Parallel superposition flows have been examined in the rheology literature since 1965; they consist of simultaneously applied steady shear and oscillatory flows. One interesting result found from the study of parallel superposition flows has been reportedly negative values of the dynamic moduli. This apparently unphysical result has led to some speculation regarding its origin. In this work, we provide mathematical and experimental evidence showing that negative moduli are potentially due to the oscillatory signal being distorted by inclusion of the steady shear component in the Fourier analysis. However, the moduli can be easily corrected to restore physical meaning.

Bacterial biofilms protected by viscoelastic extracellular polymeric substances (EPS) are highly resistant to chemical disinfectants and rapidly regenerate after treatment. While bubble-mediated mechanical disruption has emerged as an eco-friendly antifouling strategy, bubbles generated by conventional tools act on biofilm surfaces and fail to disrupt three-dimensional biofilms. Here, we demonstrate that generating bubbles within biofilms, referred to as matrix bubbles, and controlling their dynamics with temperature, enables effective matrix disruption and biofilm removal. Using P. aeruginosa biofilms as a model system, we compared H2O2 alone with MnO2 nanocatalyst-doped biosilica microparticles (MnO2-biosilica) across a range of temperatures. H2O2 alone produced catalase-driven O2 bubbles localized on the biofilm surface with minimal temperature dependence, resulting in limited biofilm removal. In contrast, MnO2-biosilica generated temperature-amplified matrix bubbles that formed swarms, penetrated biofilms, disrupted EPS, and suppressed regrowth at elevated temperatures (25 and 40 °C). Kinetic and imaging analyses revealed that this temperature-dependent behavior arises from accelerated MnO2-catalyzed H2O2 decomposition coupled with enhanced bubble expansion and rupture, which deliver strong mechanical perturbation within the biofilm matrix. Importantly, nanocatalyst-induced matrix bubbles effectively removed biofilm from complex surgical instrument geometries and acted synergistically with autoclaving. This study therefore establishes temperature-controlled, nanocatalyst-mediated matrix bubble dynamics as a physical strategy for overcoming biofilm resistance in clinical and industrial settings.

ABSTRACT Treatment‐resistant wounds driven by polymicrobial biofilms are a major clinical challenge, affecting millions globally and leading to chronic inflammation, persistent pain, and poor healing outcomes. These wounds are characterized by mature biofilms reinforced by dense extracellular polymeric substances, which confer strong tolerance to conventional treatments. Despite emerging technologies, such as nanoparticles, bacteriophages, and engineered enzymes, effective clearance of established biofilms remains challenging. Here, we develop a microblasting wound dressing (µBLAST) that delivers spatially confined mechano‐chemical disruption at the tissue‐biofilm interface to remove viscoelastic biofilm matrices and promote tissue regeneration. The µBLAST is assembled by embedding MnO 2 ‐doped diatom biosilica beneath an H 2 O 2 ‐releasing cellulose mesh, enabling localized catalytic microbubble generation within biofilm matrices. Confined expansion and rupture of oxygen bubbles produce localized mechanical stress sufficient to dislodge mature, antibiotic‐resistant polymicrobial biofilms, while sustained H 2 O 2 release prolongs particle activity. In a murine wound model infected with mature P. aeruginosa and methicillin‐resistant S. aureus biofilms, µBLAST treatment significantly reduces biofilm burden, accelerates re‐epithelialization, promotes hair regrowth, and mitigates inflammation. Moreover, µBLAST enhances antibiotic efficacy, suppressing biofilm regrowth even at ten‐fold reduced drug doses. These findings highlight confined mechano‐chemical biofilm disruption as a therapeutic strategy for treating mature, antibiotic‐resistant biofilm infections and promoting tissue regeneration.

