A wide variety of systems, including granular media, colloidal suspensions and molecular systems, exhibit non-equilibrium transitions from a fluid-like to a solid-like state, characterized solely by the sudden arrest of their dynamics. Crowding or jamming of the constituent particles traps them kinetically, precluding further exploration of the phase space(1). The disordered fluid-like structure remains essentially unchanged at the transition. The jammed solid can be refluidized by thermalization, through temperature or vibration, or by an applied stress. The generality of the jamming transition led to the proposal(2) of a unifying description, based on a jamming phase diagram. It was further postulated that attractive interactions might have the same effect in jamming the system as a confining pressure, and thus could be incorporated into the generalized description. Here we study experimentally the fluid-to-solid transition of weakly attractive colloidal particles, which undergo markedly similar gelation behaviour with increasing concentration and decreasing thermalization or stress. Our results support the concept of a jamming phase diagram for attractive colloidal particles, providing a unifying link between the glass transition(3), gelation(4,5) and aggregation(6-8).
We show that gelation of weakly attractive colloids is remarkably similar to the colloidal glass transition. Like the glass transition, dynamic light scattering functions near gelation scale with scattering vector, and exhibits a two-step decay with a power-law divergence of the final decay time. Like the glass transition, static light scattering does not change upon gelation. These results suggest that, like the glass transition, gelation results from kinetic arrest due to crowding of clusters, and that both gelation and the glass transition are manifestations of a more general jamming transition.
The slow sedimentation of suspensions of solid particles in a fluid results in complex phenomena that are poorly understood. For a low volume fraction (phi) of particles, long-range hydrodynamic interactions result in surprising spatial correlations(1) in the velocity fluctuations; these are reminiscent of turbulence, even though the Reynolds number is very low(2-4). At higher values of phi, the behaviour of sedimentation remains unclear; the upward backflow of fluid becomes increasingly important, while collisions and crowding further complicate inter-particle interactions(5-8). Concepts from equilibrium statistical mechanics could in principle be used to describe the fluctuations and thereby provide a unified picture of sedimentation, but one essential ingredient-an effective temperature that provides a mechanism for thermalization-is missing. Here we show that the gravitational energy of fluctuations in particle number can act as an effective temperature. Moreover, we demonstrate that the high-phi behaviour is in fact identical to that at low phi, provided that the suspension viscosity and sedimentation velocity are scaled appropriately, and that the effects of particle packing are included.
In order to provide depth resolution for bulk tissue imaging experiments using fluorescence or absorption signals, we have designed an internal laser point spread technique. A laser light source has been imbedded in different depths into cardiac tissue and tissue phantoms, the signal on the tissue surface detected by a CCD detector. These measurements in combination with an analytic solution of the diffusion equation allow us to estimate optical properties of the investigated tissue. We show how this information provides the core of depth quantification of fluorescence and absorption measurements in bulk tissue and investigate experimentally the transition from single scattering to diffuse photon transport in cardiac tissue and suspensions of microscopic spherical particles that serve as model systems. (C) 2001 Optical Society of America.
Times Cited: 0 Conference on Photon Migration, Optical Coherence Tomography, and Microscopy Jun 18-21, 2001 Munich, germany Opt Soc Amer; SPIE; Deutsch Gesell Lasermed eV; European Opt Soc; IEEE
We study the effect of isopropyl alcohol (IPA) and various salts (sodium chloride, sodium thiocyanate, sodium carbonate, and urea) on the rheological behavior and the conformation of aqueous guar solutions. Ultralow angle light scattering, conventional light scattering, and neutron scattering are used to probe the structure of guar over length scales spanning 5 decades, from a few angstroms to several tens of microns. Although both IPA and salts worsen solvent conditions, their effect is very different. Isopropyl alcohol promotes the formation of a network of large-scale structures via intermolecular associations, thus increasing dramatically the elastic response of guar solutions. Salts. on the contrary, affects guar on a local scale, leading to a more collapsed chain configuration, thus to a lower effective volume fraction and to reduced viscosity.
Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.
Confocal microscopy is used in the study of colloidal gels, glasses, and binary fluids. We measure the three-dimensional positions of colloidal particles with a precision of approximately 50 nm (a small fraction of each particle's radius) and with a time resolution sufficient for tracking the thermal motions of several thousand particles at once. This information allows us to characterize the structure and the dynamics of these materials in qualitatively new ways, for example, by quantifying the topology of chains and clusters of particles as well as by measuring the spatial correlations between particles with high mobilities. We describe our experimental technique and describe measurements that complement the results of light scattering. (C) 2001 Optical Society of America.
Colloidal Microscopy is a new microscopy, invented by whoever shortened the title. Paul Chaikin, the world's greatest physicist, will explain it in his talk. This talk will cover any details that Paul does not cover. (C) 2000 Optical Society of America.
Times Cited: 0 Conference on Photon Correlation and Scattering Aug 21-23, 2000 Whistler, canada Opt Soc Amer; NASA
Confocal microscopy was used to directly observe three-dimensional dynamics of particles in colloidal supercooled fluids and colloidal glasses. The fastest particles moved cooperatively; connected clusters of these mobile particles could be identified: and the cluster size distribution, structure, and dynamics were investigated. The characteristic cluster size grew markedly in the supercooled fluid as the glass transition was approached, in agreement with computer simulations; at the glass transition, however, there was a sudden drop in their size. The clusters of fast-moving particles were largest near the alpha-relaxation time scale for supercooled colloidal fluids, but were also present, albeit with a markedly different nature, at shorter beta-relaxation time scales, in both supercooled fluid and glass colloidal phases.
Using a microscope capable of simultaneous imaging and static light scattering, we measure the optical properties of different tissues. We correlate the scattering patterns to local structures and heterogeneities. (C) 1999 Optical Society of America.
Times Cited: 0 Biomedical Topical Meeting Apr 02-05, 2000 Miami beach, fl
The rheological data of weakly attractive colloidal particles are shown to exhibit a surprising scaling behavior as the particle volume fraction, phi, or the strength of the attractive interparticle interaction, U, are varied. There is a critical onset of a solid network as either phi or U increase above critical values. For all solidlike samples, both the frequency-dependent linear viscoelastic moduli, and the strain-rate dependent stress can be scaled onto universal master curves. A model of a solid network interspersed in a background fluid qualitatively accounts for this behavior.
We describe an experimental technique for the production of highly monodisperse emulsions (with minimum achievable polydispersities <3%). The phase to be dispersed is introduced into a coflowing, surfactant-laden continuous phase via a tapered capillary. Drops detach from the capillary when the streamwise forces exceed the force due to interfacial tension. Drop size is a function of the capillary tip diameter, the velocity of the continuous phase, the extrusion rate, and the viscosities and interfacial tension of the two phases. Emulsions composed of a variety of fluids and with drop sizes ranging from 2 to 200 mu m have been produced using this technique.
Rayleigh light scattering has not yet been used for quantitative investigations of heterogeneous systems. Preconditions for such an experiment are a well defined scattering geometry and independent information about the local state of the sample. We have designed a new instrument that meets these criteria: a light- scattering microscope with simultaneous imaging. We demonstrate the ability to characterize local differences within one tissue type as well as global differences between tissue types. Real space images of the sample are taken by normal video microscopy techniques. The light scattering pattern is analyzed by the evaluation of wave- vector dependence (form factor) and scattering direction of the scattered intensity. Statistical analysis of scattering patterns show what is important for the characterization and classification of tissues and heterogeneous structures. Real space images provide context for scattering analysis. The light scattering microscope is a powerful tool for characterization of local structural order in inhomogeneous structures like tissues.
