Attractive Gel Research Articles

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.

Salt and pH-Induced Attractive Interactions on the Rheology of Food Protein-Stabilized Nanoemulsions.

This research aimed to investigate the possibility of forming gelled nanoemulsions (NEs) by inducing attractive interactions among the nanodroplets. The effect of salt concentration and changes in pH on the stability and gelation behavior of 2, 4, and 5% sodium caseinate (SC) and whey protein isolate (WPI)-stabilized 40% canola oil-in-water NEs were investigated. For the effect of salt, sodium chloride was added in a concentration of 0.1, 0.5, and 1 M in the continuous phase of the NEs at neutral pH, whereas to study the effect of acidification, the pH of the NEs was adjusted to the isoelectric point (pI) of the proteins. The addition of salt led to attractive gelation in WPI NEs because of a screening of charge. In contrast, the gel strength of SC-stabilized NEs was reduced with salt, which was attributed to the loss of close packing of droplets and their surrounding repulsive barriers because of charge screening and to the steric barrier of interfacial SC preventing droplet aggregation. All the NEs with pH at the pI of proteins transformed into strong attractive gels made of droplet aggregates irrespective of the type or concentration of protein because of the complete charge neutralization. The strength of the acidified NE gels increased with a decrease in droplet size and the type of protein used. Overall, research on the effect of different environmental factors on the stability and gelation behavior of protein-stabilized NEs could be useful for possible applications of these nanoscale materials in various food systems.

Chemical Signals are Involved in the Detection and Preference of Food Sources in Blattella Germanica

The German cockroach (Blattella germanica) is a cosmopolitan omnivorous species which can take nutrients from a wide variety of sources. Its condition of pest generated interest in the study of its biology, and while there are many works regarding its feeding behavior, few approaches have been made to the relevance of the chemosensory system during foraging. Objective: In this work we studied the role of chemical stimuli in the detection and preference of food sources in B. germanica using behavioral observations. Methods: Adult males were placed in a circular experimental arena where different diet types were presented individually. Parameters associated to the detection of odors (latency to the food source, number of visits to the source) were measured in an experimental time of 15 minutes. We also measured the total amount ingested determining the weight gain of each individual, as an indicator of taste evaluation of food. Findings: Insects showed a lower latency when the source emitted a larger amount of volatile compounds, so peanut butter was detected faster compared with the other diets studied. Regarding ingestion, insects spent significantly more time on the peanut butter, indicating the presence of phagostimulants such as sugars or lipids. When sugar solutions were used as food sources, the number of visits, the time spent on the source and the amount consumed was higher when increasing sugar concentration. Insects also showed preference for the solution of higher concentration, when two sugar solutions were presented simultaneously. Latency increased and the amount of sugar ingested decreased when insects´ mouthparts were chemically blocked with N-Ethylmaleimide, a general inhibitor of chemoreception. Novelty/Improvement: These results describe how cockroaches use odors to locate food sources and chemoreceptors in their mouthparts to identify diets of higher energetic value, from information regarding the concentration and quality of the food source. Understanding the feeding behavior of cockroaches allows the design of more attractive and palatable toxic gel baits enhancing their efficacy for cockroach control.

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.

Concepts for Developing Physical Gels of Chitosan and of Chitosan Derivatives.

Chitosan macro- and micro/nano-gels have gained increasing attention in recent years, especially in the biomedical field, given the well-documented low toxicity, degradability, and non-immunogenicity of this unique biopolymer. In this review we aim at recapitulating the recent gelling concepts for developing chitosan-based physical gels. Specifically, we describe how nowadays it is relatively simple to prepare networks endowed with different sizes and shapes simply by exploiting physical interactions, namely (i) hydrophobic effects and hydrogen bonds—mostly governed by chitosan chemical composition—and (ii) electrostatic interactions, mainly ensured by physical/chemical chitosan features, such as the degree of acetylation and molecular weight, and external parameters, such as pH and ionic strength. Particular emphasis is dedicated to potential applications of this set of materials, especially in tissue engineering and drug delivery sectors. Lastly, we report on chitosan derivatives and their ability to form gels. Additionally, we discuss the recent findings on a lactose-modified chitosan named Chitlac, which has proved to form attractive gels both at the macro- and at the nano-scale.