In soft materials, a clear relationship between material properties and human sensory perception has long been desired for design of consumer products, but the link has remained evasive. Favorable perception indicates that customers enjoy a product and are likely to continue using it or purchase it again. Perception is frequently measured subjectively by consumer test panels in terms of descriptive sensory words such as softness, smoothness, thickness, etc. that lack established scientific definitions. In this work, we move beyond ambiguous definitions and detail a method to objectively measure and quantify human-material interactions using a representative series of viscoelastic putties. We show that human behaviors have direct rheological meaning with features that are characterized using transient recovery rheology. The rheology scales logarithmically at perception-relevant timescales, akin to Fechner's law. Our work explains variability in user-reported perception and demonstrates a way to construct direct relationships between user behavior and measurable rheology.

Associative dynamic covalent networks combine the elastic properties of cross-linked thermosets with the viscous flow of thermoplastics, thus enabling reprocessability while maintaining network connectivity. In this work, we investigate the effect of catalyst mobility and concentration on the linear viscoelasticity of associative thiol-thioester dynamic covalent networks. We present a simple method to covalently tether nucleophiles capable of catalyzing thiol-thioester exchange reactions to polymer networks and compare their viscoelastic behavior to their analogous untethered catalyst. We first show that the cross-linking density and bond dynamics can be independently tuned using precise stoichiometric adjustments for both catalyst systems. Notably, we find that the mobility of the catalysts does not affect the bond dynamics, as validated by similar relaxation times between the tethered and untethered catalysts for a given temperature and catalyst loading. Creep experiments show that the dynamic bonds are continuously exchanging in the polymer networks and can be well described by a single-mode Maxwell model. We find that a single set of horizontal and vertical shift factors was sufficient to construct time-temperature superposition (TTS) master curves and was directly related to the relaxation time and the cross-linking density, respectively, further validating the structure-property relations. Activation energies calculated from the Arrhenius relationship with the flow viscosity are systematically higher for the tethered catalysts, which we hypothesize is due to steric effects in the polymer networks. Finally, we propose the use of time-temperature-catalyst superposition (TTCS) as a framework to probe the bond dynamics spanning substantially longer time scales than TTS as a function of temperature and catalyst loading.

We report a combined rheology and x-ray photon correlation spectroscopy (XPCS) study of the structural and mechanical relaxation of a ductile, nanocolloidal glass following the cessation of shear flow. After the glass is sheared to 300

Viscous resuspension, the process by which sedimented particles are re-entrained into a fluid under flow, is central to numerous natural and industrial systems, including environmental contaminant transport, riverbed erosion, and biogeochemical cycling. Despite its importance, predicting when and how resuspension occurs remains challenging, particularly under oscillatory shear, where particle interactions are nonlinear, collective, and time-dependent. Here, we examine the resuspension dynamics of dense, non-Brownian suspensions under both steady and oscillatory shear using bulk rheometry and in situ rheo-microscopy over a broad range of particle volume fractions (phi = 0.30-0.55). We show that resuspension cannot be described by the shear rate alone; rather, particle motion requires a critical stress condition ( Shields Number approximate to 1), while the extent of resuspension is governed by the accumulated strain. This strain-driven evolution is mediated by effective interparticle collisions and collective particle motion. We develop a predictive model that captures the observed strain thresholds as a function of volume fraction, allowing for the construction of a state diagram delineating sedimentation, resuspension, and full suspension regimes. These findings indicate that, under the conditions investigated, strain governs the progression of resuspension once the stress threshold is exceeded and provide a framework for describing suspension behavior across steady and oscillatory flows. This framework offers mechanistic insight into particle resuspension and inform the understanding of related transport phenomena in more complex systems. (c) 2026 Published under an exclusive license by Society of Rheology. https://doi.org/10.1122/8.0001095

The nonlinear response of yield stress fluids remains difficult to predict and control. Here, we show that the height of the overshoot in the loss modulus G^{''}, a key characteristic of yielding, depends only on linear viscoelastic properties. Furthermore, the position of this overshoot depends on linear viscoelastic and flow properties, demonstrating the important and enduring role of elasticity in yielding. The physics governing linear viscoelasticity is therefore not only preserved during yielding but also controls two commonly reported yielding metrics.

