Spatially: resolved heterogeneous dynamics in a strong colloidal gel (original) (raw)
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Physical Review Letters, 2009
We introduce a new dynamic light scattering method, termed photon correlation imaging, which enables us to resolve the dynamics of soft matter in space and time. We demonstrate photon correlation imaging by investigating the slow dynamics of a quasi two-dimensional coarsening foam made of highly packed, deformable bubbles and a rigid gel network formed by dilute, attractive colloidal particles. We find the dynamics of both systems to be determined by intermittent rearrangement events. For the foam, the rearrangements extend over a few bubbles, but a small dynamical correlation is observed up to macroscopic length scales. For the gel, dynamical correlations extend up to the system size. These results indicate that dynamical correlations can be extremely long-ranged in jammed systems and point to the key role of mechanical properties in determining their nature.
Physical Review E
Relaxation of internal stresses through a cascade of microscopic restructuring events is the hallmark of many materials, ranging from amorphous solids like glasses and gels to geological structures subjected to a persistent external load. By means of photon correlation imaging, a recently developed technique that blends the powers of scattering and imaging, we provide a spatially and temporally resolved survey of the restructuring and aging processes that spontaneously occur in physical gels originating from an arrested phase separation. We show that the temporal dynamics is characterized by an intermittent sequence of spatially localized "microquakes" that eventually lead to global rearrangements occurring at a rate that scales with the gel age. Notably, these dramatic upheavals of the gel structure are heralded by a progressive acceleration of the microscopic gel dynamics that originates from recognizable active spots and then spreads at a large but finite speed through the gel. Within the "slack" phase between two of these "macroquakes," the fluctuations of the degree of temporal correlation obey a non-Gaussian statistics described by a generalized logistic distribution. The evidence we obtained bear consistent analogies with the stress relaxation processes taking place in earthquake sequences and with the intermittent restructuring of plastic crystals at the microscale.
Optical correlation techniques for the investigation of colloidal systems
2013
This review aims to provide a simple introduction to the application of optical correlation methods in colloidal science. In particular, I plan to show that full appraisal of the intimate relation between light scattering and microscopy allows designing novel powerful investigation techniques that combine their powers. An extended version of this paper will appear in "ColloidalFoundations of Nanoscience", edited by D. Berti and G. Palazzo, Elsevier (ISBN 978-0-444-59541-6). I am very grateful to the publisher for having granted me the permission to post this preprint on arXiv.
Highly Nonlinear Dynamics in a Slowly Sedimenting Colloidal Gel
Physical Review Letters, 2011
We use a combination of original light scattering techniques and particles with unique optical properties to investigate the behavior of suspensions of attractive colloids under gravitational stress, following over time the concentration profile, the velocity profile, and the microscopic dynamics. During the compression regime, the sedimentation velocity grows nearly linearly with height, implying that the gel settling may be fully described by a (time-dependent) strain rate. We find that the microscopic dynamics exhibit remarkable scaling properties when time is normalized by strain rate, showing that the gel microscopic restructuring is dominated by its macroscopic deformation.
Dynamic Light-Scattering Study of Aggregating and Gelling Colloidal Disks
Journal of Colloid and Interface Science, 2001
We have studied the relaxation of the concentration fluctuations of gelling disk-like clay particles (Laponite) in water containing 5 mM NaCl. At a Laponite concentration of 10 g/l a gel is formed that does not flow when tilted. The comparison of dynamic light scattering and dynamic mechanical measurements shows that the terminal relaxation time of concentration fluctuations diverges not at the gel point, but only when the gel is sufficiently dense to inhibit motion over the length scale on the order of the inverse scattering wave vector. The initial decay of the concentration fluctuations is due to cooperative diffusion and is insensitive to the gel formation. At a concentration of 1 g/L, large aggregates that slowly precipitate are formed. The scattering wave vector dependence of the decay of the concentration fluctuations shows that the aggregates are semi-flexible.
Optics Express, 2011
We discuss a new method for simultaneously probing translational, rotational, and vibrational dynamics in dilute colloidal suspensions using digital holographic microscopy (DHM). We record digital holograms of clusters of 1-μm-diameter colloidal spheres interacting through short-range attractions, and we fit the holograms to an exact model of the scattering from multiple spheres. The model, based on the T-matrix formulation, accounts for multiple scattering and near-field coupling. We also explicitly account for the non-asymptotic radial decay of the scattered fields, allowing us to accurately fit holograms recorded with the focal plane located as little as 15 μm from the particle. Applying the fitting technique to a time-series of holograms of Brownian dimers allows simultaneous measurement of six dynamical modes - three translational, two rotational, and one vibrational - on timescales ranging from 10(-3) to 1 s. We measure the translational and rotational diffusion constants to a precision of 0.6%, and we use the vibrational data to measure the interaction potential between the spheres to a precision of ∼50 nm in separation distance. Finally, we show that the fitting technique can be used to measure dynamics of clusters containing three or more spheres.
Slow dynamics and aging in colloidal gels studied by x-ray photon correlation spectroscopy
Physical Review E, 2007
Slow, nonequilibrium dynamics during delayed sedimentation in a colloidal depletion gel was studied by x-ray photon correlation spectroscopy. The intermediate scattering functions change during the process from stretched to compressed exponential decays, indicating a jamming transition toward full aging. A complex aging behavior follows this process; it is proposed that large-scale network deformations trigger an unjamming, leading to the final collapse of the gel.
Three-Dimensional Confocal Microscopy of Colloids
Applied Optics, 2001
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 radius͒ 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.
Differential Dynamic Microscopy for Anisotropic Colloidal Dynamics
Langmuir, 2012
Differential dynamic microscopy (DDM) is a low-cost, high-throughput technique recently developed for characterizing the isotropic diffusion of spherical colloids using white-light optical microscopy. We develop the theory for applying DDM to probe the dynamics of anisotropic colloidal samples such as various ordered phases, or particles interacting with an external field. The q-dependent dynamics can be measured in any direction in the image plane. We demonstrate the method on a dilute aqueous dispersion of anisotropic magnetic particles (hematite) aligned in a magnetic field. The measured diffusion coefficients parallel and perpendicular to the field direction are in good agreement with theoretical values. We show how these measurements allow us to extract the orientational order parameter S 2 of the system.