Times Cited: 0 Conference on Optical Biopsy III Jan 23-24, 2000 San jose, ca SPIE; Int Biomed Opt Soc
The velocity fluctuations and local strain rate in a fluidized suspension of particles are investigated using the new ultrasonic technique of diffusing acoustic wave spectroscopy. DAWS probes the relative motion of the particles at very short length scales down to the inter-particle separation, and allows the spatial correlations of the velocity fluctuations to be probed by varying the transport mean free path of the diffusing ultrasonic waves. Our results demonstrate the power of this ultrasonic technique to probe the dynamics of sedimenting particles at larger length scales and Reynolds numbers than can be achieved by light scattering methods. (C) 2000 Elsevier Science B.V. All rights reserved.
Times Cited: 11 5th International Conference on the Electrical Transport and Optical Properties of Inhomogeneous Media (ETOPIM5) Jun 21-25, 1999 Hong kong baptist univ, hong kong, hong kong Lee Hysan Fdn; Croucher Fdn; KC Wong Educ Fdn; Schlumberger Doll Res; ARO FE; AFOSR AOARD; ONRASIA
We report measurements of the frequency-dependent complex shear modulus of semidilute F-actin solutions based on optical observations of the thermally excited motion of monodisperse tracer microspheres. Because the tracer spheres cause incident laser light to be strongly scattered, we determine their average motion using diffusing wave spectroscopy. From the measured mean square displacement, we extract the retardation spectrum of the actin solution using st regularized fit based on a discretized model involving a linear superposition of harmonically bound Brownian particles. At an actin concentration of C = 1.2 mg/ml and for microspheres of radius a = 0.8 mu m, we find that the complex modulus exhibits a dominant low frequency plateau modulus and a high frequency rise with the loss modulus dominating above a crossover frequency. Over a limited range of frequencies well above the crossover frequency, the magnitude of the high frequency storage modulus G'(omega) is consistent with the power law scaling omega(3/4). The observed gradual crossover appears to be at odds with previous theoretical predictions, but it corresponds to a simple structure of the retardation spectrum. (C) 2000 The Society of Rheology. [S0148-6055(00)00904-4].
We investigate the self-assembly of colloidal spheres on periodically patterned templates. The surface potentials and the surface phases are induced entropically by the presence of dissolved, nonadsorbing polymers. A rich variety of two-dimensional fluidlike and solidlike phases was observed to form on template potentials with both one- and two-dimensional symmetry. The same methodology was then used to nucleate an oriented single fee crystal more than 30 layers thick. The general approach provides a new route for directed self-assembly of novel mesoscopic structures.
We present a novel approach to the fabrication of binary colloidal materials where specific biomolecular cross-linking drives the self-assembly of bidisperse colloidal suspensions. In particular, we have employed low-affinity immune system carbohydrate-selectin interactions to mediate the heterotypic assembly of binary colloidal structures. Using small (0.94 mum) and larger (5.5 mum) diameter particles coated with complementary chemistry, we show that a progressive series of structures, such as colloidal micelles (a large particle coated with smaller particles), colloidal clusters, rings, and elongated chains, can be made by decreasing the number fraction, N-A/N-B, Of Small (A) to large (B) particles (200 greater than or equal to N-A/N-B greater than or equal to 2) at low total volume fraction (phir = 10(-4)-10(-3)). The assembly is due to specific molecular interactions, as control experiments in which the molecules are blocked or eliminated do not lead to the assembly of these structures. The size of the structures can be modulated by time or total volume fraction. Currently, our methods employ high molecular surface densities, such that the structures result from kinetically trapped, diffusion-limited assembly. Ultimately, with the ability to control the strength of the interaction (using different chemistries and molecular surface densities) as well as the lengths of the molecular tethering arms, particle number densities, and physical properties of the colloidal components, this colloidal assembly driven by specific interactions should yield new materials with many potential technological applications including optical filters, sensors, and separation media.