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Dense suspensions of hard granular particles can transform from liquid-like to solid-like when perturbed; a state diagram is mapped out that reveals how this transformation can occur via dynamic jamming at sufficiently large shear stress while leaving the particle density unchanged. Shear jamming in dense suspensions Dense suspensions of hard granular particles exhibit a rich array of dynamical behaviour: depending on how they are perturbed, and the timescale on which they are measured, they can transform from liquid-like to solid-like. Ivo Peters et al . look specifically at the effects of dynamical shear, mapping out these behaviours as a function of particle density and shear stress. The result is a comprehensive state diagram that provides a unified picture of the steady state and transient behaviours of such systems, with dynamic shear 'jamming' playing the pivotal role. Liquid-like at rest, dense suspensions of hard particles can undergo striking transformations in behaviour when agitated or sheared 1 . These phenomena include solidification during rapid impact 2 , 3 , as well as strong shear thickening characterized by discontinuous, orders-of-magnitude increases in suspension viscosity 4 , 5 , 6 , 7 , 8 . Much of this highly non-Newtonian behaviour has recently been interpreted within the framework of a jamming transition. However, although jamming indeed induces solid-like rigidity 9 , 10 , 11 , even a strongly shear-thickened state still flows and thus cannot be fully jammed 12 , 13 . Furthermore, although suspensions are incompressible, the onset of rigidity in the standard jamming scenario requires an increase in particle density 9 , 10 , 14 . Finally, whereas shear thickening occurs in the steady state, impact-induced solidification is transient 2 , 15 , 16 , 17 . As a result, it has remained unclear how these dense suspension phenomena are related and how they are connected to jamming. Here we resolve this by systematically exploring both the steady-state and transient regimes with the same experimental system. We demonstrate that a fully jammed, solid-like state can be reached without compression and instead purely with shear, as recently proposed for dry granular systems 18 , 19 . This state is created by transient shear-jamming fronts, which we track directly. We also show that shear stress, rather than shear rate, is the key control parameter. From these findings we map out a state diagram with particle density and shear stress as variables. We identify discontinuous shear thickening with a marginally jammed regime just below the onset of full, solid-like jamming 20 . This state diagram provides a unifying framework, compatible with prior experimental and simulation results on dense suspensions, that connects steady-state and transient behaviour in terms of a dynamic shear-jamming process.

A jamming mechanism for the observed phenomenon of the sudden hardening of suspensions of micrometre-sized particles on impact (enough to enable a person to run over them) is described and quantified. Riddle of the quicksands If you walk fast enough, they say, you can walk over quicksand that would quickly trap the immobile and unwary. The forces involved in that feat are explained in this study. Liquids typically flow around an intruding object, but dense aqueous suspensions of micrometre-sized particles can harden under impact. Shear thickening — a tendency of the sheared suspension to dilate — is often invoked to explain the temporary hardening of such liquids, but is difficult to reconcile with the magnitude of such effects. Here, Scott Waitukaitis and Heinrich Jaeger demonstrate that the remarkable impact resistance is produced by a different mechanism. Using detailed imaging to capture the dynamics of the process — modelled by an aluminium rod striking a dense suspension of cornflour and water — they find that the stresses originate from an impact-generated solidification front that transforms an initially compressible particle matrix into a rapidly growing jammed region. Although liquids typically flow around intruding objects, a counterintuitive phenomenon occurs in dense suspensions of micrometre-sized particles: they become liquid-like when perturbed lightly, but harden when driven strongly 1 , 2 , 3 , 4 , 5 . Rheological experiments have investigated how such thickening arises under shear, and linked it to hydrodynamic interactions 1 , 3 or granular dilation 2 , 4 . However, neither of these mechanisms alone can explain the ability of suspensions to generate very large, positive normal stresses under impact. To illustrate the phenomenon, such stresses can be large enough to allow a person to run across a suspension without sinking, and far exceed the upper limit observed under shear or extension 2 , 4 , 6 , 7 . Here we show that these stresses originate from an impact-generated solidification front that transforms an initially compressible particle matrix into a rapidly growing jammed region, ultimately leading to extraordinary amounts of momentum absorption. Using high-speed videography, embedded force sensing and X-ray imaging, we capture the detailed dynamics of this process as it decelerates a metal rod hitting a suspension of cornflour (cornstarch) in water. We develop a model for the dynamic solidification and its effect on the surrounding suspension that reproduces the observed behaviour quantitatively. Our findings suggest that prior interpretations of the impact resistance as dominated by shear thickening need to be revisited.

