Hyper-aging dynamics of nanoclay suspension

Laponite®-From Dispersion to Gel-Structure, Properties, and Applications

Laponite® (LAP) is an intensively studied synthetic clay due to the versatility given by its layered structure, which makes it usable in various applications. This review describes the multifaceted properties and applications of LAP in aqueous dispersions and gel systems. The first sections of the review discuss the LAP structure and the interactions between clay discs in an aqueous medium under different conditions (such as ionic strength, pH, temperature, and the addition of polymers) in order to understand the function of clay in tailoring the properties of the designed material. Additionally, the review explores the aging phenomenon characteristic of LAP aqueous dispersions as well as the development of shake-gels by incorporating LAP. The second part shows the most recent studies on materials containing LAP with possible applicability in the drilling industry, cosmetics or care products industry, and biomedical fields. By elucidating the remarkable versatility and ease of integration of LAP into various matrices, this review underscores its significance as a key ingredient for the creation of next-generation materials with tailored functionalities.

Elastoviscoplasticity, hyperaging, and time-age-time-temperature superposition in aqueous dispersions of bentonite clay

Clay slurries are both ubiquitous and essential in the oil exploration industry, and are most commonly employed as drilling fluids. Due to its natural abundance, bentonite clay is often the de facto choice for these materials. Understanding and predicting the mechanical response of these fluids is critical for safe and efficient drilling operations. However, rheological modeling of bentonite clay suspensions is complicated by the fact that thermally-driven microscopic arrangements of particle aggregates lead to a continual evolution of the viscoelastic properties and the yield stress of the suspension with time. Ergodic relations fundamental to linear viscoelastic theory, such as the Boltzmann superposition principle, do not hold in this scenario of 'rheological aging'. We present an approach for modeling the linear viscoelastic response of aging bentonite suspensions across a range of temperatures that is based on the transformation from laboratory time to an effective 'material time' domain in which time-translation invariance holds, and the typical relations of non-aging linear viscoelastic theory apply. In particular, we model the constitutive relationship between stress and strain-rate in the bentonite suspensions as fractional Maxwell gels with constant relaxation dynamics in the material time domain, in parallel with a non-aging Newtonian viscous contribution to the total stress. This approach is supported by experimental measurements of the stress relaxation and rapid time-resolved measurements of the linear viscoelastic properties performed using optimized exponential chirps. This data is then reduced to master curves in the material domain using time-age-time superposition to obtain best fits of the model parameters over a range of operating temperatures.

A microstructural investigation of an industrial attractive gel at pressure and temperature

Oil-continuous drilling fluids used in the oil and gas industry are formulated to be pseudoplastic with a relatively weak yield stress. These fluids are required to maintain their properties over wide temperature and pressure ranges yet there are few methods that can sensitively study the inherent structure and mechanical properties in the fluids under such conditions. Here we study a model oil-continuous drilling fluid formulation as a function of both temperature (up to 153 °C) and pressure (up to 1330 bar) with Diffusive Wave Spectroscopy (DWS). The system comprises a colloidal gel network of clay particles and trapped emulsion droplets. As a function of temperature the system undergoes local structural changes reflected in the DWS dynamics which are also consistent with macroscopic rheological measurements. On cycling to high pressure the system exhibits similar structural and dynamic changes with a strong hysteresis. Although multiple scattering in multicomponent non-ergodic samples does not directly yield self diffusion probe dynamics, the use of microrheology analysis here appears to be in good agreement with direct rheological measurements of the sample linear viscoelasticity at ambient pressure. Thus DWS microrheology succesfully probes irreversible changes in the structure and the mechanical response of the drilling fluid formulation under a high pressure cycle.

Modulation of soft glassy dynamics in aqueous suspensions of an anisotropic charged swelling clay through pH adjustment

HYPOTHESIS

Sodium-montmorillonite (Na-Mt) particles are geometrically anisometric that carry a pH dependent anisotropic surface charge. Therefore, it should be possible to manipulate the particle-particle interaction of colloidal range Na-Mt suspensions through pH changes which in turn should alter the soft glassy dynamics of Na-Mt suspensions.

EXPERIMENTS

Rheological experiments were used to probe the impact of pH mediated colloidal particle-particle interaction on the physical aging, linear viscoelastic response, and yield stress behavior of Na-Mt suspension.