Macro- and microscale structure formation and aging in different arrested states of Laponite dispersions

Structure formation and aging in different arrested states of Laponite dispersions have been investigated at the macro- and microscale. Covering a wide range of solid content and salt concentrations at different pH glasses, strong and weak gels with prevailing edge-to-face (EF) or face-to-face (FF) layer contacts were formed. Mechanical shear and squeeze flow rheometry were combined with diffusing wave spectroscopy and multiple particle tracking (MPT) microrheology. Strong attractive gels form much more quickly than weak gels, and particularly repulsive glasses. Gels with preferred EF contacts are stronger and are created more quickly than gels with prevailing FF contacts. Strong gels show little aging and exhibit a weak increase of G′ ∼ tα with α = 0.11 ± 0.03, higher α values are found for weak gels, and the strongest aging is observed in glasses. MPT data reveal structural refinement at the submicrometer length scale during aging for gels but not for glasses. Strong structural heterogeneity most pronounced at pH = 8.5 occurs during gel or glass formation, but at longer times, all arrested states appear homogenous at the 0.2 μm length scale. Finally, all arrested states exhibit power law frequency dependence G″ ∼ ω0.75 at high frequencies attributed to internal bending modes of layers.Structure formation and aging in different arrested states of Laponite dispersions have been investigated at the macro- and microscale. Covering a wide range of solid content and salt concentrations at different pH glasses, strong and weak gels with prevailing edge-to-face (EF) or face-to-face (FF) layer contacts were formed. Mechanical shear and squeeze flow rheometry were combined with diffusing wave spectroscopy and multiple particle tracking (MPT) microrheology. Strong attractive gels form much more quickly than weak gels, and particularly repulsive glasses. Gels with preferred EF contacts are stronger and are created more quickly than gels with prevailing FF contacts. Strong gels show little aging and exhibit a weak increase of G′ ∼ tα with α = 0.11 ± 0.03, higher α values are found for weak gels, and the strongest aging is observed in glasses. MPT data reveal structural refinement at the submicrometer length scale during aging for gels but not for glasses. Strong structural heterogeneity most prono...

Thermal rheology and microstructure of shear thickening suspensions of silica nanoparticles dispersed in the ionic liquid [C4mim][BF4

A stable dispersion of silica nanoparticles in a common ionic liquid [C4mim][BF4] that exhibits significant shear thickening is formulated by controlling the strength of hydrogen bonding between the nanoparticle surface and the anion of [C4mim][BF4]. Colloidal stability is demonstrated to be due to the formation of solvation layers, the properties of which are determined by scattering measurements. Time-temperature superposition measurements and the onset of yielding behavior in steady shear indicate the increase of particle-particle attraction and loss of stability of colloidal dispersions above 30 °C. Small angle neutron scattering and dynamic light scattering measurements confirm the reduction in solvation layering at elevated temperatures, leading to a transition from stable dispersion to unstable, attractive gel with increasing temperature (i.e., an inverse melting transition). It is demonstrated how controlling specific surface-anion interactions through chemical functionality can be used to formula...

Suppression of the coffee-ring effect in a hectorite aqueous dispersion

The drying patterns of droplets of hectorite aqueous dispersions were studied. The hectorite volume fractions and salt concentrations of the dispersions ranged from 0.001 to 0.01 and from 0.001M to 0.1M, respectively. It was observed that the drying pattern was ring shaped in the absence of salt and bump shaped in the presence of salt. It is highly plausible that, in the presence of a salt, a three-dimensional network structure of hectorites (an attractive gel) is formed and the hectorites are arrested by an electrostatic attractive force among them, which leads to compression of the network structure in the drying process, resulting in the bump-shaped deposit. It was revealed that the dispersion state, “sol” or “gel,” of the hectorite mineral in the water dominates the drying pattern of the hectorite dispersion and offers a new method for suppression of the coffee-ring effect.

Rheology of Laponite-scleroglucan hydrogels

Both Laponite and scleroglucan can find several applications in various fields (from industrial to biomedical one) in virtue of their peculiar features and rheological properties displayed in aqueous phases. Structural states of Laponite dispersions strongly depend on concentration and ionic strength. When attractive and repulsive interparticle interactions are so effective that they lead to arrested states (attractive gel or repulsive glass), the rheological behavior of the dispersion undergoes a sharp transition, from quasi-Newtonian to markedly shear thinning and viscoelastic. Conversely, scleroglucan solutions gradually change to weak gels with increasing polymer concentration. The present work is concerned with aqueous Laponite-scleroglucan mixed systems, obtained according to different preparation modes, and is aimed at examining how much the content and proportion of both components affect the viscoelastic and flow properties of the mixed system.

Phase Behavior of Aqueous Suspension of Laponite: New Insights with Microscopic Evidence.

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 wt %) 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 of the rheological response leads to fractal dimension that primarily depends on the Laponite concentration. We also obtain the activation energy for gelation, which is observed to decrease with increase in Laponite as well as salt concentration. Significantly, the cryo-TEM images of the postgel state clearly show the presence of a percolated network formed by interparticle bonds. The present work therefore conclusively establishes the system to be in an attractive gel state resolving a long-standing debate in the literature.

Microstructural Rearrangements and their Rheological Implications in a Model Thixotropic Elastoviscoplastic Fluid.

