Irmgard Bischofberger

We show that filler-hydrogel composites exhibit viscoelastc properties with signatures of successive transitions from linear to non-linear elasticity to macroscopic irreversibility with increasing strain amplitude.

We show that filler-hydrogel composites exhibit viscoelastc properties with signatures of successive transitions from linear to non-linear elasticity to macroscopic irreversibility with increasing strain amplitude.

' reaches its maximum, characterized by a marked change in the stress relaxation dynamics. Beyond , the composites no longer recover their initial viscoelastic properties in repeated strain amplitude sweeps, indicating that the material has sustained macroscopically irreversible changes and a permanent loss of elasticity. We thus identify three distinct regimes in the strain-stiffening materials: nonlinear elasticity, macroscopic irreversibility, and yielding. We further suggest that the plasticity underpinning macroscopic irreversibility is due to the breaking of bonds that contribute most to the composite's strain stiffening response in the hydrogel matrix.

A drop of a colloidal suspension placed on a substrate forms a solid particle deposit as it dries. As water evaporates, large gradients in pore pressure inside the porous deposit cause shrinkage and stresses. The deposit cracks, then delaminates from the substrate, and bends out of plane, creating a striking three-dimensional structure. Previous models have attributed the out-of-plane deformation to pore pressure gradients through the deposit's thickness, a hypothesis our findings contradict. Through a combination of interference and confocal microscopy, we show that the final curvature strongly depends on the deposit thickness, with thinner deposits curving more. We propose a mechanism where the curvature is driven not by vertical pressure gradients, but by much larger radial pressure gradients across the length of the deposit. The resulting in-plane differential shrinkage creates geometric frustration that is resolved through out-of-plane buckling. We validate this mechanism using non-Euclidean plate simulations, which successfully reproduce the buckling behavior and the observed dependence of curvature on thickness.

Particulate suspensions, consisting of solid particles dispersed in a fluid, exhibit complex flow behaviors influenced by multiple factors, including particle interactions, concentration gradients, and external forces. Suspensions play an important role in diverse processes, from sediment transport to food processing, and display instabilities triggered by shear-driven effects, frictional interactions, and viscous forces. These instabilities can often be understood by identifying the key mechanical quantities that govern the dynamics. Following hydrodynamic tradition, such mechanics can be characterized by dimensionless numbers, which encapsulate the interplay between geometric, kinematic, and mechanical factors. Many of these numbers represent competitions between opposing pairs of mechanical quantities, which we discuss in detail while also considering a few phenomena that require more complex combinations. By emphasizing the underlying mechanical principles, this review provides a perspective for understanding pattern formation and flow instabilities in confined particulate suspensions across different flow geometries.

Mobula rays have evolved leaf-shaped filter structures to separate food particles from seawater, which function similarly to industrial cross-flow filters. Unlike cross-flow filtration, where permeability and selectivity are rationally designed following trade-off analyses, the driving forces underlying the evolution of mobula filter geometry have remained elusive. To bridge the principles of cross-flow and mobula filtration, we establish a universal framework for the permeability-selectivity trade-off in a leaky channel inspired by mobula filters, where permeability and selectivity are characterized by the pore-scale leaking rate and the cut-off particle size, respectively. Beyond the classic pore-flow regime in cross-flow filtration, we reveal transition and vortex regimes pertinent to mobula filtration. Combining theory, physical experiments, and simulations, we present distinct features of water permeability and particle selectivity across the three regimes. In particular, we identify an unreported 1/2-scaling law for the leaking rate in the vortex regime. We conclude by demonstrating that mobula filters strike an elegant balance between permeability and selectivity, which enables mobula rays to simultaneously satisfy biological requirements for breathing and filter feeding. By integrating cross-flow and mobula filtration into a universal framework, our findings provide fundamental insights into the physical constraints and evolutionary pressures associated with biological filtration geometries and lay the foundation for developing mobula-inspired filtration in industry.

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As sessile drops of aqueous colloidal suspensions dry, a close-packed particle deposit forms that grows from the edge of the drop toward the center. To compensate for evaporation over the solid's surface, water flows radially through the deposit, generating a negative pore pressure in the deposit associated with tensile drying stresses that induce the formation of cracks. As these stresses increase during drying, existing cracks propagate and additional cracks form, until the crack density eventually saturates. We rationalize the dynamics of crack propagation and crack densification with a local energy balance between the elastic energy released by the crack, the energetic cost of fracture, and the elastic energy released by previously formed cracks. We show that the final spacing between radial cracks is proportional to the local thickness of the deposit, while the aspect ratio of the crack segments depends on the shape of the deposit.

