Ajay Negi

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

We employ parallel superposition rheology to study the dynamics of an aging colloidal glass in the presence of a mean field stress. Over a range of intermediate stresses, the loss modulus exceeds the storage modulus at short times but develops a maximum concomitant with a crossover between the two as the system ages. This is attended by a narrowing of the loss peak on increasing stress. We show that this feature is characteristic of the structural arrest in these materials, which is made observable on reasonable timescales by the activating influence of the stress. The arrest time displays an exponential dependence on inverse stress. These results provide experimental validation of the role of stress as an effective temperature in soft glassy systems as has been advanced in recent theoretical frameworks.

We monitor the relaxation of internal stresses in a fractal colloidal gel on cessation of flow and find a weak power-law decay, σi˜t-α over five decades of time where α≈0.07 . The system exhibits physical aging of the elastic modulus, G'˜tβ , with β≈α . Imposition of zero stress after waiting time tw results in strain recovery as the system relaxes without constraint. Remarkably, recoveries at different tw can be shifted to construct a master curve where data are scaled vertically by 1/σi(tw) and plotted horizontally as (t-tw)/twμ , where μ≈1.25 , indicative of a superaging response.

The yielding behavior of colloidal suspensions is a strong function of inter-particle interactions. Recent results [Pham et al. 2006, 2008] indicate that attractive colloidal glasses display a two-step yielding due to inter-particle bond rupture followed by particle cage escape. From this perspective, we examine the yielding behavior of an oil-in-water emulsion system with attractive interactions using dynamic bulk rheology. In strain sweep experiments, after a limited linear regime, the system yields with a pronounced bump in the viscous modulus, a sharp decrease in the elastic modulus and a crossover between the two. The yielding response is marked by bond-breaking at low volume fractions and bond-breaking accompanied by cage escape above a critical concentration. An increase in the complex modulus is observed at yet higher strains (>100%), with both the elastic and viscous components showing small frequency dependent peaks. The onset, peak strains and peak stress display different dependences on volume fraction. We speculate that this display is due to the formation of shear induced structures at high strains and advance a simple model for this behavior.

The aging response of glassy particulate systems originates due to slow structural rearrangements of its constituent matter. It is reasonable to speculate that structural rearrangements on different length scales should manifest themselves in dynamical response on different timescales. Here we consider the frequency dependence of aging in a colloidal glassy system using parallel superposition bulk rheology. The aging behavior of the system is characterized by time evolution of the complex modulus in response to a sinusoidally varying probe stress of different frequencies superimposed on a steady background stress. Strikingly, the system displays more rapid aging when observed at smaller frequencies. This suggests that, by comparison, it is more arrested on shorter length scales (higher frequencies) than on the longer length scales where many-particle correlated motions are in effect. Such correlated motions are believed to be responsible for relaxation in glassy materials. The variation in the aging dynamics at different frequencies is more prominent at higher background stresses where the system is more fluidized.

Many colloidal suspensions are inherently out of equilibrium and display a slow evolution of their dynamics over time. However, many features of the glass transition as encountered in polymer and molecular glasses are not conserved. This phenomenon is still not completely understood and little is known of the connection between flow history, as a determinant of the initial system state, and subsequent aging dynamics. Further, the changes in the energy landscape during aging 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. Here we discuss the use of stress jump experiments that investigate the role of flow history on aging, and the systematic reconstruction of the frequency and strain dependence as a function of age for a repulsive colloidal glass undergoing structural arrest and aging. We uncover a connection between the aging behavior and the rate of flow cessation that is additionally reflected in the dynamics of residual stress relaxation. 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.

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.

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

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 nonstationary 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.

We employ parallel superposition rheology to study the dynamics of an aging colloidal glass in the presence of a mean field stress. Over a range of intermediate stresses, the loss modulus exceeds the storage modulus at short times but develops a maximum concomitant with a crossover between the two as the system ages. This is attended by a narrowing of the loss peak on increasing stress. We show that this feature is characteristic of the structural arrest in these materials, which is made observable on reasonable timescales by the activating influence of the stress. The arrest time displays an exponential dependence on inverse stress. These results provide experimental validation of the role of stress as an effective temperature in soft glassy systems as has been advanced in recent theoretical frameworks.

