Structure of propagating high-stress fronts in a shear-thickening suspension.

We report direct measurements of spatially resolved stress at the boundary of a shear-thickening cornstarch suspension revealing persistent regions of high local stress propagating in the flow direction at the speed of the top boundary. The persistence of these propagating fronts enables precise measurements of their structure, including the profile of boundary stress measured by boundary stress microscopy (BSM) and the nonaffine velocity of particles at the bottom boundary of the suspension measured by particle image velocimetry (PIV). In addition, we directly measure the relative flow between the particle phase and the suspending fluid (fluid migration) and find the migration is highly localized to the fronts and changes direction across the front, indicating that the fronts are composed of a localized region of high dilatant pressure and low particle concentration. The magnitude of the flow indicates that the pore pressure difference driving the fluid migration is comparable to the critical shear stress for the onset of shear thickening. The propagating fronts fully account for the increase in viscosity with applied stress reported by the rheometer and are consistent with the existence of a stable jammed region in contact with one boundary of the system that generates a propagating network of percolated frictional contacts spanning the gap between the rheometer plates and producing strong localized dilatant pressure.

Dynamics and memory of boundary stresses in discontinuous shear thickening suspensions during oscillatory shear.

We report direct measurements of spatially resolved surface stresses of a dense suspension during large amplitude oscillatory shear (LAOS) in the discontinuous shear thickening regime using boundary stress microscopy. Consistent with previous studies, bulk rheology shows a dramatic increase in the complex viscosity above a frequency-dependent critical strain. We find that the viscosity increase is coincident with that appearance of large heterogeneous boundary stresses, indicative of the formation of transient solid-like phases (SLPs) on spatial scales large compared to the particle size. The critical strain for the appearance of SLPs is largely determined by the peak oscillatory stress, which depends on the peak shear rate and the frequency-dependent suspension viscosity. The SLPs dissipate and reform on each cycle, with a spatial pattern that is highly variable at low frequencies but remarkably persistent at the highest frequency measured (ω = 10 rad s-1).

Shear-Induced Gelation of Self-Yielding Active Networks.

An enticing feature of active materials is the possibility of controlling macroscale rheological properties through the activity of the microscopic constituents. Using a unique combination of microscopy and rheology we study three dimensional microtubule-based active materials whose autonomous flows are powered by a continually rearranging connected network. We quantify the relationship between the microscopic dynamics and the bulk mechanical properties of these nonequilibrium networks. Experiments reveal a surprising nonmonotonic viscosity that strongly depends on the relative magnitude of the rate of internally generated activity and the externally applied shear. A simple two-state mechanical model that accounts for both the solidlike and yielded fluidlike elements of the network accurately describes the rheological measurements.

Role of particle orientational order during shear thickening in suspensions of colloidal rods.

Rheology of dense anisotropic colloidal suspensions often exhibits unsteady flow at constant imposed shear stress and/or shear rate. Using simultaneous high-resolution confocal microscopy and rheology, we find that the temporal behavior arises due to a strong coupling between shear flow and particle orientation. At smaller applied stresses, the orientation of rods fluctuates around the flow direction. A transition to an intermittent disordered state is observed at higher stresses when the angle between the flow and the rod orientation reaches a critical value. This disordered state is associated with transient drop in shear rate and an increase in viscosity. Simultaneous visualization of boundary stresses and orientation shows that the disordered regions lead to heterogeneous stresses and positive normal forces at the boundary, indicating the formation of systems spanning disordered particle contact networks.

Contact Networks Enhance Shear Thickening in Attractive Colloid-Polymer Mixtures.

Increased shear thinning arising due to strong attractive interactions between colloidal particles is thought to obscure shear thickening. Here, we demonstrate how moderate attractions, induced by adding a nonadsorbing polymer, can instead enhance shear thickening. We measure the rheology of colloidal suspensions at a constant particle volume fraction of ϕ=0.40 with dilute to weakly semidilute concentrations of three polyacrylamide depletants of different molecular weights. Suspensions containing large polymer exhibit increased shear thickening and positive first normal stress differences at high shear stress, and increased heterogeneous fluctuations in the boundary stress. These results are consistent with a friction-based model for shear thickening, suggesting that the presence of large, extended polymers induces the formation of near-spanning networks of interparticle contacts.

Silk Molecular Weight Influences the Kinetics of Enzymatically Cross-linked Silk Hydrogel Formation.