Submissions to the third Gallery of Soft Matter include stunning and thoughtfully crafted images and videos which we hope will captivate both scientists and the general public. It is our pleasure to share the five winning entries in this PRE Guest Editorial.

Supramolecular chirality typically originates from either chiral molecular building blocks or external chiral stimuli. Generating chirality in achiral systems in the absence of a chiral input, however, is non-trivial and necessitates spontaneous mirror symmetry breaking. Achiral nematic lyotropic chromonic liquid crystals have been reported to break mirror symmetry under strong surface or geometric constraints. Here we describe a previously unrecognised mechanism for creating chiral structures by subjecting the material to a pressure-driven flow in a microfluidic cell. The chirality arises from a periodic double-twist configuration of the liquid crystal and manifests as a striking stripe pattern. We show that the mirror symmetry breaking is triggered at regions of flow-induced biaxial-splay configurations of the director field, which are unstable to small perturbations and evolve into lower energy structures. The simplicity of this unique pathway to mirror symmetry breaking can shed light on the requirements for forming macroscopic chiral structures.

Dense suspensions exhibit the remarkable ability to switch dynamically and reversibly from a fluid-like to a solid-like, shear-jammed (SJ) state. Here, we show how this transition has important implications for the propensity for forming fractures. We inject air into bulk dense cornstarch suspensions and visualize the air invasion into the opaque material using time-resolved X-ray radiography. For suspensions with cornstarch mass fractions high enough to exhibit discontinuous shear thickening and shear jamming, we show that air injection leads to fractures in the material. For high mass fractions, these fractures grow quasistatically as rough cavities with fractured interfaces. For lower mass fractions, remarkably, the fractures can relax to smooth bubbles that then rise under buoyancy. We show that the onset of the relaxation occurs as the shear rate induced by the air cavity growth decreases below the critical shear rate denoting the onset of discontinuous shear thickening, which reveals a structural signature of the SJ state.

Supramolecular chirality typically originates from either chiral molecular building blocks or external chiral stimuli. Generating chirality in achiral systems in the absence of a chiral input, however, is non-trivial and necessitates spontaneous mirror symmetry breaking. Achiral nematic lyotropic chromonic liquid crystals have been reported to break mirror symmetry under strong surface or geometric constraints. Here we describe a previously unrecognised mechanism for creating chiral structures by subjecting the material to a pressure-driven flow in a microfluidic cell. The chirality arises from a periodic double-twist configuration of the liquid crystal and manifests as a striking stripe pattern. We show that the mirror symmetry breaking is triggered at regions of flow-induced biaxial-splay configurations of the director field, which are unstable to small perturbations and evolve into lower energy structures. The simplicity of this unique pathway to mirror symmetry breaking can shed light on the requirements for forming macroscopic chiral structures.

Supramolecular chirality typically originates from either chiral molecular building blocks or external chiral stimuli. Generating chirality in achiral systems in the absence of a chiral input, however, is non-trivial and necessitates spontaneous mirror symmetry breaking. Achiral nematic lyotropic chromonic liquid crystals have been reported to break mirror symmetry under strong surface or geometric constraints. Here we describe a previously unrecognised mechanism for creating chiral structures by subjecting the material to a pressure-driven flow in a microfluidic cell. The chirality arises from a periodic double-twist configuration of the liquid crystal and manifests as a striking stripe pattern. We show that the mirror symmetry breaking is triggered at regions of flow-induced biaxial-splay configurations of the director field, which are unstable to small perturbations and evolve into lower energy structures. The simplicity of this unique pathway to mirror symmetry breaking can shed light on the requirements for forming macroscopic chiral structures.

We reveal the mechanism for the strong reinforcement of attractive nanofiller-hydrogel composites. Measuring the linear viscoelastic properties of hydrogels containing filler nanoparticles, we show that a significant increase of the modulus can be achieved at unexpectedly low volume fractions of nanofillers when the filler-hydrogel interactions are attractive. Using three-dimensional numerical simulations, we identify a general microscopic mechanism for the reinforcement, common to hydrogel matrices of different compositions and concentrations and containing nanofillers of varying sizes. The attractive interactions induce a local increase in the gel density around the nanofillers. The effective fillers, composed of the nanofillers and the densified regions around them, assemble into a percolated network, which constrains the gel displacement and enhances the stress coupling throughout the system. A global reinforcement of the composite is induced as the stresses become strongly coupled. This physical mechanism of reinforcement, which relies only on attractive filler-matrix interactions, provides design strategies for versatile composites that combine low nanofiller fractions with an enhanced mechanical strength.

