arXiv subjects

Heinrich M. Jaeger

Publications and source records attributed to Heinrich M. Jaeger.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear force response of modular lattice-based metamaterials

Lattice-based metamaterials provide lightweight platforms where local instabilities can govern the global mechanical response, enabling applications in energy routing, vibration isolation, and impact mitigation. Although much progress has been made in controlling deformation and buckling sequences through geometric design, the behavior of coupled nonlinear units over a large range of strain rates and their history-dependent response is less explored. Here, we investigate lattice-based mechanical metamaterials whose nonlinear buckling behavior can be harnessed through modular architectures. By combining modular units in series, we show that their interaction gives rise to emergent force responses, including transient weakening and enhanced force attenuation, that are absent in the individual modules. Furthermore, selected designs exhibit training behavior under cyclic loading, transitioning between distinct buckling states and revealing a history-dependent mechanical response. Our results demonstrate that modular, instability-driven metamaterials can be programmed and tuned not only through geometry but also through loading history, opening new avenues for designing a nonlinear stress-response in mechanical systems.

cond-mat.soft↗

arXiv2026

Structure Selection by Non-Conservative 3-Body Acoustic Interactions

Non-conservative multi-body interactions are typically associated with instabilities and activity in driven, field-mediated systems. Here we show that they can also promote stable static structures. Combining experiments and simulations in a minimal, acoustically levitated three-particle system, we tune the relative strength of conservative and non-conservative contributions to the force field. The conservative component favors a symmetric equilibrium configuration, whereas the non-conservative 3-body contribution selects a flattened isosceles triangle. Our results identify non-conservative multi-body forces as a mechanism for static structure selection in driven-dissipative matter in the absence of an effective-energy landscape.

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arXiv2026

Nonreciprocal multi-body interactions activate liquid state of acoustically levitated particle ensembles

Nonreciprocal forces are often a consequence of asymmetry in the properties of the interacting objects. However. even if all objects are identical and isotropic, and the pairwise interactions between two objects are completely reciprocal, nonreciprocal forces can still appear when an arrangement of many objects breaks configurational symmetry in the presence of non-pairwise, multi-body interactions. Here we demonstrate that such emergent nonreciprocity can activate a particle ensemble to behave like a liquid, albeit with unique traits. In our experiments, passive microspheres are acoustically levitated in air, where they form a freely floating monolayer containing up to a couple hundred particles and collectively behave like a two-dimensional liquid droplet. The particles interact via nonreciprocal multi-body forces that arise from the combination of acoustic scattering and sound-induced viscous microstreaming. We find that these forces drive superdiffusive particle motion with non-Gaussian tails in the particles' speed distribution. Using probes that reach laterally into the levitation plane, we perform liquid pendant drop and pinch-off experiments. Compared to ordinary liquids, the droplets are found to have a kinematic viscosity similar to that of water, but in combination with an extremely low interfacial tension. The pinching-off is driven by nonreciprocity-induced active fluctuations and exhibits the self-similar double-cone neck profile seen also in liquid nanojets close to rupture, however here characterized by power law behavior with a scaling exponent that is anomalously small.

cond-mat.soft↗

arXiv2026

Irregular Metamaterial Networks

Metamaterials can achieve exceptional functionality through careful engineering of their mesoscale structure. Although appropriately introduced irregularities can be advantageous, current approaches largely conform to regular structures to preserve tractability. Here, we contend that network theory, enriched with geometry and physics, provides a natural framework for designing metamaterials with controlled irregularities at relevant scales, thereby enabling the discovery of new property-enhancing structures. We examine how this augmented network theory can facilitate the creation of irregular metamaterials with enhanced or novel properties and how metamaterial research, in turn, is opening new directions in network science. Supported by machine learning and advanced self-assembly, the emerging field of irregular metamaterial networks is poised to transform inverse design and scalable manufacturing of novel materials.

