Highly Nonlinear Dynamics in a Slowly Sedimenting Colloidal Gel

Abstract

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.PACS numbers: 47.57.ef, 64.70.pv, 82.70.Dd Gels and attractive glasses resulting from the aggregation of colloidal particles are the subject of extensive studies because their physical behavior often results from a complex interplay between equilibrium thermodynamics and nonequibrium dynamic processes [1][2][3], and because they are relevant for understanding networkforming biological systems [4] and for industrial applications. Although they exhibit solid-like mechanical properties, colloidal gels are easily disrupted by small perturbations, such as gravitational forces. While a large body of macroscopic observations of gels under gravitational stress exists [5][6][7][8][9][10][11][12][13], very little is known on the microscopic processes at play during sedimentation, thus limiting our ability to understand and predict the behavior of sedimenting gels.Here, we use a novel light scattering method to gain access to the dynamics of a slowly settling colloidal system from the macroscopic deformation of the sample down to the relaxational behavior at the particle scale. We find that the very slow macroscopic deformation occurs via irreversible plastic events at the microscopic scale. Remarkably, the gel behavior at all scales is controlled by a single parameter, the time-dependent compression rate, in striking analogy with recent observations on deformed polymer [14] and colloidal [15] glasses.We study gels formed by attractive colloidal hard spheres with radius R = 82 ± 3 nm, suspended in an aqueous solvent at an initial volume fraction ϕ 0 = 0.123 (more details can be found in [16][17][18]). Gelation is induced by attractive depletion forces obtained by adding micelles of a nonionic surfactant. The interaction between colloids is well described by the Asakura-Oosawa depletion potential [16], with a range r ≈ 3 nm. The potential can be mapped on the Adhesive Hard Sphere model, with a stickiness parameter τ ≃ 0.01 [16,17]. The density mismatch between the particles and the solvent is ∆ρ = 1.12 g/cm 3 . The particles have an intrinsic optical anisotropy; accordingly, they scatter light with polarization both parallel and perpendicular ("depolarized") to that of the incident radiation. The depolarized scattered intensity is an accurate probe of the local particle concentration [16].To probe the sedimentation process in great detail, we use...

Biopolymer gels with “physical” cross-links: gelation kinetics, aging, heterogeneous dynamics, and macroscopic mechanical properties

“…6 In this paper, through a detailed investigation performed by photon correlation imaging, a novel light scattering technique particularly suitable to investigate the microscopic dynamics of spatially inhomogeneous systems, we highlight and discuss several features of the gelation kinetics and the restructuring processes in alginate gels generated by the slow perfusion of CaCl 2 . 22 In particular, the advancement of the gelation front, accurately marked by a dramatic increase of dynamic time correlations, displays a very surprising, non-diffusive time behavior, which can hardly be reconciled with the existing gelation model. Aer the formation of a dynamically quasiarrested structure, the gel undergoes a time evolution characterized by a rst consistent restructuring and strengthening accompanied by a progressive slowing-down of the local dynamics, followed by a much longer period marked by drastic and abrupt global "uidization" events, which do not apparently lead to signicant changes of the gel structure, but presumably contribute to a slow "creeping" behavior observed for the gel at very long time.…”

Equilibrium concentration profiles and sedimentation kinetics of colloidal gels under gravitational stress

“…Additionally, we have shown in Ref. [16] thatε governs also the microscopic dynamics, since structural rearrangements occur on a time scale τ α such that τ αε = ε y , where ε y ≈ 0.02 is the typical yield strain beyond which irreversible rearrangements occur. In view of the key role played byε, it is interesting to test whether a macroscopic measurement of the global strain rate, |ḣ|/h, reflects accurately the microscopic strain rate.…”

Strong and Weak Chaos in Weakly Nonintegrable Many-Body Hamiltonian Systems

“…Recent experiments on gel formed by attractive colloidal hard spheres, suspended in an aqueous solvent, show that the spreading of gel is indeed well described by such type of a nonlinear diffusion equation (17) with a nonlinear flux term [53]. The theoretical models of rheology flow try to explain such a spreading by phenomenological statistical models with disorder and metastability (see e.g.…”