When a hyperelastic hydrogel confined between two parallel glass plates begins to dry from a lateral boundary, the volume lost by evaporation is accommodated by an inward displacement of the air-hydrogel interface that induces an elastic deformation of the hydrogel. Once a critical front displacement is reached, we observe intermittent fracture events initiated by a geometric instability resulting in localized bursts at the interface. These bursts relax the stresses and irreversibly form air cavities that lead to cellular networks. We show that the spatial extent of the strain field prior to a burst, influenced by the air-hydrogel interfacial tension and the confinement of the gel, determines the characteristic size of the cavities.

It is known that inertial lift forces can lead to particle focusing in channel flows; yet oscillatory straining effects have also been suggested as a mechanism for particle focusing in wavy channels. To explore the synergy between these two mechanisms, we analytically and experimentally investigate the focusing behaviour of rigid neutrally buoyant particles in a wavy channel. We decompose the particle-free channel flow into a primary Poiseuille flow and secondary eddies induced by the waviness. We calculate the perturbation of the particle on the particle-free flow and the resulting lateral lift force exerted on the particle using the method of matched asymptotic expansions. This yields a zeroth-order lift force arising from the Poiseuille flow and a first-order lift force due to the waviness of the channel. We further incorporate the inertial lift force into the Maxey–Riley equation and simulate particle trajectories in wavy channels. The interactions between the zeroth-order lift force and the particle-free flow largely determine the focusing locations. Experiments in wavy channels with varying amplitudes at channel Reynolds numbers ranging from 5 to 250 are consistent with the predictions of the focusing locations, which are mainly governed by the channel Reynolds number as well as the competition between the inertial lift and the oscillatory straining effects.

Second Annual APS DSOFT Gallery of Soft MatterSoft materials are ubiquitous in our lives: we eat them, our body is largely composed of them, and they shape our landscapes with avalanches and dunes.And often, they contain an astounding hidden beauty.The Second Annual Gallery of Soft Matter, held at the American Physical Society (APS) March Meeting 2023 in Las Vegas, organized by the APS Division of Soft Matter, showcases the aesthetic appeal and elegance of soft matter systems.The submissions were judged for their combination of striking visual qualities and scientific interest.

The interplay between viscoelasticity and inertia in dilute polymer solutions at high deformation rates can result in inertio-elastic instabilities. We show how fluid elasticity has a nonmonotonic effect on jet stability depending on magnitude, creating two distinct regimes. The nonlinear evolution of these instabilities generates a state of elasto-inertial turbulence (EIT) with different spatiotemporal features than Newtonian turbulence. We use high-speed digital schlieren imaging and dynamic mode decomposition to quantify EIT and identify two modes of instability which can lead to a transition to turbulence at a lower Reynolds number with flow-aligned structures in the turbulent region.

of soft matter is amply studied and understood, novel tools are needed to capture the complexity of out-of-equilibrium soft materials. A particular challenge is getting access to and understanding the fast processes occurring at the nano-and micro-scale of the elementary constituents, while simultaneously capturing the slower evolution of bulk properties in non-equilibrium conditions. To take on this task, new theoretical, numerical and experimental methodologies need to be developed that enable the investigation of the structure, dynamics, thermodynamics, and rheology of out-of-equilibrium soft matter. Adopting a comprehensive view that covers fundamental topics as well as research focusing on processes and issues faced in applications, this Research Topic aims to showcase some of the latest advancements and innovations in the field.Living and active materials are intrinsically non-equilibrium systems due to metabolic activity and/or energy consumption. Using newly developed fabrication and image processing tools, Chang et al.

Controlling the growth morphology of fluid instabilities is challenging because of their self-amplified and nonlinear growth. The viscous fingering instability, which arises when a less viscous fluid displaces a more viscous one, transitions from exhibiting dense-branching growth characterized by repeated tip splitting of the growing fingers to dendritic growth characterized by stable tips in the presence of anisotropy. We controllably induce such a morphology transition by shear-enhancing the anisotropy of nematic liquid crystal solutions. For fast enough flow induced by the finger growth, the intrinsic tumbling behavior of lyotropic chromonic liquid crystals can be suppressed, which results in a flow alignment of the material. This microscopic change in the director field occurs as the viscous torque from the shear flow becomes dominant over the elastic torque from the nematic potential and macroscopically enhances the liquid crystal anisotropy to induce the transition to dendritic growth.