cond-mat.dis-nn↗

arXiv2026

Diffusive buckling fronts in lattice-based metamaterials

Mechanical metamaterials can be designed to exhibit unique mechanical properties, including tunable auxetic behavior as well as multi-stability, which arise from the geometry and configuration of the constituent building blocks. Lattice-based metamaterials, in particular, provide lightweight platforms where local instabilities can dictate the global response, with applications in energy routing and vibration isolation. In underdamped structures, perturbations have been found to propagate as nonlinear waves, e.g., transition waves or solitons. Here we investigate the opposite limit of overdamped, highly dissipative lattice metamaterials. Focusing on three-dimensional structures, we uncover how buckling instabilities, triggered by compression, propagate as fronts that shape the macroscopic behavior. We demonstrate in experiments on 3D-printed simple cubic lattices how global and local buckling modes can be controlled via the lattice geometry. By incorporating viscoelastic dissipation into a 3D-continuum model, we show that strain-driven buckling fronts obey coupled reaction-diffusion equations. The diffusion and reaction coefficients, determined by local geometry, material properties, and strain, select the propagation direction and enable steering of the fronts. This establishes a predictive and experimentally validated framework for the control of cascading mechanical instabilities in lattice-based metamaterials.

cond-mat.soft↗

arXiv2026

The Role of Hydrogen Bridging Bonds in the Shear-Thickening and Jamming of Dense Suspensions

Strong shear thickening and jamming in dense suspensions are driven by friction as particles are sheared into contact. Control over these frictional interactions can be achieved via particle shape and roughness, and also via the particles' surface chemistry and interactions with the surrounding solvent. We report on experiments with cornstarch suspensions where friction is enhanced by molecular bridging when hydrogen atoms at the ends of solvent molecules bond with hydroxyl groups on the surfaces of adjacent particles. We systematically vary the hydrogen bonding propensity by increasing the size of the backbone of the solvent molecule, from water to diols with up to 4 carbon atoms. For a fixed particle weight fraction, we find a sudden transition from strong shear thickening (in water and ethylene glycol) to shear thinning (in propanediol and butanediol). Combining data from rheology, density functional theory simulations, and fixed-rate pull tests, our results show how changes in the solvent's molecular structure affect both particle-solvent and solvent-solvent interactions, and how this can be used to tailor the shear thickening and jamming behavior of suspensions.

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arXiv2026

Constraint ratio controls viscosity in shear thickening suspensions

The dramatic viscosity increase observed in dense suspensions under shear poses a major challenge in our understanding of how microscopic contact mechanics translate into macroscopic flow resistance. Here, we introduce a constraint-counting model that incorporates friction and dimensionality naturally without additional assumptions and allows for collapsing of rheological data onto a universal master curve. In this model, we borrow ideas from dry granular jamming physics and classify contacts as either locked or non-locked to define a single state variable, the constraint ratio, which measures the average strength of mechanical constraint per particle. By identifying the constraint ratio as the key control parameter, our framework provides a unifying route toward predictive modeling and rational design of shear-thickening materials.

cond-mat.soft↗

arXiv2025

Fracture and failure of shear-jammed dense suspensions under impact

Impacted with sufficiently large stress, a dense, initially liquid-like suspension can be forced into a solid-like state through the process of shear jamming. While the onset of shear jamming has been investigated extensively, less is known about the resulting solid-like state in the high stress limit and its failure. We experimentally produce such high-stress failure by impacting dense suspensions at a controlled speed. Using cornstarch suspensions we vary impact speed over several orders of magnitude and change fluid viscosity and surface tension in order to identify the conditions for failure. The results are compared with dense suspensions of potato starch or silica particles. In the case of fracture, we observe two types of cracks: a primary circular crack around the impactor followed by secondary radial cracks. Mapping out the onset of radial fracturing for different volume fractions and impact speeds, we identify the requirements for failure via crack formation to occur with at least 50% likelihood. We find that this likelihood is not sensitive to changes in particle diameter, but increases when the solvent's viscosity or surface tension are reduced. In the state diagram for dense suspensions we delineate the upper limit of shear-jammed rigidity and the crossover into a fracture regime at large volume fraction and normal stress, several orders of magnitude above the onset stress for shear-jamming. We find that the onset of fracturing in many cases is correlated with internal ductile deformation of the shear-jammed material underneath the impactor, observable in normal stress as a function of axial strain. For small suspension volumes and large impact speeds, we find strain-hardening up until fracturing. This more brittle behavior results in a modulus that, just before crack formation, is an order of magnitude larger than in shear-jammed suspensions undergoing ductile deformation.

