Tribological and Rheological Study of Thixotropic Gels of 2D Nanoparticles.

A thixotropic colloidal gel constituting an aqueous dispersion of synthetic clay Laponite with varying concentrations of salt has been studied for its rheological and tribological performance as a lubricant. We observed that the incorporation of NaCl induces notable enhancements in the colloidal gel's relaxation time, elastic modulus, and yield stress. Although an increase in NaCl concentration decreases the material's relaxation time dependence on waiting time (tw), overall, the strength of its thixotropic character has been observed to increase with an increase in salt concentration. The analysis of friction and wear indicated that the utilization of a thixotropic colloidal gel of Laponite with a higher concentration of NaCl resulted in progressively greater reductions in both the coefficient of friction and specific wear rates under various load-speed conditions. Severe abrasive wear on disc surface under dry test, gradually mitigated upon the introduction of these lubricants. Two simultaneous lubricating mechanisms, first, the smooth sliding of the friction pair, facilitated by the alignment of Laponite particles in the direction of shear forces, and second, the stable structure of Laponite, coupled with the addition of NaCl, enabling continuous replenishment of the wear track with lubricant, are attributed to lubrication effectiveness.

Large amplitude oscillatory shear study of a colloidal gel near the critical state.

A system undergoing sol-gel transition passes through a unique point, known as the critical gel state, where it forms the weakest space spanning percolated network. We investigate the nonlinear viscoelastic behavior of a colloidal dispersion at the critical gel state using large amplitude oscillatory shear rheology. The colloidal gel at the critical point is subjected to oscillatory shear flow with increasing strain amplitude at different frequencies. We observe that the first harmonic of the elastic and viscous moduli exhibits a monotonic decrease as the material undergoes a linear to nonlinear transition. We analyze the stress waveform across this transition and obtain the nonlinear moduli and viscosity as a function of frequency and strain amplitude. The analysis of the nonlinear moduli and viscosities suggests intracycle strain stiffening and intracycle shear thinning in the colloidal dispersion. Based on the insights obtained from the nonlinear analysis, we propose a potential scenario of the microstructural changes occurring in the nonlinear region. We also develop an integral model using the time-strain separable Kaye-Bernstein-Kearsley-Zapas constitutive equation with a power-law relaxation modulus and damping function obtained from experiments. The proposed model with a slight adjustment of the damping function inferred using a spectral method, compares well with experimental data at all frequencies.

Advances and challenges in the high-pressure rheology of complex fluids.

Complex fluids and soft materials are ubiquitous in nature and industry. In industrial processes, these materials often get exposed to high hydrostatic pressures. Some examples include polymer melts, crude oils, gas hydrates, food systems, foams, motor oils, lubricants, etc. In spite of the relevance and utilization of hydrostatic pressure in many industrial applications, the role of pressure on the rheological properties has not been examined extensively in the literature. We review the high-pressure rheometric systems and present advantages and drawbacks of various kinds of rheometers such as capillary rheometer, sliding plate rheometer, falling ball viscometer, and rotational rheometer. By outlining the design complexities, precision, low-torque resolution limits and the inherent error sources of each type are critically evaluated. Furthermore, the high-pressure rheology data, chosen to cover a broad range of pressures and material class ranging from simple Newtonian fluids (incompressible), complex non-Newtonian fluids and compressible fluids featuring various key applications from different industries, are reviewed. The literature suggests, while effect of pressure on the rheological behavior is vital for many applications, compared to the effects of temperature on the rheological behavior, knowledge of the effect of pressure is still in its infancy.

Signatures of Overaging in an Aqueous Dispersion of Carbopol.