FINDINGS

The temporal evolution of the storage modulus (G') was stronger in the acid regime (pH < 9.5) than the base (pH ≥ 9.5) pH regime. Horizontal shifting of the aging curves in the acid and base regimes led to aging time-H⁺ concentration and aging time-OH^- concentration superposition. An aging time-Na-Mt concentration superposition was also observed in both pH regimes. The critical stress associated with the viscosity bifurcation behavior increased linearly with G' but with different slopes for acid and base regime. We propose that positively charged patches on the Na-Mt particle edge merge with the characteristic surface as a function of H⁺ ions in the system. This leads to a strongly associated microstructure at low pH and a relatively weak but associated microstructure at natural pH, hence confirming the hypothesis.

Effect of sodium pyrophosphate and understanding microstructure of aqueous LAPONITE® dispersion using dissolution study

In this work, we investigate the physical origin of ergodicity breaking in an aqueous colloidal dispersion of synthetic hectorite clay, LAPONITE^®, by performing dissolution and rheological experiments with monovalent salt and tetrasodium pyrophosphate solution. We also study the effect of pH and nature of interface, nitrogen and paraffin oil on the same. Dissolution experiments carried out for dispersions with both the interfaces show similar results. However, for samples with a nitrogen interface, all the effects are observed to get expedited in time compared to a paraffin oil interface. When kept in contact with water, 1.5 wt.% and 2.8 wt.% colloidal dispersion at pH 10 swells at small ages, while it does not swell at large ages. The solution of tetrasodium pyrophosphate, interestingly, dissolves the entire colloidal dispersion sample with pH 10 irrespective of the concentration of clay. Experiments carried out on colloidal dispersions prepared in water having pH 13 demonstrate no effect of water as well as sodium pyrophosphate solution on the same suggesting a possibility of the presence of negative charge on edge at that pH. We believe that all the behaviors observed for samples at pH 10 can be explained by an attractive gel microstructure formed by edge-to-face contact. Furthermore, the absence of swelling in old colloidal dispersion at pH 10 and dissolution of the same by sodium pyrophosphate solution cannot be explained by merely repulsive interactions. This behavior suggests that attractive interactions originating from edge-to-face contact play an important role in causing ergodicity breaking in the colloidal dispersions at pH 10 at all the ages irrespective of the clay concentration. We further substantiate the presence of a fractal network structure formed by interparticle edge-face association using rheological tools and cryo-TEM imaging. We also conduct a comprehensive study of the effect of tetrasodium pyrophosphate on the sol-gel transition of LAPONITE^® dispersion.

Microstructure and Soft Glassy Dynamics of an Aqueous Laponite Dispersion

Synthetic hectorite clay Laponite RD/XLG is composed of disk-shaped nanoparticles that acquire dissimilar charges when suspended in an aqueous medium. Owing to their property to spontaneously self-assemble, Laponite is used as a rheology modifier in a variety of commercial water-based products. In particular, an aqueous dispersion of Laponite undergoes a liquid-to-solid transition at about 1 vol % concentration. The evolution of the physical properties as the dispersion transforms to the solid state is reminiscent of physical aging in molecular as well as colloidal glasses. The corresponding soft glassy dynamics of an aqueous Laponite dispersion, including the rheological behavior, has been extensively studied in the literature. In this feature article, we take an overview of recent advances in understanding soft glassy dynamics and various efforts taken to understand the peculiar rheological behavior. Furthermore, the continuously developing microstructure that is responsible for the eventual formation of a soft solid state that supports its own weight against gravity has also been a topic of intense debate and discussion. In particularly, extensive experimental and theoretical studies lead to two types of microstructures for this system: an attractive gel-like or a repulsive glass-like structure. We carefully examine and critically analyze the literature and propose a state (phase) diagram that suggests an aqueous Laponite dispersion to be present in an attractive gel state.

Nanoengineered Colloidal Inks for 3D Bioprinting

Nanoengineered hydrogels offer the potential to design shear-thinning bioinks for three-dimensional (3D) bioprinting. Here, we have synthesized colloidal bioinks composed of disk-shaped two-dimensional (2D) nanosilicates (Laponite) and poly(ethylene glycol) (PEG). The addition of Laponite reinforces the PEG network and increases viscosity, storage modulus, and network stability. PEG-Laponite hydrogels display shear-thinning and self-recovery characteristics due to rapid internal phase rearrangement. As a result, a range of complex patterns can be printed using PEG-Laponite bioinks. The 3D bioprinted structure has similar mechanical properties compared to the as-casted structure. In addition, encapsulated cells within the PEG-Laponite bioink show high viability after bioprinting. Overall, this study introduces a new class of PEG-Laponite colloidal inks for bioprinting and cell delivery.