We transform reconstituted silk solutions into robust hydrogels through covalent dityrosine cross-linking resulting from an enzymatic reaction. The bulk rheological properties and the covalent dityrosine bond formation of these gels are measured during polymerization. We compare the time-resolved bond formation to the mechanical properties, where we find that the gelation process is consistent with a model of percolation. The molecular weight of the protein determines whether a secondary mode of growth postpercolation exists, indicating that molecular weight changes affect the mechanisms by which these gels polymerize.

Rate Dependence of Elementary Rearrangements and Spatiotemporal Correlations in the 3D Flow of Soft Solids.

We use a combination of confocal microscopy, rheology, and molecular dynamics simulations to investigate jammed emulsions under shear, by analyzing the 3D droplets rearrangements in the shear frame. Our quantitative analysis of local dynamics reveals elementary nonaffine rearrangements that underlie the onset of the flow at small strains. We find that the mechanism of unjamming and the upturn in the material flow curve are associated to a qualitative change in spatiotemporal correlations of such rearrangements with the applied shear rate. At high shear rates, droplet clusters follow coordinated, stringlike motion. Conversely, at low shear rates, the elementary nonaffine rearrangements exhibit longer-ranged correlations, with complex spatiotemporal patterns. The 3D microscopic details provide novel insights into the specific features of the material flow curve, common to a large class of technologically relevant soft disordered solids and new fundamental ingredients for constitutive models.

Acid-induced assembly of a reconstituted silk protein system.

Silk cocoons are reconstituted into an aqueous suspension, and protein stability is investigated by comparing the protein's response to hydrochloric acid and sodium chloride. Aggregation occurs for systems mixed with hydrochloric acid, while sodium chloride over the same range of concentrations does not cause aggregation. We measure the structures present on the protein and aggregate length scales in these solutions using both optical and small-angle neutron scattering, while mass spectrometry techniques shed light on a possible mechanism for aggregate formation. We find that the introduction of acid modulates the aggregate size and pervaded volume of the protein, an effect that is not observed with salt.

Localized stress fluctuations drive shear thickening in dense suspensions.

Dense particulate suspensions exhibit a dramatic increase in average viscosity above a critical, material-dependent shear stress. This thickening changes from continuous to discontinuous as the concentration is increased. Using direct measurements of spatially resolved surface stresses in the continuous thickening regime, we report the existence of clearly defined dynamic localized regions of substantially increased stress that appear intermittently at stresses above the critical stress. With increasing applied stress, these regions occupy an increasing fraction of the system, and the increase accounts quantitatively for the observed shear thickening. The regions represent high-viscosity fluid phases, with a size determined by the distance between the shearing surfaces and a viscosity that is nearly independent of shear rate but that increases rapidly with concentration. Thus, we find that continuous shear thickening arises from increasingly frequent localized discontinuous transitions between distinct fluid phases with widely differing viscosities.

Rheological Signature of Frictional Interactions in Shear Thickening Suspensions.

Colloidal shear thickening presents a significant challenge because the macroscopic rheology becomes increasingly controlled by the microscopic details of short ranged particle interactions in the shear thickening regime. Our measurements here of the first normal stress difference over a wide range of particle volume fractions elucidate the relative contributions from hydrodynamic lubrication and frictional contact forces, which have been debated. At moderate volume fractions we find N_{1}<0, consistent with hydrodynamic models; however, at higher volume fractions and shear stresses these models break down and we instead observe dilation (N_{1}>0), indicating frictional contact networks. Remarkably, there is no signature of this transition in the viscosity; instead, this change in the sign of N_{1} occurs while the shear thickening remains continuous. These results suggest a scenario where shear thickening is driven primarily by the formation of frictional contacts, with hydrodynamic forces playing a supporting role at lower concentrations. Motivated by this picture, we introduce a simple model that combines these frictional and hydrodynamic contributions and accurately fits the measured viscosity over a wide range of particle volume fractions and shear stress.

Silk Fibroin Degradation Related to Rheological and Mechanical Properties.

