We study the time evolution of different Laponite suspensions from a low-viscosity ergodic state to a viscoelastic non-ergodic state over a wide range of volume fractions and salt contents. We find that the evolution of non-ergodicity parameter (Debye-Waller factor) splits into two branches for all the samples, which correspond to two distinct dynamically arrested states. At moderately high salt concentrations, on the other hand, a third and new nonergodic state appears that are different from the above two nonergodic states. Measurement of the conductivity of Laponite solutions in pure water shows that the contribution of counterions in the ionic strength is considerable and their role should be taken into account in interpretations of aging dynamics and the phase diagram. Based on these data and available data in the literature, we propose a new (non-equilibrium) phase diagram for Laponite suspensions.
We have studied the effect of temperature on aging dynamics of laponite suspensions by carrying out the rheological oscillatory and creep experiments. We observed that at higher temperatures the mechanism responsible for aging became faster thereby shifting the evolution of elastic modulus to lower ages. Significantly, in the creep experiments, all the aging time and the temperature dependent strain data superposed to form a master curve. Possibility of such superposition suggests that the rheological behavior depends on the temperature and the aging time only through the relaxation processes …
In this paper we investigate glass transition in aqueous suspension of synthetic hectorite clay, laponite. We believe that upon dispersing laponite clay in water, system comprises of clusters (agglomerates) of laponite dispersed in the same. Subsequent osmotic swelling of these clusters leads to increase in their volume fraction. We propose that this phenomenon is responsible for slowing down of the overall dynamics of the system. As clusters fill up the space, system undergoes glass transition. Along with the mode coupling theory, proposed mechanism rightly captures various characteristic fea…
Aqueous suspensions of Laponite, a system composed of disk-like nanoparticles, are found to develop optical birefringence over several days, well after the suspensions solidified due to jamming. The optical anisotropy is particularly enhanced near the air Laponite suspension interface over length-scales of several millimetres, which is beyond five orders of magnitude larger than the particle length scale, suggestive of large scale ordering influenced by the interface. The orientational order increases with time and is always greater for higher concentration of salt, higher concentration of Lap…
The primary and secondary relaxation timescales of aging colloidal suspensions of Laponite are estimated from intensity autocorrelation functions obtained in dynamic light scattering (DLS) experiments. The dynamical slowing down of these relaxation processes are compared with observations in fragile supercooled liquids by establishing a one-to-one mapping between the waiting time since filtration of a Laponite suspension and the inverse of the temperature of a supercooled liquid that is rapidly quenched towards its glass transition temperature. New timescales, such as the Vogel time and the Ka…
We investigate the nonlinear rheological behavior of colloidal suspensions of Laponite, a synthetic clay, driven by a steady and homogeneous shear strain. We show that the external drive leads to a drastic slowing down of the aging dynamics or even, in some cases, in the rejuvenation of the system. Under shear, after a surprisingly long time, the spontaneous aging process observed at rest is suppressed. The system then reaches a non-equilibrium stationary state, characterised by a complex viscosity depending on the applied shear rate. In addition, the glass exhibits a non-Newtonian shear-thinn…
The settling dynamics of falling spheres inside a Laponite suspension is studied. Laponite is a colloidal synthetic clay that shows physical aging in aqueous suspension due to the spontaneous evolution of inter-particle electrostatic interactions. In our experiments, millimeter-sized steel balls are dropped in aqueous Laponite suspensions of different ages (i.e., time elapsed since sample preparation). The motion of the falling balls are captured using a high-speed camera and the velocities of their centroids are estimated from the images. Interestingly, we observe that balls of larger diamete…
We investigate pressure driven pipe flow of Laponite suspension, as a model thixotropic fluid. The tendency of the suspension to age is controlled by addition of sodium chloride salt to vary the ionic strength. We use a syringe pump to prescribe the flow and observe that a steady state flow is obtained. Unusually, the steady state pressure drop required to maintain a constant flow rate decreases with an increase in the flow rate, in qualitative contrast to the expectation for Poiseuille flow. We demonstrate that experimental results obtained by varying the flow rate, salt concentration, and fl…