cond-mat.soft↗

arXiv2025

Non-reciprocity and multibody interactions in acoustically levitated particle systems: A three body problem

In active fluids and active solids the constituents individually generate movement by each extracting energy from their environment or from their own source. Non-reciprocal interactions among these active constituents then enable novel collective behavior that often can be strikingly counterintuitive. However, non-reciprocity in these cases typically requires that the interacting bodies have different physical properties or it needs to be programmed explicitly into all pairwise interactions. Here we show that collective activity in a driven system can emerge spontaneously through multibody nonreciprocal forces, even if all bodies are individually non-active and have identical properties. We demonstrate this with as few as three identical spheres, acoustically levitated in air, which exhibit collective activity as they interact through non-pairwise forces: similar to the classic gravitational three-body problem, the interaction between two spheres depends sensitively on the relative position of the third sphere. Non-reciprocity arises naturally from both near-field sound scattering and microstreaming forces among the spheres. The underdamped dynamics in air furthermore make it possible to go beyond collective center-of-mass propulsion or rotation and observe internal, engine-like reconfigurations that follow limit cycles. These findings open up new possibilities for self-assembly, where now multibody interactions not only determine the resulting structure but also drive the spontaneously emerging dynamics.

cond-mat.soft↗

arXiv2025

Dense suspensions as trainable rheological metafluids

Memory-forming properties introduce a new paradigm to the design of adaptive materials. In dense suspensions, an adaptive response is enabled by non-Newtonian rheology; however, typical suspensions have little memory, which implies rapid cessation of any adapted behavior. Here we show how multiple adaptive responses can be achieved by designing suspensions where different stress levels trigger different memories. This is enabled by the interplay of interactions based on frictional contact and dynamic chemical bridging. These two interactions lead to novel rheology with several well-delineated shear thinning and thickening regimes, which enable stress-activated memories associated with opposite time-dependent trends. As a result, in response to different stress levels, the suspension can evolve by either softening or stiffening and is trainable, exhibiting targeted viscosity and energy dissipation with repeated low-velocity impact. Such behavior, usually associated with mechanical metamaterials, suggests that dense suspensions with multiple memories can be viewed as trainable rheological metafluids.

cond-mat.soft↗

arXiv2025

Strain stiffening due to stretching of entangled particles in random packings of granular materials

Stress-strain relations for random packings of entangling chains under triaxial compression can exhibit strain stiffening and sustain stresses several orders-of-magnitude beyond typical granular materials. X-ray tomography reveals the transition to this strong strain stiffening occurs when chains are long enough to entangle an average of about one chain each, which results in system-filling clusters of entangled chains. The number of entanglements is nearly proportional to the area surrounded by entangling particles with an excluded volume effect. A tendency was found for chain links to stretch when the packing was strained. The slope of the stress-strain relation of the packing can be calculated from a mean-field model consisting of the product of the effective extensional modulus of the chain, packing fraction, probability of stretched links, and the ratio of strain of stretched links to packing strain. The stress-strain model requires as input measurements of the ratio between local particle deformation and global average strain, and the probability of stretching for non-rigid particles. This results in a quadratic prediction for the stress-strain curve, with a curvature that agrees with experiments within the model uncertainties. This model explains that the strength of these packings comes from stretching of the links of chains, but only when the system-filling network of entanglements provides constraints that prevents failure by shear banding, so that particles must be deformed to move further under strain. In this model, the increasing slope of the stress-strain curve is mainly due to the fraction of stretched links increasing with strain. This model for the stress-strain relation is shown to be generalizable to different shapes of entangling particles by applying it to staples.