Passive microrheology in the effective time domain: analyzing time dependent colloidal dispersions

We studied the aging dynamics of an aqueous suspension of LAPONITE®, a model time dependent soft glassy material, using a passive microrheology technique. This system is known to undergo physical aging during which its microstructure evolves progressively to explore lower free energy states. Optical microscopy is used to monitor the motion of micron-sized tracer probes embedded in a sample kept between two glass plates. The mean square displacements (MSD) obtained from the motion of the tracer particles show a systematic change from a purely diffusive behavior at short aging times to a subdiffusive behavior as the material ages. Interestingly, the MSDs at all the aging times as well as different LAPONITE® concentrations superpose remarkably to show a time-aging time master curve when the system is transformed from the real time domain to the effective time domain, which is obtained by rescaling the material clock to account for the age dependent relaxation time. The transformation of the master curve from the effective time domain to the real time domain leads to the prediction of the MSD in real time over a span of 5 decades when the measured data at individual aging times are only over 2 decades. Since the MSD obtained from microrheology is proportional to the creep compliance of a material, by using the Boltzmann superposition principle along with the convolution relation in the effective time domain, we predict the stress relaxation behavior of the system in real time. This work shows that the effective time approach applied to microrheology facilitates the prediction of long time creep and relaxation dynamics of a time dependent soft material by carrying out short time experiments at different aging times.

Analyzing a fractal gel of charged oblate nanoparticles in a suspension using time-resolved rheometry and DLVO theory

The disk-like nanoparticles of LAPONITE® are known to self-assemble to form a fractal gel within hours after a sufficiently large concentration of LAPONITE® is dispersed in water containing salt. The concentration of sodium counterions associated with LAPONITE® particles, however, continues to increase over a period of days, suggesting that delamination of LAPONITE® disks from stacks is sluggish and/or dissociation of counterions is slow. In either case, spontaneous self-assembly of LAPONITE® particles occurs even though delamination and/or counterion dissociation has not reached its equilibrium state. In order to determine the nature of the fractal gel as the extent of delamination and/or dissociation progresses towards equilibrium, we subject the LAPONITE® suspension to a freezing–defrosting cycle, which interestingly reinitiates the gelation process in suspension afresh. Application of time-resolved rheometry to a defrosted suspension shows that iso-frequency loss tangent curves intersect at an identical point, validating the Winter–Chambon criterion for a critical fractal gel state. Interestingly, while the time required to form a critical gel is observed to decrease with increased time elapsed since preparation, at which freezing–defrosting is carried out, the fractal dimension of the critical gel is observed to remain unaffected. We also solve DLVO theory for free energy interactions between the negatively charged LAPONITE® particle faces and analyze the observed phenomena.

A model for aging under deformation field, residual stresses and strains in soft glassy materials

A model is proposed that considers aging and rejuvenation in a soft glassy material as, respectively, a decrease and an increase in free energy. The aging term is weighted by an inverse of characteristic relaxation time suggesting that greater mobility of the constituents induces faster aging in a material. A dependence of relaxation time on free energy is proposed, which under quiescent conditions leads to a power law dependence of relaxation time on waiting time as observed experimentally. The model considers two cases, namely, a constant modulus when aging is entropy controlled and a time dependent modulus. In the former and the latter cases the model has, respectively, two and three experimentally measurable parameters that are physically meaningful. Overall, the model predicts how the material undergoes aging and approaches a rejuvenated state under the application of a deformation field. In particular, the model proposes distinctions between various kinds of rheological effects for different combinations of parameters. Interestingly, when the relaxation time evolution is stronger than linear, the model predicts various features observed in soft glassy materials such as thixotropic and constant yield stress, thixotropic shear banding, and the presence of residual stress and strain.