A universal secondary relaxation process, known as the Johari-Goldstein (JG) $β$-relaxation process, appears in glass formers. It involves all parts of the molecule and is particularly important in glassy systems because of its very close relationship with the $α$-relaxation process. However, the absence of a J-G $β$-relaxation mode in colloidal glasses raises questions regarding its universality. In the present work, we study the microscopic relaxation processes in Laponite suspensions, a model soft glassy material, by dynamic light scattering (DLS) experiments. $α$ and $β$-relaxation timesca…
We study aging behavior of aqueous suspension of Laponite as a function of concentration of Laponite, concentration of salt, time elapsed since preparation of suspension (idle time) and temperature by carrying extensive rheological and conductivity experiments. We observe that temporal evolution of elastic modulus, which describes structural buildup and aging, shifts to low times for experiments carried out for higher concentration of Laponite, higher concentration of salt, greater temperature and on higher idle time while preserving the curvature of evolution in the solid regime. Consequently…
Hypothesis: Aging in colloidal suspensions manifests as a reduction in kinetic freedom of the colloids. In aqueous suspensions of charged colloids, the role of inter-particle electrostatics interactions on the aging dynamics is well debated. Despite water being the dispersion medium, the influence of water structure on the physicochemical properties of aging colloids has never been considered before. Laponite, a model hectorite clay, could be used to evaluate the relative contributions of medium structure and electrostatics in determining the physicochemical properties of aging colloidal suspe…
Investigating microstructure of suspensions with particles having anisotropic shape that share complex interactions is a challenging task leading to competing claims. This work investigates phase behavior of one such system: aqueous Laponite suspension, which is highly contested in the literature, using rheological and microscopic tools. Remarkably, we observe that over a broad range of Laponite (1.4 to 4 weight %) and salt concentrations (0 to 7 mM), the system overwhelmingly demonstrates all the rheological characteristics of the sol-gel transition leading to a percolated network. Analysis o…
This work is devoted to the study of displacement fluctuations of micron-sized particles in an aging colloidal glass. We address the issue of the validity of the fluctuation dissipation theorem (FDT) and the time evolution of viscoelastic properties during aging of aqueous suspensions of a clay (Laponite RG) in a colloidal glass phase. Given the conflicting results reported in the literature for different experimental techniques, our goal is to check and reconcile them using \emph{simultaneously} passive and active microrheology techniques. For this purpose we measure the thermal fluctuations …
We report Brownian dynamics simulation of the out-of-equilibrium dynamics (aging) in a colloidal suspension composed of rigid charged disks, one possible model for Laponite, a synthetic clay deeply investigated in the last few years by means of various experimental techniques. At variance with previous numerical investigations, mainly focusing on static structure and equilibrium dynamics, we explore the out-of-equilibrium aging dynamics. We analyze the wave-vector and waiting time dependence of the dynamics, focusing on the single-particle and collective density fluctuations (intermediate scat…
We study dilute aqueous solutions of charged disk-like mineral particles (Laponite) by a combination of static light scattering (SLS) and small-angle x-ray scattering (SAXS). Laponite solutions are known to form gels above a certain critical concentration that must be described as non-equilibrium states. Here we focus on the investigation by SLS and SAXS at concentrations below gelation (c < 0.016 g/L) and at low concentrations of added salt (0.001 and 0.005 mM). Thus, we have obtained the scattering function of single Laponite platelets as well as the structure factor describing their interac…
Aqueous colloidal Laponite clay suspensions transform spontaneously to a soft solid-like arrested state as its aging or waiting time increases. This article reports the rapid transformation of aqueous Laponite suspensions into soft solids due to the application of a DC electric field. A substantial increase in the speed of solidification at higher electric field strengths is also observed. The electric field is applied across two parallel brass plates immersed in the Laponite suspension. The subsequent solidification that takes place on the surface of the positive electrode is attributed to th…