Bottlebrush block copolymers (BBCPs), characterized by densely grafted side chains along their backbone, have emerged as promising materials for structural color applications. Their unique architecture prevents entanglement and facilitates rapid assembly kinetics, enabling the formation of various photonic crystals with high tunability of structural color from the visible to the infrared range. However, accessing non-1D structures has been largely limited to synthetic approaches. In this paper, we report large modulation of a microphase separated morphology of the polystyrene-b-polylactide (PS-b-PLA) BBCP using a single material by exploiting selective solubility of the two blocks in a series of structurally similar solvents. Combining optical spectroscopy, electron microscopy, photoinduced force microscopy and X-ray scattering, we unveil the microstructural evolution as dependent on solvent selectivity. Furthermore, we uncovered the mechanisms underlying the differences in assembly, where highly selective solvents induce sidechain aggregation, altering the volume fraction and facilitating the formation of non-1D structures.

The complex two-step yielding observed in some soft materials under oscillatory shearing is shown to result from two independent phenomena. The first step at small deformations corresponds to elastic softening, while the larger deformation feature corresponds to yielding. This interpretation is supported by experimental recovery rheology data and the construction of a rheo-physical model. Our findings elucidate the mechanisms governing complex yielding and underscore the insight afforded by recovery experiments, which decouple the underlying physics between recoverable and unrecoverable processes.

The topic of thixotropy has historically received much attention due to its importance in a wide range of complex fluids and their applications. However, a thorough understanding of the phenomenon and how to model it remain outstanding challenges. In this work, we examine two materials that exhibit phenomenology often referred to as thixotropic through the lens of stress-controlled recovery rheology. When subjected to an oscillatory shear stress, the materials, an aqueous surfactant system that structurally forms multilamellar vesicles as well as a frequently studied fumed silica suspension, show a transient increase in the resulting strain amplitude. We use both creep and oscillatory tests in conjunction with recovery rheology to measure the elastic and viscous contributions to flow and deformation and find that the elastic contributions remain constant, even at larger amplitudes where nonlinear responses are induced. We conclude that the observed behavior is, therefore, strictly a viscous phenomenon, in contrast with common modeling efforts that describe both the viscous and elastic behaviors as being transient. We additionally examine how typical use of the dynamic moduli can give a misleading description of the material’s behavior, whereas examination of the energetic contributions provides a description consistent with the recovery measurements.

Nanostructured epoxy composite resins have broad usage in adhesives, coatings, composites, and 3D printing. With these materials, careful control of the rheological properties is critical to ensuring that the properties meet their required performance targets. However, it can be difficult to accurately measure the rheological properties. In this work, we establish a method to develop a reliable pre-shear (PS) procedure to repeatably measure the apparent yield stress of the resins, which is critical to ensure the accurate understanding of the material behavior. The resins in this study consisted of an epoxy resin with nanoclay as a shear thinning agent, ionic liquid (1-ethyl-3-methylimidazolium dicyanamide) as a latent curing agent, and poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) block copolymer (BCP) as a nanostructured component. We establish a methodology to evaluate the effectiveness of a pre-shear protocol and evaluate several methods to identify a pre-shear procedure that resulted in repeatable transient creep results on a rheometer. We identified that large amplitude oscillatory shear was the most effective method for these materials, and the optimal magnitude of the shear was dependent on the composition of the epoxy resins. Through the consistent application of this approach, we were able to use transient creep testing to identify the phase boundaries in the epoxy/BCP resins when the BCP micelles undergo an order-order transition from spherical to hexagonal micelles through changes in the yield stress of the material. This work adds to the new growing body of literature demonstrating the importance of establishing rigorous pre-shear conditions to improve the accuracy of structured yield stress fluids.

