensions varies by orders of magnitude depending on how quickly they are sheared. Such non- Newtonian behavior arises from the arrangement of suspended particles and their mutual interactions. Although numerical simulations and various scattering experiments have revealed much about the local and average suspension structures, particle dynamics at mesoscopic length scales, where non- Newtonian behaviors are believed to originate, are still poorly understood. Here, by combining fast confocal microscopy with simultaneous rheological measurements, we systematically investigate changes in suspension structure over a range of length scales, as the suspension transitions through regimes with different rheological signatures. Our measurements bridge previous simulation and scattering results, and unambiguously show that shear thinning is coupled to particle layering, that shear thickening is decoupled from suspension order-to-disorder transitions, and that there exists a novel phase where particles self-assemble into strings oriented normal to the plane of shear. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H9.00014: Cubic crystals from cubic colloids Laura Rossi, Stefano Sacanna, William Irvine, Paul Chaikin, David Pine, Albert Philipse We have studied the crystallization behavior of colloidal cubes by means of tunable depletion interactions. The colloidal system consists of novel micron-sized cubic particles prepared by silica deposition on hematite templates and various non-adsorbing water-soluble polymers as depletion agents. We show that under certain conditions the cubes can self-organize into crystals with a simple cubic symmetry, which is set by the size of the depletant. The dynamic of crystal nucleation and growth is investigated monitoring the samples in time by optical microscopy. Furthermore, by using temperature sensitive microgel particles as depletant it is possible to fine tune depletion interactions as to induce crystal melting. Assisting crystallization with an alternating electric field improves the uniformity of the cubic pattern allowing the preparation of macroscopic (almost defect-free) crystals that show visible Bragg colors. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H9.00015: Spin-coating of rapidly dried colloidal suspensions: model and experiments Maximiliano Giuliani, Wenceslao Gonz\'alez-Vi\~nas, Anand Yethiraj The study of the formation of colloidal crystals has been a very active field in recent years. The spin-coating technique has proven to be a highly reproducible process to form large area colloidal crystals. Here, we present recent results on spin- coating of rapidly dried colloidal suspension. We show that the dynamics observed can be represented by an extension of classical Emslie model to highly volatile fluids. We obtained this extension while maintaining the explicit solution for the temporal evolution of the fluid thickness. We observed that the dynamics can be separated in two regimes: one dominated by non-evaporative effects and a second dominated by evaporative effects. The transition between these two dynamical regimes corresponds well with the transition between two symmetries observed during the fluid phase (six and four-fold). Similarly, the quality of the deposited structure is also well correlated to the relative strength of the capillary forces with respect to the viscous ones. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700