We investigate the rheology of dilute dispersions of fumed colloidal particles with attractive interactions in hydrocarbon liquids. Surprisingly, these systems display shear thickening due to the breakdown of densified flocs and a concomitant increase in the effective volume fraction of the fractal particles in the fluid. We show that this shear thickening is controlled by a critical stress and accompanied by a positive increase in the first normal stress difference, N 1, at the shear thickening transition. This is in contrast to the well-known hydrocluster mechanism of shear thickening in concentrated hard-sphere and repulsive systems. Gel elasticity depends strongly on the stress applied to suspensions in preshear, scaling roughly as $G'\sim\sigma_{\text{preshear}}^{2}$ . We propose a simple model to account for these results in terms of the cluster number density determined by the preshear stress. At low shear rates, vorticity-aligned aggregates are present at $\dot\gamma\approx 10^0 {\rm{s}}^{-1}$  . In this regime, the system displays a small but noticeable increase in viscosity on increasing shear rate. We investigate the effect of tool roughness and find that wall slip is not responsible for the observed phenomena. Instead, the increase in the apparent viscosity results from increased flow resistance due to the presence of gap-spanning log-like flocs in rolling flow.

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.

Dilute Laponite suspensions in water at low salt concentration form repulsive colloidal glasses which display physical aging. This phenomenon is still not completely understood and in particular, little is known about the connection between the flow history, as a determinant of the initial state of the system, and the subsequent aging dynamics. Using a stress controlled rheometer, we perform stress jump experiments to observe the elastic component of the flow stress that remains on cessation of flow or flow quenching. We investigate the connection between the dynamics of these residual stresses and the rate of physical aging upon quenching from different points on the steady state flow curve. Quenching from high rates produces a fluid state, G">G', with small, fast relaxing residual stresses and rapid, sigmoidal aging of the complex modulus. Conversely, quenching from lower shear rates produces increasingly jammed states featuring slowly relaxing stresses and a slow increase of the complex modulus with system age. Flow cessation from a fixed shear rate with varying quench durations shows that slower quenches produce smaller residual stresses at short times which relax at long times by smaller extents, by comparison with faster quenches. These smaller stresses are correlated with a higher modulus but slower physical aging of the system. The characteristic time for the residual stress relaxation scales inversely with the quench rate. This implies a frustrated approach to any ideal stress-free state that succinctly reflects the frustrated nature of these glassy systems.

We investigate the dynamics of aging in a repulsive colloidal glass composed of charged clay particles in aqueous suspension. Dynamic rheological measurements show a power law evolution of the elastic modulus of the system with sample age, measured as time elapsed after the cessation of a rejuvenating shear flow. We show that the scaling exponent is dependent on the rate of flow cessation or the flow quench rate. Comparatively fast quenches lead to systems with a smaller elastic modulus and accelerated aging whereas slower quenches result in higher modulus but correspondingly less rapid aging. We apply a recently proposed technique to follow the dynamics of residual or internal elastic stresses immediately after the flow arrest and find striking parallels between the relaxation of these stresses and the aging of the system. These results indicate that the evolution of the slow dynamics is strongly coupled to the internal stress state of the system and point to the identification of the flow quench rate as a mechanical variable that characterizes the system's departure from equilibrium.

Dilute Laponite suspensions in water at low salt concentration form repulsive colloidal glasses which display physical aging. This phenomenon is still not completely understood and in particular, little is known about the connection between the flow history, as a determinant of the initial state of the system, and the subsequent aging dynamics. Using a stress controlled rheometer, we perform stress jump experiments to observe the elastic component of the flow stress that remains on cessation of flow or flow quenching. We investigate the connection between the dynamics of these residual stresses and the rate of physical aging upon quenching from different points on the steady state flow curve. Quenching from high rates produces a fluid state, G">G', with small, fast relaxing residual stresses and rapid, sigmoidal aging of the complex modulus. Conversely, quenching from lower shear rates produces increasingly jammed states featuring slowly relaxing stresses and a slow increase of the complex modulus with system age. Flow cessation from a fixed shear rate with varying quench durations shows that slower quenches produce smaller residual stresses at short times which relax at long times by smaller extents, by comparison with faster quenches. These smaller stresses are correlated with a higher modulus but slower physical aging of the system. The characteristic time for the residual stress relaxation scales inversely with the quench rate. This implies a frustrated approach to any ideal stress-free state that succinctly reflects the frustrated nature of these glassy systems.

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.

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 vorticity aligned log-like structures. We study the dynamics of these elongated flocs using optical microscopy in situ with bulk rheology. The appearance of the vorticity aligned aggregates is attended by an increase in the suspension viscosity which peaks quickly and then gradually recedes with passage of time under flow. The occurrence in time of the viscosity maximum scales inversely with the shear rate applied to the system. This emergence of the peak 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.