Linking the macroscopic flow properties and nanoscopic structure is a fundamental challenge to understanding, predicting, and designing disordered soft materials. Under small stresses, these materials are soft solids, while larger loads can lead to yielding and the acquisition of plastic strain, which adds complexity to the task. In this work, we connect the transient structure and rheological memory of a colloidal gel under cyclic shearing across a range of amplitudes via a generalized memory function using rheo-X-ray photon correlation spectroscopy (rheo-XPCS). Our rheo-XPCS data show that the nanometer scale aggregate-level structure recorrelates whenever the change in recoverable strain over some interval is zero. The macroscopic recoverable strain is therefore a measure of the nano-scale structural memory. We further show that yielding in disordered colloidal materials is strongly heterogeneous and that memories of prior deformation can exist even after the material has been subjected to flow.

Synthesis of high-purity bottlebrush block copolymers (BBCPs) with high degrees of polymerization via graft-through ring-opening metathesis polymerization of norbornene-based macromonomers faces difficulties due to the lower reactivity of these macromonomers compared to smaller monomers. Herein, we report a scalable synthetic methodology to access polystyrene-b-polylactide BBCPs with a high degree of polymerization (PS796-b-PLA1114) and high brush densities (2 brushes per norbornene repeat unit, PS201-b-diPLA229) with high purity (as illustrated by the monomodal molecular weight distribution). This methodology combines graft-through (GT) polymerization of macromonomer polymerization and multifunctional monomer polymerization to synthesize a bottlebrush-linear block copolymer as an intermediate. The brushes of the second block are then synthesized using a graft-from (GF) polymerization. The synthesized BBCPs self-assemble into periodic structures with photonic properties showing wavelengths of reflection nearing the IR region (similar to 1500 nm). No difference between the traditional GT and this combined GT-GF polymerization was identified spectroscopically or in their self-assembled structures. However, the viscoelastic behaviors suggest that GT-GF-based BBCPs are more flexible in comparison to purely GT-based polymers. This difference in molecular flexibility is putatively attributed to a difference in chemical structure at the interface between the two blocks. Moreover, we noted that BBCPs with higher brush density exhibited greater elasticity when compared to those with lower density. In summary, we successfully created a range of BBCPs characterized by a high degree of polymerization and high brush density and examined how the variations in their topology influenced the structural, photonic, and viscoelastic properties of the materials.

Material extrusion is an additive manufacturing technique that enables the creation of reproducible and complex hardware by depositing a viscous, shear-thinning ink onto a substrate in a custom-pattern via extrusion through a syringe. The ability of an ink to be extruded onto a substrate in many layers and maintain the desired shape is what defines printability. Printability has historically been investigated in an iterative manner by formulating and printing inks and then performing postmortem analysis of final parts. Highly concentrated pastes continue to pose issues for practitioners as the effect of filler morphology and size dispersity on the ink rheology and corresponding printability is not well understood. A printability criterion based on the particle filler’s maximum packing fraction was recently proposed to provide a general framework to understand printability of particle-filled inks. Inks were found to be printable if the particle loading was within 90-94% of the maximum packing fraction of the particle. Here we expand on that work to validate the generality of the maximum packing fraction criterion by testing with 10 new single and multimodal particle fillers. The maximum packing fraction calculated from small amplitude oscillatory shear experiments and is found to correctly predict the printability range for all inks. We then utilize statistical methods to develop a filler characteristics model to predict the maximum packing fraction from particle analysis alone. These two methods paired together can significantly speed up development of new inks, increase the performance of material extrusion printing, and improve the stability of printed parts, with less wasted time and materials.