In this work, we study the effect of the deformation field on the physical aging behavior of an aqueous Carbopol dispersion. It is composed of soft swollen particles of gel that get deformed and acquire a polygonal shape, with flat interfaces rendering the dispersion a soft solid-like consistency as filled volume fraction approaches unity. It has been proposed that owing to release of stored elastic energy in the deformed particles, Carbopol dispersion undergoes microstructural evolution that is reminiscent of physical aging in soft glassy materials. We observe that application of moderate magnitude of oscillatory strain to Carbopol dispersion slows down its relaxation dynamics, thereby showing characteristics of overaging. On the other hand, the sufficiently high magnitude of strain makes the relaxation dynamics faster, causing rejuvenation. We also solve the soft glassy rheology model, which, when subjected to the same flow field, corroborates with experimental observations on the Carbopol dispersion. This behavior, therefore, suggests that in a system of jammed soft particles of Carbopol, the particles occupying shallow energy wells upon application of moderate strain field adjust themselves in such a manner that they predominantly occupy the deeper energy wells leading to observe the overaging dynamics.

Two step yielding in soft materials.

A review is presented on the topic of two-step yielding observed in complex fluids that cover a broad variety of materials ranging from colloidal gels, attractive glasses, emulsions, suspensions, and several commercial paste-like materials. The common features in various systems displaying two-step yielding behavior are the presence of two characteristic forces between the interacting particles or two varying representative length or time scales. This focused review aims to provide physical insights, mechanistic understanding of the two-step yielding and other associated rheological consequences of this nonlinear behavior. A discussion is provided on the microstructural details with an overview of different experimental systems exhibiting double-yielding studied so far highlighting the similarities and differences among them. Particularly, the effects of continuous phase properties, dispersed particle phase factors (size, shape, softness and surface charge) and external force field (electric, magnetic, thermal and shear flows) on two-step yielding are considered.

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.

Adhesion strength and viscoelastic properties of polydimethylsiloxane (PDMS) based elastomeric nanocomposites with embedded electrospun nanofibers.

In the present study, the adhesive and viscoelastic properties of polydimethylsiloxane (PDMS) based nanocomposite pressure sensitive adhesives (PSAs) with embedded electrospun polyacrylonitrile (PAN) and polyvinyl alcohol (PVA) nanofibers as fillers were investigated. PDMS nanocomposite adhesive films using PAN and PVA nanofibers were synthesized by dispersing fillers in the matrix by a solvent mixing process. The adhesion strength and reusability of the prepared nanocomposite PSA films were measured using peel tests as the fraction of nanofibers in the polymer matrix is increased. The variations of the adhesive properties of the PSAs as function of the type and loading of filler were related to their rheological properties in terms of shear and elastic moduli. Although 3-fold enhancement of the adhesion strength was achieved with 0.5 wt% loading for both types (PAN and PVA) of nanocomposites as compared to elastic PDMS, the composite adhesive with PAN nanofibers can provide a superior balance of rheological properties, resulting in improved reusability over other PSAs. The differences in the adhesion and viscoelastic properties of the composite PSAs are attributed to the polymer chemistry, processability, and architecture of the electrospun nanofibers in the soft PDMS matrix.

Rheological Behavior and in Situ Confocal Imaging of Bijels Made by Mixing.

Bijels (bicontinuous interfacially jammed emulsion gels) have the potential to be useful in many different applications due to their internal connectivity and the possibility of efficient mass transport through the channels. Recently, new methods of making the bijel have been proposed, which simplify the fabrication process, making commercial application more realistic. Here, we study the flow properties of bijels prepared by mixing alone using oscillatory rheology combined with confocal microscopy and also squeezing flow experiments. We found that the bijel undergoes a two-step yielding process where the first step corresponds to the fluidizing of the interface, allowing the motion of the structure, and the second step corresponds to the breaking of the structure. In the squeeze flow experiments, the yield stress of the bijel is observed to show a power law dependence on squeezing speed. However, when stress in excess of yield stress is plotted against shear rate, all the different squeeze flow data show a superposition.

Three-dimensional yielding in anisotropic materials: validation of Hill's criterion.

Yielding transition in isotropic soft materials under the superposition of orthogonal deformation fields is known to follow von Mises' criterion. However, in anisotropic soft materials, von Mises' criterion fails owing to the preferred directions associated with the system. In this work we study a model anisotropic yield stress system: electrorheological (ER) fluids which show structural formation in the direction of an electric field. We subject the ER fluids to the superposition of orthogonal stress fields which leads to different yield stress values. We obtain a yielding state diagram by plotting the normalized rotational shear stress against the normalized radial shear stress corresponding to a yield point for a given electric field. Remarkably, the state diagram validates the Hill yielding criterion, which is a general yielding criterion for materials with anisotropy along three orthogonal directions, originally developed for metallic systems. Validation of Hill's criterion suggests the universality of its application in anisotropic systems including conventional anisotropic soft materials having yield stress.