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

We employ parallel superposition rheology to study the dynamics of an aging colloidal glass in the presence of a mean field stress. Over a range of intermediate stresses, the loss modulus exceeds the storage modulus at short times but develops a maximum concomitant with a crossover between the two as the system ages. This is attended by a narrowing of the loss peak on increasing stress. We show that this feature is characteristic of the structural arrest in these materials, which is made observable on reasonable timescales by the activating influence of the stress. The arrest time displays an exponential dependence on inverse stress. These results provide experimental validation of the role of stress as an effective temperature in soft glassy systems as has been advanced in recent theoretical frameworks.

We monitor the relaxation of internal stresses in a fractal colloidal gel on cessation of flow and find a weak power-law decay, σi˜t-α over five decades of time where α≈0.07 . The system exhibits physical aging of the elastic modulus, G'˜tβ , with β≈α . Imposition of zero stress after waiting time tw results in strain recovery as the system relaxes without constraint. Remarkably, recoveries at different tw can be shifted to construct a master curve where data are scaled vertically by 1/σi(tw) and plotted horizontally as (t-tw)/twμ , where μ≈1.25 , indicative of a superaging response.

The yielding behavior of colloidal suspensions is a strong function of inter-particle interactions. Recent results [Pham et al. 2006, 2008] indicate that attractive colloidal glasses display a two-step yielding due to inter-particle bond rupture followed by particle cage escape. From this perspective, we examine the yielding behavior of an oil-in-water emulsion system with attractive interactions using dynamic bulk rheology. In strain sweep experiments, after a limited linear regime, the system yields with a pronounced bump in the viscous modulus, a sharp decrease in the elastic modulus and a crossover between the two. The yielding response is marked by bond-breaking at low volume fractions and bond-breaking accompanied by cage escape above a critical concentration. An increase in the complex modulus is observed at yet higher strains (>100%), with both the elastic and viscous components showing small frequency dependent peaks. The onset, peak strains and peak stress display different dependences on volume fraction. We speculate that this display is due to the formation of shear induced structures at high strains and advance a simple model for this behavior.

The aging response of glassy particulate systems originates due to slow structural rearrangements of its constituent matter. It is reasonable to speculate that structural rearrangements on different length scales should manifest themselves in dynamical response on different timescales. Here we consider the frequency dependence of aging in a colloidal glassy system using parallel superposition bulk rheology. The aging behavior of the system is characterized by time evolution of the complex modulus in response to a sinusoidally varying probe stress of different frequencies superimposed on a steady background stress. Strikingly, the system displays more rapid aging when observed at smaller frequencies. This suggests that, by comparison, it is more arrested on shorter length scales (higher frequencies) than on the longer length scales where many-particle correlated motions are in effect. Such correlated motions are believed to be responsible for relaxation in glassy materials. The variation in the aging dynamics at different frequencies is more prominent at higher background stresses where the system is more fluidized.

Many colloidal suspensions are inherently out of equilibrium and display a slow evolution of their dynamics over time. However, many features of the glass transition as encountered in polymer and molecular glasses are not conserved. This phenomenon is still not completely understood and little is known of the connection between flow history, as a determinant of the initial system state, and subsequent aging dynamics. Further, the changes in the energy landscape during aging 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. Here we discuss the use of stress jump experiments that investigate the role of flow history on aging, and the systematic reconstruction of the frequency and strain dependence as a function of age for a repulsive colloidal glass undergoing structural arrest and aging. We uncover a connection between the aging behavior and the rate of flow cessation that is additionally reflected in the dynamics of residual stress relaxation. 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.

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.

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

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 nonstationary 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.

We employ parallel superposition rheology to study the dynamics of an aging colloidal glass in the presence of a mean field stress. Over a range of intermediate stresses, the loss modulus exceeds the storage modulus at short times but develops a maximum concomitant with a crossover between the two as the system ages. This is attended by a narrowing of the loss peak on increasing stress. We show that this feature is characteristic of the structural arrest in these materials, which is made observable on reasonable timescales by the activating influence of the stress. The arrest time displays an exponential dependence on inverse stress. These results provide experimental validation of the role of stress as an effective temperature in soft glassy systems as has been advanced in recent theoretical frameworks.