The behaviour of a single droplet in an immiscible external fluid, submitted to shear flow is investigated using numerical simulations. The surface of the droplet is modelled by a Boussinesq–Scriven constitutive law involving the interfacial viscosities and a constant surface tension. A numerical method using Loop subdivision surfaces to represent droplet interface is introduced. This method couples boundary element method for fluid flows and finite element method to take into account the stresses due to the surface dilational and shear viscosities and surface tension. Validation of the numerical scheme with respect to previous analytic and computational work is provided, with particular attention to the viscosity contrast and the shear and dilational viscosities characterized both by a Boussinesq number $B_{q}$ . Then, influence of equal surface viscosities on steady-state characteristics of a droplet in shear flow are studied, considering both small and large deformations and for a large range of bulk viscosity contrast. We find that small deformation analysis is surprisingly predictive at moderate and high surface viscosities. Equal surface viscosities decrease the Taylor deformation parameter and tank-treading angle and also strongly modify the dynamics of the droplet: when the Boussinesq number (surface viscosity) is large relative to the capillary number (surface tension), the droplet displays damped oscillations prior to steady-state tank-treading, reminiscent from the behaviour at large viscosity contrast. In the limit of infinite capillary number $Ca$ , such oscillations are permanent. The influence of surface viscosities on breakup is also investigated, and results show that the critical capillary number is increased. A diagram $(B_{q};Ca)$ of breakup is established with the same inner and outer bulk viscosities. Additionally, the separate roles of shear and dilational surface viscosity are also elucidated, extending results from small deformation analysis. Indeed, shear (dilational) surface viscosity increases (decreases) the stability of drops to breakup under shear flow. The steady-state deformation (Taylor parameter) varies nonlinearly with each Boussinesq number or a linear combination of both Boussinesq numbers. Finally, the study shows that for certain combinations of shear and dilational viscosities, drop deformation for a given capillary number is the same as in the case of a clean surface while the inclination angle varies.

A remarkable property of dense suspensions is that they can transform from liquid-like at rest to solid-like under sudden impact. Previous work showed that this impact-induced solidification involves rapidly moving jamming fronts; however, details of this process have remained unresolved. Here we use high-speed ultrasound imaging to probe non-invasively how the interior of a dense suspension responds to impact. Measuring the speed of sound we demonstrate that the solidification proceeds without a detectable increase in packing fraction, and imaging the evolving flow field we find that the shear intensity is maximized right at the jamming front. Taken together, this provides direct experimental evidence for jamming by shear, rather than densification, as driving the transformation to solid-like behaviour. On the basis of these findings we propose a new model to explain the anisotropy in the propagation speed of the fronts and delineate the onset conditions for dynamic shear jamming in suspensions. Suspensions of particles at high volume fractions are subject to discontinuous shear thickening or even turn into solid upon impact, yet the underlying mechanism remains elusive. Here, Han et al . follow the propagation of shear bands at jamming fronts in three dimensions and show no sign of densification.

