High-bandwidth viscoelastic properties of aging colloidal glasses and gels (original) (raw)

Microrheology of an aging colloidal glass

2006

We report measurements of the frequency-dependent shear moduli of aging colloidal systems that evolve from a purely low-viscosity liquid to a predominantly elastic glass or gel. Using microrheology, we measure the local complex shear modulus G*͑͒ over a very wide range of frequencies ͑from 1 Hz to 100 kHz͒. The combined use of one-and two-particle microrheology allows us to differentiate between colloidal glasses and gels-the glass is homogenous, whereas the colloidal gel shows a considerable degree of heterogeneity on length scales larger than 0.5 m. Despite this characteristic difference, both systems exhibit similar rheological behaviors which evolve in time with aging, showing a crossover from a single-power-law frequency dependence of the viscoelastic modulus to a sum of two power laws. The crossover occurs at a time t 0 , which defines a mechanical transition point. We found that the data acquired during the aging of different samples can be collapsed onto a single master curve by scaling the aging time with t 0 . This raises questions about the prior interpretation of two power laws in terms of a superposition of an elastic network embedded in a viscoelastic background.

Time resolved viscoelastic properties during structural arrest and aging of a colloidal glass

2010

Evolution of the energy landscape during physical aging of glassy materials can be understood from the frequency and strain dependence of the shear modulus but the non-stationary nature of these systems frustrates investigation of their instantaneous underlying properties. Using a series of time dependent measurements we systematically reconstruct the frequency and strain dependence as a function of age for a repulsive colloidal glass undergoing structural arrest. In this manner, we are able to unambiguously observe the structural relaxation time, which increases exponentially with sample age at short times. The yield stress varies logarithmically with time in the arrested state, consistent with recent simulation results, whereas the yield strain is nearly constant in this regime. Strikingly, the frequency dependence at fixed times can be rescaled onto a master curve, implying a simple connection between the aging of the system and the change in the frequency dependent modulus.

Aging and rheology in soft materials

Journal of Rheology, 2000

We study theoretically the role of ageing in the rheology of soft materials. We define several generalized rheological response functions suited to ageing samples (in which time translation invariance is lost). These are then used to study ageing effects within a simple scalar model (the "soft glassy rheology" or SGR model) whose constitutive equations relate shear stress to shear strain among a set of elastic elements, with distributed yield thresholds, undergoing activated dynamics governed by a "noise temperature", x. (Between yields, each element follows affinely the applied shear.) For 1 < x < 2 there is a power-law fluid regime in which transients occur, but no ageing. For x < 1, the model has a macroscopic yield stress. So long as this yield stress is not exceeded, ageing occurs, with a sample's apparent relaxation time being of order its own age. The (age-dependent) linear viscoelastic loss modulus G ′′ (ω, t) rises as frequency is lowered, but falls with age t, so as to always remain less than G ′ (ω, t) (which is nearly constant). Significant ageing is also predicted for the stress overshoot in nonlinear shear startup and for the creep compliance. Though obviously oversimplified, the SGR model may provide a valuable paradigm for the experimental and theoretical study of rheological ageing phenomena in soft solids.

Rheological signatures of gelation and effect of shear melting on aging colloidal suspension

Journal of Rheology, 2014

Colloidal suspensions that are out of thermodynamic equilibrium undergo physical aging wherein their structure evolves to lower the free energy. In aqueous suspension of Laponite, physical aging accompanies enhancement of elastic and viscous moduli as a function of time. In this work we study temporal evolution of elastic and viscous moduli at different frequencies and observe that freshly prepared aqueous suspension of Laponite demonstrates identical rheological behavior reported for the crosslinking polymeric materials undergoing chemical gelation. Consequently at a certain time tanδ is observed to be independent of frequency. However, for samples preserved under rest condition for longer duration before applying the shear melting, the liquid to solid transition subsequent to shear melting shows greater deviation from classical gelation. We also obtain continuous relaxation time spectra from the frequency dependence of elastic and viscous moduli. We observe that, with increase in the rest time, continuous relaxation time spectrum shows gradual variation from negative slope, describing dominance of fast relaxation modes to positive slope representing dominance of slow relaxation modes. We propose that the deviation from gelation behavior for the shear melted suspensions originates from inability of shear melting to completely break the percolated structure thereby creating unbroken aggregates. The volume fraction of such unbroken aggregates increases with the rest time. For small rest times presence of fewer number of unbroken aggregates cause deviation from the classical gelation. On the other hand, at high rest times presence of greater fraction of unbroken aggregates subsequent to shear melting demonstrate dynamic arrest reminiscent in that of colloidal glasses.