Dynamic light scattering study and DLVO analysis of physicochemical interactions in colloidal suspensions of charged disks

The interparticle interactions in colloidal suspensions of charged disks of Laponite clay in water were investigated using dynamic light scattering (DLS) and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. We studied the effects of clay concentration (C(L)), the concentration of externally added salt (C(S)), and temperature (T) on the microscopic dynamics of the clay suspensions. The fast (τ1) and mean slow relaxation times (⟨τ(ww)⟩) of Laponite suspensions were extracted from intensity autocorrelation functions measured at different waiting times (t(w)) after sample preparation. Comprehensive Laponite concentration-salt concentration-temperature-time superpositions of both the microscopic diffusive time scales and the stretching exponent corresponding to the slow relaxation process highlight the self-similar nature of the energy landscapes of the Laponite suspensions. The evolution of the sodium ion concentration in the aging suspension with tw, measured for several values of CL, CS, and T, was used in a DLVO analysis of the free energy of the suspension for two charged disks parallely approaching one another. This analysis confirms that, in addition to repulsive interparticle interactions, attractive interactions also play a pivotal role in the microscopic dynamics of spontaneously evolving Laponite suspensions.

Comparison of the physical aging behavior of a colloidal glass after shear melting and concentration jumps

Colloidal systems are considered good models of molecular glasses and we further explore the range of validity of this paradigm using a thermosensitive core-shell particle dispersion to study the aging response of a colloidal glass subsequent to both shear-melting and temperature (concentration)-jump perturbations in the vicinity of the glass transition concentration or temperature. Sequential creep experiments were used to probe the different aging responses of the system. The colloidal glass displays aging behavior after both types of perturbation and our results indicate that this colloidal glass is similar to a molecular glass, in that shift rates are found to be below unity and to decrease towards zero as the glass temperature (or concentration) is approached as temperature increases. However, the kinetics of the aging in the two cases are different indicating that the structural changes induced by the mechanical perturbation are different from those induced by the temperature or concentration jump-similar to findings on mechanical rejuvenation of molecular glasses. We also find differences between the colloidal glass and molecular glasses: In the case of the colloidal glass the structural recovery or equilibration times do not diverge, while the mechanical relaxation times do. On the other hand, for the molecular glass, both times change very rapidly with decreasing temperature, apparently towards a distant point of divergence.

Dynamics of colloidal glasses and gels

Many household and industrially important soft colloidal materials, such as pastes, concentrated suspensions and emulsions, foams, slurries, inks, and paints, are very viscous and do not flow over practical timescales until sufficient stress is applied. This behavior originates from restricted mobility of the constituents arrested in disordered structures of varying length scales, termed colloidal glasses and gels. Usually these materials are thermodynamically out of equilibrium, which induces a time-dependent evolution of the structure and the properties. This review presents an overview of the rheological behavior of this class of materials. We discuss the experimental observations and theoretical developments regarding the microstructure of these materials, emphasizing the complex coupling between the deformation field and nonequilibrium structures in colloidal glasses and gels, which leads to a rich array of rheological behaviors with profound implications for various industrial processes and products.

Kinetics of anisotropic ordering in Laponite dispersions induced by a water-air interface

In this work, we report the kinetics of ordering occurring at the water-air interface of Laponite dispersions. Propagation of such ordering into the bulk and its relaxation dynamics were systematically studied through light scattering measurements. Depolarization ratio D(p), which accounted for the optical anisotropy, was measured as a function of depth from the interface and aging of the samples. The extent of spatial ordering was found to be several decades larger than the typical particle size. Spatial ordering originated from the interface and percolated into the bulk with aging time t(w). Growth in D(p) with waiting time was found to follow power-law behavior given as D(p)~t(w)(n), with n increasing from 0.1 to 4 as one moved away from the interface into the bulk. D(p) decreased exponentially with depth h given as D(p)~e(-(h/h(0))), where h(0) is the decay length, increasing from 0.4 to 0.75 mm with aging time. Dynamic structure factor measurements performed on the samples at various aging times, depths, and temperatures yielded two distinct relaxation times: one fast mode followed by a slow mode. The fast mode remained invariant while slow mode relaxation time followed an exponential decay with depth. This study indicated that the arrested phase nucleated from the interface and propagated into the bulk, which was not observed when the surface was insulated with a layer of hydrophobic liquid. Dilution of the concentrated samples destroyed the aforesaid ordering and made the dispersion homogeneous implying the ordered state was a glass.