Regenerated silk fibroin has been proposed as a material substrate for biomedical, optical, and electronic applications. Preparation of the silk fibroin solution requires extraction (degumming) to remove contaminants, but results in the degradation of the fibroin protein. Here, a mechanism of fibroin degradation is proposed and the molecular weight and polydispersity is characterized as a function of extraction time. Rheological analysis reveals significant changes in the viscosity of samples while mechanical characterization of cast and drawn films shows increased moduli, extensibility, and strength upon drawing. Fifteen minutes extraction time results in degraded fibroin that generates the strongest films. Structural analysis by wide angle X-ray scattering (WAXS) and Fourier transform infrared spectroscopy (FTIR) indicates molecular alignment in the drawn films and shows that the drawing process converts amorphous films into the crystalline, ß-sheet, secondary structure. Most interesting, by using selected extraction times, films with near-native crystallinity, alignment, and molecular weight can be achieved; yet maximal mechanical properties for the films from regenerated silk fibroin solutions are found with solutions subjected to some degree of degradation. These results suggest that the regenerated solutions and the film casting and drawing processes introduce more complexity than native spinning processes.

Rheology and dynamics of colloidal superballs.

Recent advances in colloidal synthesis make it possible to generate a wide array of precisely controlled, non-spherical particles. This provides a unique opportunity to probe the role that particle shape plays in the dynamics of colloidal suspensions, particularly at higher volume fractions, where particle interactions are important. We examine the role of particle shape by characterizing both the bulk rheology and micro-scale diffusion in a suspension of pseudo-cubic silica superballs. Working with these well-characterized shaped colloids, we can disentangle shape effects in the hydrodynamics of isolated particles from shape-mediated particle interactions. We find that the hydrodynamic properties of isolated superballs are marginally different from comparably sized hard spheres. However, shape-mediated interactions modify the suspension microstructure, leading to significant differences in the self-diffusion of the superballs. While this excluded volume interaction can be captured with a rescaling of the superball volume fraction, we observe qualitative differences in the shear thickening behavior of moderately concentrated superball suspensions that defy simple rescaling onto hard sphere results. This study helps to define the unknowns associated with the effects of shape on the rheology and dynamics of colloidal solutions.

Stress heterogeneities in sheared type-I collagen networks revealed by Boundary Stress Microscopy.

Disordered fiber networks provide structural support to a wide range of important materials, and the combination of spatial and dynamic complexity may produce large inhomogeneities in mechanical properties, an effect that is largely unexplored experimentally. In this work, we introduce Boundary Stress Microscopy to quantify the non-uniform surface stresses in sheared collagen gels. We find local stresses exceeding average stresses by an order of magnitude, with variations over length scales much larger than the network mesh size. The strain stiffening behavior observed over a wide range of network mesh sizes can be parameterized by a single characteristic strain and associated stress, which describes both the strain stiffening regime and network yielding. The characteristic stress is approximately proportional to network density, but the peak boundary stress at both the characteristic strain and at yielding are remarkably insensitive to concentration.

Rheology of peptide- and protein-based physical hydrogels: are everyday measurements just scratching the surface?

Rheological characterization of physically crosslinked peptide- and protein-based hydrogels is widely reported in the literature. In this review, we focus on solid injectable hydrogels, which are commonly referred to as 'shear-thinning and rehealing' materials. This class of what sometimes also are called 'yield-stress' materials holds exciting promise for biomedical applications that require well-defined morphological and mechanical properties after delivery to a desired site through a shearing process (e.g., syringe or catheter injection). In addition to the review of recent studies using common rheometric measurements on peptide- and protein-based, physically crosslinked hydrogels, we provide experimentally obtained visual evidence, using a rheo-confocal microscope, of the fracture and subsequent flow of physically crosslinked ß-hairpin peptide hydrogels under steady-state shear mimicking commonly conducted experimental conditions using bench-top rheometers. The observed fracture demonstrates that the supposed bulk shear-thinning and rehealing behavior of physical gels can be limited to the yielding of a hydrogel layer close to the shearing surface with the bulk of the hydrogel below experiencing negligible shear. We suggest some measures to be taken while acquiring and interpreting data using bench-top rheometers with a particular focus on physical hydrogels. In particular, the use of confocal-rheometer assembly is intended to inspire studies on yielding behavior of hydrogels perceived as shear-thinning and rehealing materials. A deeper insight into their yielding behavior will lead to the development of yield-stress, injectable, solid biomaterials, and hopefully inspire the design of new shear-thinning and rehealing hydrogels and more thorough physical characterization of such systems. Finally, more examples of bulk fracture in some physical hydrogels based on peptides and proteins are explored in the light of their behavior as yield-stress materials.

Rheology of reconstituted silk fibroin protein gels: the epitome of extreme mechanics.