Key Points For neurodegenerative diseases such as Huntington's disease, spinocerebellar muscular atrophy, amyotrophic lateral sclerosis, Parkinson's disease and Alzheimer's disease there is a lack of effective treatments that directly address the underlying biochemical aetiology of neuronal dysfunction and cell death. Protein misfolding, cellular stress and neuronal cell death are common features of neurodegenerative diseases. A diverse set of chaperone proteins act in concert to fold misfolded proteins, disaggregate damaged proteins and prevent programmed cell death. Heat shock transcription factor 1 (HSF1) coordinately activates the expression of chaperone protein gene expression. Genetic and pharmacological experiments in cell culture, fruitfly and mouse models of neurodegenerative disease suggest that enhancing the cellular protein folding and anti-apoptotic machinery by elevating levels of chaperone proteins could have potential therapeutic efficacy in neurodegenerative diseases. Current small-molecule HSF1 activators have undesirable properties — including direct proteotoxicity, inhibition of the central cellular chaperone heat shock protein 90 and other characteristics — that limit their development for clinical use. As the master activator of chaperone protein expression, HSF1 is an attractive pharmacological target for the development of optimized small-molecule activators for therapeutic intervention in neurodegenerative diseases. Neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease are associated with the accumulation of misfolded proteins, resulting in neuronal dysfunction and cell death. Thiele and colleagues discuss the therapeutic potential of combating protein misfolding by harnessing the natural cellular protein-folding machinery through pharmacological activation of heat shock transcription factor 1, the master regulator of chaperone protein expression. Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and prion-based neurodegeneration are associated with the accumulation of misfolded proteins, resulting in neuronal dysfunction and cell death. However, current treatments for these diseases predominantly address disease symptoms, rather than the underlying protein misfolding and cell death, and are not able to halt or reverse the degenerative process. Studies in cell culture, fruitfly, worm and mouse models of protein misfolding-based neurodegenerative diseases indicate that enhancing the protein-folding capacity of cells, via elevated expression of chaperone proteins, has therapeutic potential. Here, we review advances in strategies to harness the power of the natural cellular protein-folding machinery through pharmacological activation of heat shock transcription factor 1 — the master activator of chaperone protein gene expression — to treat neurodegenerative diseases.

Gripping and holding of objects are key tasks for robotic manipulators. The development of universal grippers able to pick up unfamiliar objects of widely varying shape and surface properties remains, however, challenging. Most current designs are based on the multifingered hand, but this approach introduces hardware and software complexities. These include large numbers of controllable joints, the need for force sensing if objects are to be handled securely without crushing them, and the computational overhead to decide how much stress each finger should apply and where. Here we demonstrate a completely different approach to a universal gripper. Individual fingers are replaced by a single mass of granular material that, when pressed onto a target object, flows around it and conforms to its shape. Upon application of a vacuum the granular material contracts and hardens quickly to pinch and hold the object without requiring sensory feedback. We find that volume changes of less than 0.5% suffice to grip objects reliably and hold them with forces exceeding many times their weight. We show that the operating principle is the ability of granular materials to transition between an unjammed, deformable state and a jammed state with solid-like rigidity. We delineate three separate mechanisms, friction, suction, and interlocking, that contribute to the gripping force. Using a simple model we relate each of them to the mechanical strength of the jammed state. This advance opens up new possibilities for the design of simple, yet highly adaptive systems that excel at fast gripping of complex objects.

When an amorphous material is strained beyond the point of yielding, it enters a state of continual reconfiguration via dissipative, avalanchelike slip events that relieve built-up local stress. However, how the statistics of such events depend on local interactions among the constituent units remains debated. To address this we perform experiments on granular material in which we use particle shape to vary the interactions systematically. Granular material, confined under constant pressure boundary conditions, is uniaxially compressed while stress is measured and internal rearrangements are imaged with x rays. We introduce volatility, a quantity from economic theory, as a powerful new tool to quantify the magnitude of stress fluctuations, finding systematic, shape-dependent trends. In particular, packings of flatter, more oblate shapes exhibit more catastrophic plastic deformation events and thus higher volatility, while rounder and also prolate shapes produce lower volatility. For all 22 investigated shapes the magnitudesof relaxation events is well fit by a truncated power-law distributionP(s)∼s−τexp(−s/s*), as has been proposed within the context of plasticity models. The power-law exponentτfor all shapes tested clusters aroundτ=1.5, within experimental uncertainty covering the range 1.3–1.7. The shape independence ofτand its compatibility with mean-field models indicate that the granularity of the system, but not particle shape, modifies the stress redistribution after a slip event away from that of continuum elasticity. Meanwhile, the characteristic maximum event sizes*changes by 2 orders of magnitude and tracks the shape dependence of volatility. Particle shape in granular materials is therefore a powerful new factor influencing the distance at which an amorphous system operates from scale-free criticality. These experimental results are not captured by current models and suggest a need to reexamine the mechanisms driving mesoscale plastic deformation in amorphous systems.