Polymer-mediated clustering of charged anisotropic colloids

Formation of stable, dense nanoparticle clusters is interesting due to both the underlying physics and use of nanoclusters in applications such as digital printing, imaging and biosensing, and energy storage. Here, we explore formation of nanoparticle clusters in dispersions of the model disk-shaped colloid Laponite. Under basic conditions, the model disk-shaped colloid Laponite forms a repulsive glass in water due to strong electrostatic interactions. Addition of a nonadsorbing polymer, the sodium salt of poly(acrylic acid) (PAA), induces a depletion attraction between particles. Through dynamic light scattering (DLS) and rheology, we see that the polymer initially causes a transition from the glassy phase to an ergodic fluid. Samples at higher particle concentration age to a weak nonergodic state, while samples at lower Laponite remain as fluids. As the strength of attraction between particles is increased, we find an increase in the fast relaxation time measured via dynamic light scattering (e.g., slowing of the short-time diffusion of a single particle). While this may in part be attributed to an increase in the ionic strength, the aging behavior and glass-fluid transition we observe appear to be unique to the presence of polymer, suggesting that depletion plays an important role. DLS data on the fluid samples were consistent with two widely spaced diffusive relaxation modes, corresponding to motion of single particles and motion of large clusters, although other slow dynamic processes may be present. On the basis of the estimated volume fraction and depletion attraction, we believe the Laponite-PAA suspensions to be either fluids of stable clusters or glasses of clusters, although it is possible that the nonergodic state we observe is instead a gel of clusters. Additionally, the cluster size was found to be stable for at least 120 days and was directly related to the polymer concentration. This may serve as an important means of tuning cluster size in products and processes based on dense nanoparticle assemblies.

The dynamics of polymer bridge formation and disruption and its effect on the bulk rheology of suspensions

Bridge-flocculated colloidal gels are used in many important processes and products such as gel casting for advanced ceramics, precursor inks for 3D printing, and waste treatment strategies. An important aspect of polymer bridged gels that makes them excellent candidates for these applications is the precise control it affords for control of rheological properties. Recent studies have shown that adhesion between bridged surfaces increases with time as the number of polymer bridges formed grows. However, the consequences of the dynamics of these processes toward bulk rheological properties have not been studied. Here we investigate the link between the dynamics of polymer bridging and disruption and bulk rheology in dense colloidal silica particle suspensions flocculated by polyethylene oxide (PEO). Microscale pull-off force measurements using atomic force microscope (AFM) show that upon repeated disruption and establishment of bridged contact, the adhesion between the surfaces is reduced. During contact disruption, the polymer chains bridging the two surfaces are stretched leading to chain scission. On the re-establishment of contact, these fragmented polymer chains are unable to fully re-establish the adhesion. Macroscale measurements using oscillatory rheology show that this reduced adhesion results in reduction of both the storage modulus and the yield stress. If the slurry is subjected to high shear for long periods, polymer chain scission is amplified, and the fragmented polymer chains are unable to bridge the particles again, resulting in free-flowing slurries.

Physicochemical effects in aging aqueous Laponite suspensions

We study aging behavior of an 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 moduli, which describes structural build-up and aging, shifts to low times for experiments carried out for higher concentration of Laponite, higher concentration of salt, greater temperature, and longer idle time while preserving the curvature of evolution in the solid regime (elastic modulus greater than viscous modulus). Consequently appropriate shifting of evolution of elastic modulus in the solid regime leads to aging time-idle time-salt concentration-Laponite concentration-temperature superposition. The existence of such a superposition suggests the generic nature of microstructure buildup irrespective of mentioned variables in the explored range. The behavior of shift factors needed to obtain the superposition indicate that the energy barrier associated with structural buildup decreases with an increase in idle time and temperature and decreases linearly with an increase in concentration of Laponite and that of salt. The conductivity experiments show that ionic conductivity of the suspension increases with increasing Laponite concentration, salt concentration, temperature, and very importantly the idle time. We also analyze the interparticle interactions using DLVO theory that suggests an increase in idle time, temperature, and salt concentration increases the height of the repulsive energy barrier while it decreases the width of the same when particles approach each other in a parallel fashion. However when particles approach each other in a perpendicular fashion, owing to dissimilar charges on edge and face, the energy barrier for the attractive interaction is expected to decrease with an increase in idle time, temperature, and salt concentration. Analysis of rheological and conductivity experiments suggests a strong influence of attractive interactions on the low energy structures in an aqueous suspension of Laponite.