In nature, silk fibroin proteins assemble into hierarchical structures with dramatic mechanical properties. With the hope of creating new classes of on demand silk-based biomaterials, Bombyx mori silk is reconstituted back into stable aqueous solutions that can be reassembled into functionalized materials; one strategy for reassembly is electrogelation. Electrogels (e-gels) are particularly versatile and can be produced using electrolysis with small DC electric fields. We characterize the linear and nonlinear rheological behavior of e-gels to provide fundamental insights into these distinct protein-based materials. We observe that e-gels form robust biopolymer networks that exhibit distinctive strain hardening and are recoverable from strains as large as γ=27, i.e. 2700%. We propose a simple microscopic model that is consistent with local restructuring of single proteins within the e-gel network.

Shear-driven aggregation of SU-8 microrods in suspension.

A non-Brownian suspension of micron scale rods exhibits reversible shear-driven formation of disordered aggregates resulting in dramatic viscosity enhancement at low shear rates. Aggregate formation is imaged using a combined rheometer and fluorescence microscope. The size and structure of these aggregates are found to be a function of shear rate and concentration, with larger aggregates present at lower shear rates and higher concentrations. Quantitative measurements of the early-stage aggregation process are modeled by collision driven growth of porous structures which suggest that the aggregate density increases with shear rate. This result is combined with a Krieger-Dougherty type constitutive relationship and steady-state viscosity measurements to estimate the intrinsic viscosity of complex structures developed under shear. These results represent a direct, quantitative, experimental demonstration of the association between aggregation and viscosity enhancement for a rod suspension, and demonstrate a way of inferring microscopic geometric properties of a dynamic system through the combination of quantitative imaging and rheology.

Development of a confocal rheometer for soft and biological materials.

We discuss the design and operation of a confocal rheometer, formed by integrating an Anton Paar MCR301 stress-controlled rheometer with a Leica SP5 laser scanning confocal microscope. Combining two commercial instruments results in a system which is straightforward to assemble that preserves the performance of each component with virtually no impact on the precision of either device. The instruments are configured so that the microscope can acquire time-resolved, three-dimensional volumes of a sample whose bulk viscoelastic properties are being measured simultaneously. We describe several aspects of the design and, to demonstrate the system's capabilities, present the results of a few common measurements in the study of soft materials.

Size-dependent rheology of type-I collagen networks.

We investigate the system size-dependent rheological response of branched type I collagen gels. When subjected to a shear strain, the highly interconnected mesh dynamically reorients, resulting in overall stiffening of the network. When a continuous shear strain is applied to a collagen network, we observe that the local apparent modulus, in the strain-stiffening regime, is strongly dependent on the gel thickness. In addition, we demonstrate that the overall network failure is determined by the ratio of the gel thickness to the mesh size. These findings have broad implications for cell-matrix interactions, the interpretation of rheological tissue data, and the engineering of biomimetic scaffolds.

Robust organogels from nitrogen-containing derivatives of (R)-12-hydroxystearic acid as gelators: comparisons with gels from stearic acid derivatives.

Thirteen members of a new class of low molecular-mass organogelators (LMOGs), amides, and amines based on (R)-12-hydroxystearic acid (HSA; i.e., (R)-12-hydroxyoctadecanoic acid) and the properties of their gels have been investigated by a variety of structural and thermal techniques. The abilities of these LMOGs, molecules with primary and secondary amide and amine groups and the ammonium carbamate salt of 1-aminooctadecan-12-ol, to gelate a wide range of organic liquids have been ascertained. Their gelating efficiencies are compared with those of HSA and the corresponding nitrogen-containing molecules derived from stearic acid (i.e., HSA that lacks a 12-hydroxyl group). Several of the HSA-derived molecules are exceedingly efficient LMOGs, with much less than 1 wt % being necessary to gelate several organic liquids at room temperature. Generally, the self-assembled fibrillar networks of the gels consist of spherulitic objects whose dimensions depend on the protocol employed to cool the precursor sol phases. X-ray studies indicate that the LMOG molecules are packed in lamellae within the fibers that constitute the spherulites. In addition, some of the organogels exhibit unusual thixotropic properties: they recover a large part of their viscoelasticity within seconds of being destroyed by excessive strain shearing. This recovery is at least an order of magnitude faster than for any other organogel with a crystalline fibrillar network reported to date. Correlations of these LMOG structures (as well as with those that lack a hydroxyl group along the n-alkyl chain, a headgroup at its end, or both) with the properties of their gels, coupled with the unusual theological properties of these systems, point to new directions for designing LMOGs and organogels.