We identify the sequence of microstructural changes that characterize the evolution of an attractive particulate gel under flow and discuss their implications on macroscopic rheology. Dissipative particle dynamics is used to monitor shear-driven evolution of a fabric tensor constructed from the ensemble spatial configuration of individual attractive constituents within the gel. By decomposing this tensor into isotropic and nonisotropic components we show that the average coordination number correlates directly with the flow curve of the shear stress versus shear rate, consistent with theoretical predictions for attractive systems. We show that the evolution in nonisotropic local particle rearrangements are primarily responsible for stress overshoots (strain-hardening) at the inception of steady shear flow and also lead, at larger times and longer scales, to microstructural localization phenomena such as shear banding flow-induced structure formation in the vorticity direction.

Rheological reversibility and long-term stability of repulsive and attractive nanoemulsion gels

The storage modulus (G′) of a canola oil nanoemulsion gel depends on the storage time and SDS emulsifier concentration.

Structural fingerprints of yielding mechanisms in attractive colloidal gels.

Core-Modified Dissipative Particle Dynamics (CM-DPD) with a modified depletion potential and full hydrodynamics description is used to study non-equilibrium properties of colloidal gels with short range attraction potentials at an intermediate volume fraction (ϕ = 0.2) under start-up shear deformation. Full structural and rheological analysis using the stress fabric tensor complemented by bond number and bond distribution evolution under flow reveals that similarly to dilute colloidal gels, flow-induced anisotropy and strain-induced stretching of bonds are present during the first yielding transition. Unlike in low volume fraction depletion gels however, a small fraction of bond dissociation is required to facilitate bond rotation at intermediate volume fractions. The strain at which structural stretching and anisotropy in bond distribution emerge coincides with the first maximum in the shear stress (first yielding transition). At higher strains, depending on flow strength, a second peak in stress signal appears which is attributed to the compaction and melting of clusters. In this work, for the first time we provide evidence that multibody hydrodynamic interactions are essential to predict the correct dynamics of depletion gels under flow, namely two-step yielding process.

Transition from glass- to gel-like states in clay at a liquid interface.

Colloidal clay in water suspensions are known to exhibit a multitude of bulk phases depending on initial colloidal concentration and ionic strength, and examples of this include repulsive Wigner colloidal glasses at low ionic strength and attractive gels at higher ionic strength due to screened electrostatic forces by the electrolyte. From confocal Raman microscopy combined with elasticity measurements, we infer that clay trapped at quasi two-dimensional interfaces between oil and water also exhibit confined glass-like or gel-like states. The results can be important for the preparation of particles stabilized colloidal emulsions or colloidal capsules, and a better understanding of this phenomenon may lead to new emulsion or encapsulation technologies.

Ionic strength control of sulfated cellulose nanocrystal suspension viscosity

The effect of added electrolyte on the viscosity behavior of 3–9 wt% cellulose nanocrystals (CNC) suspensions was investigated for three different grades of CNCs extracted from wood pulp with sulfuric acid at increasing hydrolysis temperature. The viscosity of aqueous CNC suspensions decreases significantly when small amounts of electrolyte are added because of a reduction in electroviscous effects caused by compression of the electrical double layer surrounding the particles. As the ionic strength increases further, the suspension viscosity reaches a minimum and then increases again with the formation of an attractive gel.

Mechanics at the glass-to-gel transition of thermoresponsive microgel suspensions.

We study the rheology of systems of thermoresponsive microgels which can transition between a repulsive glass and an attractive gel state. We find marked differences between these two colloidal solids, within the same experimental system, due to the different origins for their dynamic arrest. While the rigidity of the repulsive systems depends solely on particle volume fraction, we find that the change in linear elasticity upon introducing attractive bonds in the system scales linearly with the adhesive bond strength which can be tuned with the temperature in our experiments. And while the glasses yield reversibly and with a rate-dependent energy dissipation, bond-reorganisation in the gels is suppressed so that their rupture is irreversible and accompanied by a high, but rate-independent, dissipation. These results highlight how colloids with responsive interactions can be employed to shed new light onto solid-solid transitions.

Behaviour of LAPONITE® gels: Rheology, ageing, pH effect and phase state in the presence of dispersant

The zeta potential result showed that the nano-discotic LAPONITE® particles in dispersion were negatively charged across the whole pH range of 3–12. For the 3 wt% gel, no dispersed region was observed within this pH range. The yield stress decreased with increasing pH from 8 to 12. Particle agglomeration affected the yield stress result at low pH. Addition of 6 dwb% pyrophosphate peptizer increased the particle negative zeta potential and caused the 3 wt% gel to be completely dispersed at pH greater than 7. Again the yield stress increased with decreasing pH from 7 to 5. The critical volume fraction equation for the repulsive gel state or EDL caging effect to operate was found to be very useful for determining the phase state of the LAPONITE® gel by the method of exclusion. A liquid sol state was deduced for the 0.45 and 0.89 wt% LAPONITE® dispersions having conductivities equivalent to 0.0015 and 0.003M KCl respectively. An attractive gel state was deduced for the 1.8 dwb% P2O74− shear-thinning LAPONITE® gels with concentration ranging from 1.5 to 5.5 wt% solids. The sol state was attributed to the Newtonian dispersions containing 1.1 to 1.5 wt% solids with 1.8 dwb% P2O74−. The structural recovery behaviour of 3 wt% LAPONITE® gels with 0.001 and 0.01M KNO3, was well described by the two-parameter model of Rich et al. (2011b) and the three-parameter Leong model. The yield stress of the aged gel decreased with increasing temperature which contradicted a theoretical prediction.