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.

To study surface and sub-surface nanomechanical properties of electrospun polyacrylonitrile (PAN) nanofibers/polydimethylsiloxane (PDMS) composites.

We investigate surface and sub-surface nanomechanical properties of nanocomposites based on a crosslinked polydimethylsiloxane (PDMS) elastomer and electrospun polyacrylonitrile (PAN) nanofibers. Fabrication of PDMS substrates with anisotropy with respect to surface elasticity and their characterization in terms of local nanomechanical properties are important for many areas of adhesion applications. PDMS nanocomposite substrates with variations in surface elasticity over large areas are prepared by controllably embedding electrospun PAN nanofibers (∼600 nm) in a PDMS matrix using the solution casting technique. Variations of local surface stiffness properties of prepared composites are measured using force spectroscopy and force mapping modes of atomic force microscopy and compared with their macroscopic (bulk) mechanical properties. Since the surface of the prepared nanocomposite is elastically non-homogeneous, our studies are mainly focused on the investigation of the hysteresis (plasticity index) between loading and unloading curves which is a measure of energy dissipation in AFM indentation experiments. The distribution of the local plasticity index in the PAN/PDMS composites is related to the specific organization of electrospun nanofibers at the surface and sub-surface layers of the PDMS matrix. We observed that embedding 0.1-1% PAN nanofibers induces anti-plasticization effects for lower (0.1%) and higher (1%) concentrations of PAN nanofibers which represent the formation of interpenetrating networks and mat-like blended structures of PAN nanofibers within the PDMS matrix.

Making Nonsticky Surfaces of Sticky Materials: Self-Organized Microtexturing of Viscoelastic Elastomeric Layers by Tearing.

Fabrication of large area, multiscale microtextured surfaces engineered for antiadhesion properties remains a challenge. Compared to an elastic surface, viscoelastic solids show much higher surface stickiness, tack, and adhesion owing to the increased contact area and energy dissipation. Here, we show a simple, low cost, large-area and high throughput method with roll-to-roll compatibility to fabricate multiscale, rough microstructures resistant to adhesion in a viscoelastic layer by controlled tearing of viscous film. Even a high adhesive strength viscoelastic solid layer, such as partially cured PDMS, is made nonsticky simply by its controlled tearing. The torn surface shows a fracture induced, self-organized leaflike micropattern resistant to sticking. The topography and adhesion strength of these structures are readily tuned by changing the tearing speed and the film thickness. The microtexture displays a springlike recovery, low adhesive strength, and easy release properties even under the high applied loads.

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.

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 aging under oscillatory strain field through the soft glassy rheology model.

In this work, we solve the Soft Glassy Rheology (SGR) model under application of oscillatory deformation field with varying magnitudes of strain as well as frequency for different noise temperatures. In the glassy domain, the SGR model undergoes time evolution of elastic modulus. Increase in strain magnitude beyond the linear regime is observed to enhance the rate of aging as manifested by a faster evolution of elastic modulus with increase in strain amplitude due to overaging. However at higher strain magnitudes, the rejuvenation effect starts dominating over the aging, thereby reducing the rate at which elastic modulus evolves. We also plot the aging phase diagram describing an occurrence of the linear, the overaging, and the rejuvenation regimes as a function of strain and frequency for different noise temperatures. The aging phase diagram suggests that while the linear regime remains unaffected by the changes in frequency and noise temperature, the width of the overaging regime increases with increase in frequency and noise temperature. We also study the time evolution of the shapes of relaxation time spectra as a function of strain amplitude, which renders further insight into the overaging and the rejuvenation behavior. While the phenomenon of overaging is observed to be an inherent character of the SGR model, experimentally not all the materials demonstrate overaging. Such a discrepancy suggests that the energy well depths before and after a yielding event may not be completely uncorrelated as assumed in the SGR formalism.