Microscopic Viscoelasticity: Shear Moduli of Soft Materials Determined from Thermal Fluctuations

Physical Review Letters, 1997

We describe a high-resolution, high-bandwidth technique for determining the local viscoelasticity of soft materials such as polymer gels. Loss and storage shear moduli are determined from the power spectra of thermal fluctuations of embedded micron-sized probe particles, observed with an interferometric microscope. This provides a passive, small-amplitude measurement of rheological properties over a much broader frequency range than previously accessible to microrheology. We study both F-actin biopolymer solutions and polyacrylamide (PAAm) gels, as model semiflexible and flexible systems, respectively. We observe high-frequency v 3͞4 scaling of the shear modulus in F-actin solutions, in contrast to v 1͞2 scaling for PAAm. [S0031-9007(97)

Shear banding, aging and noise dynamics in soft glassy materials

Soft Matter, 2009

The 'soft glassy rheology' (SGR) model gives an appealing account of the flow of nonergodic soft materials in terms of the local yield dynamics of mesoscopic elements. Newtonian, power-law, and yield-stress fluid regimes arise on varying a 'noise temperature', x. Here we extend the model, to capture the idea that the noise is largely caused by yield itself. The extended model can account for the viscosity-bifurcation and shear-banding effects reported recently in a wide range of soft materials. A variant model may shed light on shear banding and strain-rate hysteresis seen in glassy star polymers solutions.

Rheology of Soft Glassy Materials

Physical Review Letters, 1997

We attribute similarities in the rheology of many soft materials (foams, emulsions, slurries, etc.) to the shared features of structural disorder and metastability. A generic model for the mesoscopic dynamics of "soft glassy matter" is introduced, with interactions represented by a mean-field noise temperature x. We find power law fluid behavior either with (x < 1) or without (1 < x < 2) a yield stress. For 1 < x < 2, both storage and loss modulus vary with frequency as ω x−1 , becoming flat near a glass transition (x = 1). Values of x ≈ 1 may result from marginal dynamics as seen in some spin glass models.

Shearing or compressing a soft glass in 2D: time-concentration superposition

Physical review letters, 2003

We report surface shear rheological measurements on dense insoluble monolayers of micron sized colloidal spheres at the oil/water interface and of the protein β-lactoglobulin at the air/water surface. As expected, the elastic modulus shows a changing character in the response, from a viscous liquid towards an elastic solid as the concentration is increased, and a change from elastic to viscous as the shear frequency is increased. Surprisingly, above a critical packing fraction, the complex elastic modulus curves measured at different concentrations can be superposed to form a master curve, by rescaling the frequency and the magnitude of the modulus. This provides a powerful tool for the extrapolation of the material response function outside the experimentally accessible frequency range. The results are discussed in relation to recent experiments on bulk systems, and indicate that these two dimensional monolayers should be regarded as being close to a soft glass state.

Chapter 5 SOFT GLASSY RHEOLOGY

2006

We review models for the rheology of soft glasses, a class of materials including e.g. emulsions, foams, colloidal glasses and possibly-but with substantial caveats-gels. The main focus is on the soft glassy rheology (SGR) model, and in particular on the occurrence of rheological aging effects. We first review appropriate definitions of rheological response functions suited to aging samples (in which time translation invariance is lost). These are then used to study aging effects within the SGR model. Its constitutive equations relate shear stress to shear strain among a set of elastic elements, with distributed yield thresholds, undergoing activated dynamics governed by a "noise temperature", x. For 1 < x < 2 there is a power-law fluid regime in which transients occur, but no aging. For x < 1, the model has a macroscopic yield stress. So long as this yield stress is not exceeded, aging occurs, with a sample's apparent relaxation time being of the order of its own age. The (age-dependent) linear viscoelastic loss modulus G (ω, t) rises as frequency is lowered, but falls with age t, so as to always remain less than G (ω, t) (which is nearly constant). Significant aging is also predicted for the stress overshoot in nonlinear shear startup and for the creep compliance. We discuss an extension of the model to include a proper tensorial description of stress and strain, and survey some related rheological models that have recently been developed.

Dynamics of Internal Stresses and Scaling of Strain Recovery in Aging Colloidal Gels

Physical Review E, 2009

We monitor the relaxation of internal stresses in a fractal colloidal gel on cessation of flow and find a weak power law decay, sigmaisimt−alpha\sigma_i \sim t^{-\alpha}sigmaisimt−alpha over 5 decades of time where alphaapprox0.07\alpha \approx 0.07alphaapprox0.07. The system exhibits physical aging of the elastic modulus, G′simtbetaG' \sim t^{\beta}G′simtbeta, with betaapproxalpha\beta \approx \alphabetaapproxalpha. Imposition of zero stress after waiting time twt_wtw results in strain recovery as the system relaxes without constraint. Remarkably, recoveries at different twt_wtw can be shifted to construct a master curve where data are scaled vertically by 1/sigmai(tw)1/\sigma_i(t_w)1/sigmai(tw) and plotted horizontally as (t−tw)/twmu(t-t_w)/t_w^{\mu}(t−tw)/twmu where muapprox1.25\mu\approx 1.25muapprox1.25, indicative of a super-aging response.