Two-step yielding in surfactant suspension pastes

In this work, we investigate two-step yielding behavior of pastes containing anionic surfactants, clay, and abrasive particles of calcite. Such pastes are commercially used as hand dishwash paste. The surfactant suspension pastes have soft solid-like consistency with finite elastic modulus. The total volume fraction of the particulate matter in the same is around 36 %; consequently, we attribute the elastic modulus of the same to the bonding between abrasive particles leading to attractive gel phase. We observe that the surfactant suspension pastes undergo physical aging during which evolution of relaxation time takes place. In dynamic strain sweep experiments, the pastes demonstrate clear signature of two-step yielding. It is observed that during the first yielding event, elastic modulus decreases while viscous modulus shows a maximum. For the second yielding event, however, elastic modulus demonstrates plateau before decreasing. While the first yielding event is observed to remain unaffected by the frequency of oscillations, the plateau values of elastic modulus, before the second yielding process, is observed to increase with frequency. We attribute the first yielding event in surfactant suspension pastes to rupture of network, while the second yielding event to breakage of the aggregates.

Surface chemistry and rheology of Laponite dispersions — Zeta potential, yield stress, ageing, fractal dimension and pyrophosphate

The ageing behaviour of Laponite gels at the phase boundary of attractive gel and flocculated state (ionic strength~0.01M 1:1 electrolyte) was investigated due to a lack of study in this region. Zeta potential and yield stress measurement revealed that freshly prepared Laponite dispersion took time to reach the surface chemical equilibrium (SCE) state. The higher the ionic strength, the shorter was the time needed. This well-defined structural state was employed at the commencement of ageing study. The ageing behaviour was characterised by a rapidly increasing yield stress region followed by a gradually increasing region and then by a plateau region. Both the initial and fully aged yield stress of Laponite gels increased with ionic strength and solid loading. Both the Leong and the two-parameter logarithmic time models described the ageing behaviour quite well. The relationship between the yield stress at the SCE state and at the fully recovered or rejuvenated state, and volume fraction obeyed a power law model. The fractal dimension of the two states was the same 2.0. This study also investigated the yield stress-pH behaviour of Laponite gel with and without pyrophosphate additive. Pure Laponite dispersion displayed a maximum yield stress at high pH. A drop in yield stress occurred at low pH region and eventually approaching zero yield stress. A totally opposite trend was observed with pyrophosphate additive. No yield stress was detected at high pH region and a maximum yield stress was located at pH5 followed by a drop in yield stress until pH2. This drop in the yield stress regardless of the presence of pyrophosphate, was due to the particle agglomeration promoted by low pH.

Avalanche-like fluidization of a non-Brownian particle gel.

We report on the fluidization dynamics of an attractive gel composed of non-Brownian particles made of fused silica colloids. Extensive rheology coupled to ultrasonic velocimetry allows us to characterize the global stress response together with the local dynamics of the gel during shear startup experiments. In practice, after being rejuvenated by a preshear, the gel is left to age for a time tw before being subjected to a constant shear rate [small gamma, Greek, dot above]. We investigate in detail the effects of both tw and [small gamma, Greek, dot above] on the fluidization dynamics and build a detailed state diagram of the gel response to shear startup flows. The gel may display either transient shear banding towards complete fluidization or steady-state shear banding. In the former case, we unravel that the progressive fluidization occurs by successive steps that appear as peaks on the global stress relaxation signal. Flow imaging reveals that the shear band grows until complete fluidization of the material by sudden avalanche-like events which are distributed heterogeneously along the vorticity direction and correlated to large peaks in the slip velocity at the moving wall. These features are robust over a wide range of tw and [small gamma, Greek, dot above] values, although the very details of the fluidization scenario vary with [small gamma, Greek, dot above]. Finally, the critical shear rate [small gamma, Greek, dot above]* that separates steady-state shear-banding from steady-state homogeneous flow depends on the width of the shear cell and exhibits a nonlinear dependence with tw. Our work brings about valuable experimental data on transient flows of attractive dispersions, highlighting the subtle interplay between shear, wall slip and aging whose modeling constitutes a major challenge that has not been met yet.