Simultaneous rheological and velocity profile measurements are performed in a smooth Couette geometry on Laponite suspensions seeded with glass microspheres and undergoing the shear-induced solid-to-fluid (or yielding) transition. Under these slippery boundary conditions, a rich temporal behaviour is uncovered, in which shear localization is observed at short times, that rapidly gives way to a highly heterogeneous flow characterized by intermittent switching from plug-like flow to linear velocity profiles. Such a temporal behaviour is linked to the fragmentation of the initially solid sample i…
The gelation of water suspension of a synthetic clay (Laponite) has been studied by dynamic light scattering in a wide range of clay weight concentration (Cw = 0.003-0.031). At variance with previous determination, indicating a stable liquid phase for Cw < Cw*=0.015-0.018, we find that the gelation takes actually place in the whole examined Cw range. More importantly, we find that Cw* marks the transition between two different routes to gelation. We hypothesize that at low concentration Laponite suspension behaves as an attractive colloid and that the slowing down of the dynamics is attained b…
Clay-polymer dispersions in aqueous solutions have attracted a great interest in recent years due to their industrial applications and intriguing physical properties. Aqueous solutions of bare Laponite particles are known to age spontaneously from an ergodic state to a non ergodic state in a time varying from hours to months depending on Laponite concentration. When a polymer species like Polyethylene Oxide (PEO) is added to the solution, it weakly adsorbs on clay particle surfaces modifying the effective interaction potential between Laponite particles. A dynamic light scattering study, varyi…
In this work we study the aging behavior of aqueous suspension of Laponite having 2.8 weight % concentration using rheological tools. At various salt concentration all the samples demonstrate orientational order when observed using crossed polarizers. In rheological experiments we observe inherent irreversibility in the aging dynamics which forces the system not to rejuvenate to the same state in the shear melting experiment carried out at a later date since preparation. The extensive rheological experiments carried out as a function of time elapsed since preparation demonstrate the self simil…
In this paper we study deformation, failure and breakage of visco-elastic thin films of aging laponite suspension under tensile deformation field. Aqueous suspension of laponite is known to undergo waiting time dependent evolution of its micro-structure, also known as aging, which is accompanied by an increase in the elastic modulus and relaxation time. In the velocity controlled tensile deformation experiments, we observed that the dependence of force and dissipated energy on velocity and initial thickness of the film is intermediate to a Newtonian fluid and a yield stress fluid. For a fixed …
In this paper, ageing behavior of suspensions of laponite with varying salt concentration is investigated using rheological tools. It is observed that the ageing is accompanied by an increase in the complex viscosity. The succeeding creep experiments performed at various ages showed damped oscillations in the strain. The characteristic time-scale of the damped oscillations, retardation time, showed a prominent decrease with the age of the system. However, this dependence weakens with an increase in the salt concentration, which is known to change microstructure of the system from glass-like to…
We study diffusion of heat in an aqueous suspension of disc shaped nanoparticles of Laponite, which has finite elasticity and paste-like consistency, by using the Mach-Zehnder interferometer. We estimate the thermal diffusivity of the suspension by comparing the experimentally obtained temperature distribution to that with analytical solution. We observe that despite highly constrained Brownian diffusivity of particles owing to its soft glassy nature, suspensions at very small concentrations of Laponite demonstrates significant enhancement in thermal diffusivity. We correlate the observed enha…
Aqueous suspensions of Laponite with discotic particles are well-studied and find a wide range of applications in industry. A new direction of their implementation is polymer composites that can exhibit improved physical properties.We have studied the aging of aqueous suspensions of Laponite and sodium polystyrene sulfonate (PSS-Na) and both their microscopic (small-angle X-ray scattering, SAXS) and macroscopic (small amplitude oscillatory shear (SAOS) rheometry) properties. The concentration of Laponite, $C_L$, was fixed at 2.5% wt and concentration of PSS-Na, $C_p$, was varied within 0-0.5% …