We investigate the rheology of dilute dispersions of fumed colloidal particles with attractive interactions in hydrocarbon liquids. Surprisingly, these systems display shear thickening due to the breakdown of densified flocs and a concomitant increase in the effective volume fraction of the fractal particles in the fluid. We show that this shear thickening is controlled by a critical stress and accompanied by a positive increase in the first normal stress difference, N 1, at the shear thickening transition. This is in contrast to the well-known hydrocluster mechanism of shear thickening in concentrated hard-sphere and repulsive systems. Gel elasticity depends strongly on the stress applied to suspensions in preshear, scaling roughly as $G'\sim\sigma_{\text{preshear}}^{2}$ . We propose a simple model to account for these results in terms of the cluster number density determined by the preshear stress. At low shear rates, vorticity-aligned aggregates are present at $\dot\gamma\approx 10^0 {\rm{s}}^{-1}$  . In this regime, the system displays a small but noticeable increase in viscosity on increasing shear rate. We investigate the effect of tool roughness and find that wall slip is not responsible for the observed phenomena. Instead, the increase in the apparent viscosity results from increased flow resistance due to the presence of gap-spanning log-like flocs in rolling flow.

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.

Dilute Laponite suspensions in water at low salt concentration form repulsive colloidal glasses which display physical aging. This phenomenon is still not completely understood and in particular, little is known about the connection between the flow history, as a determinant of the initial state of the system, and the subsequent aging dynamics. Using a stress controlled rheometer, we perform stress jump experiments to observe the elastic component of the flow stress that remains on cessation of flow or flow quenching. We investigate the connection between the dynamics of these residual stresses and the rate of physical aging upon quenching from different points on the steady state flow curve. Quenching from high rates produces a fluid state, G">G', with small, fast relaxing residual stresses and rapid, sigmoidal aging of the complex modulus. Conversely, quenching from lower shear rates produces increasingly jammed states featuring slowly relaxing stresses and a slow increase of the complex modulus with system age. Flow cessation from a fixed shear rate with varying quench durations shows that slower quenches produce smaller residual stresses at short times which relax at long times by smaller extents, by comparison with faster quenches. These smaller stresses are correlated with a higher modulus but slower physical aging of the system. The characteristic time for the residual stress relaxation scales inversely with the quench rate. This implies a frustrated approach to any ideal stress-free state that succinctly reflects the frustrated nature of these glassy systems.

We investigate the dynamics of aging in a repulsive colloidal glass composed of charged clay particles in aqueous suspension. Dynamic rheological measurements show a power law evolution of the elastic modulus of the system with sample age, measured as time elapsed after the cessation of a rejuvenating shear flow. We show that the scaling exponent is dependent on the rate of flow cessation or the flow quench rate. Comparatively fast quenches lead to systems with a smaller elastic modulus and accelerated aging whereas slower quenches result in higher modulus but correspondingly less rapid aging. We apply a recently proposed technique to follow the dynamics of residual or internal elastic stresses immediately after the flow arrest and find striking parallels between the relaxation of these stresses and the aging of the system. These results indicate that the evolution of the slow dynamics is strongly coupled to the internal stress state of the system and point to the identification of the flow quench rate as a mechanical variable that characterizes the system's departure from equilibrium.

Dilute Laponite suspensions in water at low salt concentration form repulsive colloidal glasses which display physical aging. This phenomenon is still not completely understood and in particular, little is known about the connection between the flow history, as a determinant of the initial state of the system, and the subsequent aging dynamics. Using a stress controlled rheometer, we perform stress jump experiments to observe the elastic component of the flow stress that remains on cessation of flow or flow quenching. We investigate the connection between the dynamics of these residual stresses and the rate of physical aging upon quenching from different points on the steady state flow curve. Quenching from high rates produces a fluid state, G">G', with small, fast relaxing residual stresses and rapid, sigmoidal aging of the complex modulus. Conversely, quenching from lower shear rates produces increasingly jammed states featuring slowly relaxing stresses and a slow increase of the complex modulus with system age. Flow cessation from a fixed shear rate with varying quench durations shows that slower quenches produce smaller residual stresses at short times which relax at long times by smaller extents, by comparison with faster quenches. These smaller stresses are correlated with a higher modulus but slower physical aging of the system. The characteristic time for the residual stress relaxation scales inversely with the quench rate. This implies a frustrated approach to any ideal stress-free state that succinctly reflects the frustrated nature of these glassy systems.

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.

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 vorticity aligned log-like structures. We study the dynamics of these elongated flocs using optical microscopy in situ with bulk rheology. The appearance of the vorticity aligned aggregates is attended by an increase in the suspension viscosity which peaks quickly and then gradually recedes with passage of time under flow. The occurrence in time of the viscosity maximum scales inversely with the shear rate applied to the system. This emergence of the peak 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.