Self-assembly is emerging as an elegant, ‘bottom-up’ method for fabricating nanostructured materials 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . This approach becomes particularly powerful when the ease and control offered by the self-assembly of organic components is combined with the electronic, magnetic or photonic properties of inorganic components 2 , 5 , 9 . Here we demonstrate a versatile hierarchical approach for the assembly of organic–inorganic, copolymer–metal nanostructures in which one level of self-assembly guides the next. In a first step, ultrathin diblock copolymer films form a regular scaffold of highly anisotropic, stripe-like domains 10 , 11 , 12 . During a second assembly step, differential wetting guides diffusing metal atoms to aggregate selectively along the scaffold, producing highly organized metal nanostructures. We find that, in contrast to the usual requirement of near-equilibrium conditions for ordering 2 , 3 , 13 , the metal arranged on the copolymer scaffold produces the most highly ordered configurations when the system is far from equilibrium. We delineate two distinct assembly modes of the metal component—chains of separate nanoparticles and continuous wires—each characterized by different ordering kinetics and strikingly different current–voltage characteristics. These results therefore demonstrate the possibility of guided, large-scale assembly of laterally nanostructured systems.

Processes from crystallization to protein folding to micro-robot self-assembly rely on achieving specific configurations of microscopic objects with short-ranged interactions. However, the small scales and large configuration spaces of such multi-body systems render targeted control challenging. Inspired by optical pumping manipulation of quantum states, we develop a method using parametric pumping to selectively excite and destroy undesired structures to populate the targeted one. This method does not rely on free energy considerations and therefore works for systems with non-conservative and even non-reciprocal interactions, which we demonstrate with an acoustically levitated five-particle system in the Rayleigh limit. With results from experiments and simulations on three additional systems ranging up to hundreds of particles, we show the generality of this method, offering a new path for non-invasive manipulation of strongly interacting multi-particle systems. Configuration control of non-conservative multi-body systems is challenging. Here, the authors develop a general method using parametric pumping to selectively excite and destroy undesired structures to populate a targeted one, and demonstrate it with acoustically levitated particle systems.

In most suspensions viscosity decreases with increasing shear rate. The opposite effect, shear thickening, is a problem for industrial applications. An understanding of how particle interactions in suspensions influence shear thickening may lead to a solution of this problem through the design of smart suspensions. Suspensions are of wide interest and form the basis for many smart fluids 1 , 2 , 3 , 4 , 5 , 6 , 7 . For most suspensions, the viscosity decreases with increasing shear rate, that is, they shear thin. Few are reported to do the opposite, that is, shear thicken, despite the longstanding expectation that shear thickening is a generic type of suspension behaviour 8 , 9 . Here we resolve this apparent contradiction. We demonstrate that shear thickening can be masked by a yield stress and can be recovered when the yield stress is decreased below a threshold. We show the generality of this argument and quantify the threshold in rheology experiments where we control yield stresses arising from a variety of sources, such as attractions from particle surface interactions, induced dipoles from applied electric and magnetic fields, as well as confinement of hard particles at high packing fractions. These findings open up possibilities for the design of smart suspensions that combine shear thickening with electro- or magnetorheological response.