We have studied the effect of an external DC electric field (~kV/mm) on the rheological properties of colloidal suspensions consisting of aggregates of laponite particles in a silicone oil. Microscopy observations show that under application of an electric field greater than a triggering electric field E_c~0.6 kV/mm, laponite aggregates assemble into chain- and/or column-like structures in the oil. Without an applied electric field, the steady state shear behavior of such suspensions is Newtonian-like. Under application of an electric field larger than E_c, it changes dramatically as a result …
An aqueous suspension of the synthetic clay Laponite undergoes a transition from a liquid-like ergodic state to a glass-like nonergodic arrested state. In an observation that closely resembles the dynamical slowdown observed in supercooled liquids, the phenomenon of kinetic arrest in Laponite suspensions is accompanied by a growth in the $α$-relaxation time with increasing sample aging time, $t_{w}$. The ubiquitous dynamic slowdown and fragile behavior observed in glass forming liquids approaching the glass transition is typically ascribed to the growth in the size of distinct dynamical hetero…
Optical tweezers (OTs) can detect pico-Newton range forces operating on a colloidal particle trapped in a medium and have been successfully utilized to investigate complex systems with internal structures. Laponite clay particles in an aqueous medium self-assemble to form microscopic networks over time as electrostatic interactions between the particles gradually evolve in a physical aging process. We investigate the forced movements of an optically trapped micron-sized colloidal probe particle, suspended in an aging Laponite suspension, as the underlying Laponite microstructures gradually dev…
In this work, we investigate the ageing behavior of soft glassy solids of aqueous suspension of laponite under shear flow. We observe that when an imposed time is normalized by a dominating relaxation time of the system, the rheological response at different ages shows superposition. Analysis of this behavior suggests that the structural evolution with age under a deformation field, as represented by the dependence of dominant relaxation mode on age, becomes weaker as the system becomes progressively less homogeneous due to enhanced attractive interactions caused by addition of salt. Creep-rec…
Dynamic Light Scattering (DLS) measurements were performed on colloidal suspensions of Laponite\textsuperscript{\textregistered} at different concentrations in the range $C_\text{w}= (1.5÷3.0)$%. The slowing down of the dynamics induced by aging was monitored by following the temporal evolution of autocorrelation functions at different concentrations towards the gel and glass transition. Exploiting analogies with supercooled liquids approaching their glass transitions, an Angell plot for the structural relaxation times was drawn. Finally, the fragility of Laponite\textsuperscript{\textregister…
Since the early observation of nematic phases of disc-like clay colloids by Langmuir in 1938, the phase behaviour of such systems has resisted theoretical understanding. The main reason is that there is no satisfactory generalization for charged discs of the isotropic DLVO potential describing the effective interactions between a pair of spherical colloids in an electrolyte. In this contribution, we show how to construct such a pair potential, incorporating approximately both the non-linear effects of counter-ion condensation (charge renormalization) and the anisotropy of the charged platelets…
The falling of an object through a non-Newtonian fluid is an interesting problem, depending on the details of the rheology of the fluid. In this paper we report on the settling of spherical objects through two non-Newtonian fluids: Laponite and hair Gel. A falling object's behavior in passing through a thixotropic colloidal suspension of synthetic clay, Laponite, has been reported to have many behavioral regimes. Here we report observation of a new regime where irregular motion is observed. We argue that this irregular motion may be interpreted as onset of chaos. Observation of this regime dep…
For over 30 years, mode-coupling theory (MCT) has been the de facto theoretic description of dense fluids and the liquid-glass transition. MCT, however, is limited by its ad hoc construction and lacks a mechanism to institute corrections. We use recent results from a new theoretical framework--developed from first principles via a self-consistent perturbation expansion in terms of an effective two-body potential--to numerically explore the kinetics of systems of classical particles, specifically hard spheres obeying Smoluchowski dynamics. We present here a full solution to the kinetic equation…