ABSTRACT Self‐stratifying polymer systems are of great interest for coatings, as such systems reduce the time, cost, and environmental impact associated with the application of multilayered coatings by providing several layers in a single coating step. We have developed an understanding of self‐stratification in polyurethane systems that occurs when hydrophobic and hydrophilic polyols containing ethylene oxide, propylene oxide, and butylene oxide mers and prepolymers containing toluene diisocyanate and methylene diphenyl diisocyanate are mixed and cured. When these components are mixed in appropriate proportions, self‐stratification occurs where the hydrophobic component migrates to the air interface and the hydrophilic component to the substrate interface, with a thin hydrophobic layer present at the substrate walls when the substrate is hydrophobic. Self‐stratification requires less than 60 min, significantly less than the time required for the storage modulus to crossover the loss modulus (∼5 h). SIMS, XPS, and confocal Raman show that the stratification process at the air and substrate interfaces is dependent on interfacial surface energies, with the thickness and composition of the up to 10 µm thick interfacial region at the substrate controlled by the substrate surface energy. Self‐stratification is observed in both the bulk and thicknesses conventionally associated with coatings (10s of µm).

Injuries to musculoskeletal interfaces, such as the tendon-to-bone insertion of the rotator cuff, present significant physiological and clinical challenges for repair due to complex gradients of structure, composition, and cellularity. Advances in interface tissue engineering require stratified biomaterials able to both provide local instructive signals to support multiple tissue phenotypes while also reducing the risk of strain concentrations and failure at the transition between dissimilar materials. Here, we describe adaptation of a thiolated gelatin (Gel-SH) hydrogel via selective amination of carboxylic acid subunits on the gelatin backbone. The magnitude and kinetics of HRP-mediated primary crosslinking and carbodiimide-mediated secondary crosslinking reactions can be tuned through amination and thiolation of carboxylic acid subunits on the gelatin backbone. We also show that a stratified biomaterial comprised of mineralized (bone-mimetic) and non-mineralized (tendon-mimetic) collagen scaffold compartments linked by an aminated Gel-SH hydrogel demonstrate improved mechanical performance and reduced strain concentrations. Together, these results highlight significant mechanical advantages that can be derived from modifying the gelatin macromer via controlled amination and thiolation and suggest an avenue for tuning the mechanical performance of hydrogel interfaces within stratified biomaterials.

Yielding of dynamically crosslinked hydrogels, or the transition between a solid-like and liquid-like state, allows facile injection and utility in translational biomedical applications including delivery of therapeutic cells. Unfortunately, the time-varying nature of the transition is not well understood, nor are there design rules for understanding the effects of yielding on encapsulated cells. Here, we unveil underlying molecular mechanisms governing the yielding transition of dynamically crosslinked gels currently being researched for use in cell therapy. We demonstrate through nonlinear rheological characterization that the network dynamics of the dynamic hydrogels dictate the speed and character of their yielding transition. Rheological testing of these materials reveals unexpected elastic strain stiffening during yielding, as well as characterization of the rapidity of the yielding transition. A slower yielding speed explains enhanced protection of directly injected cells from shear forces, highlighting the importance of mechanical characterization of all phases of yield-stress biomaterials.

While granular hydrogels are increasingly used in biomedical applications, methods to capture their rheological behavior generally consider shear-thinning and self-healing properties or produce ensemble metrics (e.g., dynamic moduli) while neglecting transient yielding and unyielding processes. Combining oscillatory shear testing with Brittility (Bt) via the Kamani-Donley-Rogers (KDR) model, this work shows that granular hydrogels behave as brittle yield stress fluids. This work quantifies steady and transient rheology as a function of microgel properties and granular composition for polyethylene glycol and gelatin microgels. The KDR model with Bt captures granular hydrogel behavior for a wide range of design parameters, reducing the complex rheology to a determination of model parameters. In granular mixtures, this work observes monotonic dependencies of the elastic modulus, structural viscosity, and brittility upon granular composition, while the yield stress is lower for mixtures. Microgel size distribution and polymer fraction are the most influential parameters in monolithic granular hydrogels, while microgel size and packing density are less impactful. The model robustly captures self-healing behavior and reveals that granular hydrogel relaxation accelerates with an increased small-amplitude strain rate. This quantitative framework is an important step toward rational design of granular hydrogels for applications ranging from injection and in situ stabilization to 3D bioprinting.