cond-mat.soft↗

arXiv2025

On the addition of micron-size intruders in a shear-thickening suspension of nanoparticles

This study investigates the rheological behavior of shear-thickening suspensions made of different types of nanoparticles upon the addition of large intruders referred to as granules. The size ratio ranges from 20 to 120. We examine the effects of granule size, volume fraction, and surface properties on shear-thickening characteristics. Starting with a fumed silica suspension exhibiting discontinuous shear thickening (DST) without granules, the addition of granules at different volume fractions, shifts the onset of thickening to lower shear rates. Concomitantly, the strength of the thickening, quantified by the thickening index, decreases, transitioning from DST to continuous shear thickening (CST). Comparison with suspensions of nanosilica spheres reveals a similar trend, suggesting generality across different systems. However, these results contrast with cornstarch-based suspensions, where granule addition enhances thickening. This difference is attributed to the large size ratio studied here: When the granules are much larger than the particles in the interstitial suspension, the granules introduce a spread in the local shear rate and disrupt the particles' ability to form an extended fabric of force chains. The findings highlight the critical role of particle size ratio in determining the rheology of complex suspensions, paving the way for tailoring material properties in industrial and scientific applications.

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arXiv2024

Tunable mechanical properties and air-based lubrication in an acoustically levitated granular material

Cohesive granular materials are found in many natural and industrial environments, but experimental platforms for exploring the innate mechanical properties of these materials are often limited by the difficulty of adjusting cohesion strength. Granular particles levitated in an acoustic cavity form a model system to address this. Such particles self-assemble into free-floating, quasi-two-dimensional raft structures which are held together by acoustic scattering forces; the strength of this attraction can be changed simply by modifying the sound field. We investigate the mechanical properties of acoustically bound granular rafts using substrate-free micro-scale shear tests. We first demonstrate deformation of rafts of spheres and the dependence of this deformation on acoustic pressure. We then apply these methods to rafts composed of anisotropic sand grains and smaller spheres, in which the smaller spheres have a thin layer of air separating them from other grain surfaces. These spheres act as soft, effectively frictionless particles that populate the interstices between the larger grains, which enables us to investigate the effect of lubricating the mixture in the presence of large-grain cohesion.

cond-mat.soft↗

arXiv2024

Shear thickening inside elastic open-cell foams under dynamic compression

We measure the response of open-cell polyurethane foams filled with a dense suspension of fumed silica particles in polyethylene glycol at compression speeds spanning several orders of magnitude. The gradual compressive stress increase of the composite material indicates the existence of shear rate gradients in the interstitial suspension caused by wide distributions in pore sizes in the disordered foam network. The energy dissipated during compression scales with an effective internal shear rate, allowing for the collapse of three data sets for different pore-size foams. When scaled by this effective shear rate, the most pronounced energy increase coincides with the effective shear rate corresponding to the onset of shear thickening in our bulk suspension. Optical measurements of the radial deformation of the foam network and of the suspension flow under compression provide additional insight into the interaction between shear thickening fluid and foam. This optical data, combined with a simple model of a spring submerged in viscous flow, illustrates the dynamic interaction of viscous drag with foam elasticity as a function of compression rate, and identifies the foam pore size distribution as a critically important model parameter. Taken together, the stress measurements, dissipated energy, and relative motion of the fluid and the foam can be rationalized by knowing the pore size distribution and the average pore size of the foam.

cond-mat.soft↗

arXiv2024

Direct measurement of forces in air-based acoustic levitation systems

Acoustic levitation is frequently used for non-contact manipulation of objects and to study the impact of microgravity on physical and biological processes. While the force field produced by sound pressure lifts particles against gravity (primary acoustic force), multiple levitating objects in the same acoustic cavity interact via forces that arise from scattered sound (secondary acoustic forces). Current experimental techniques for obtaining these force fields are not well-suited for mapping the primary force field at high spatial resolution and cannot directly measure the secondary scattering force. Here we introduce a method that can measure both acoustic forces in situ, including secondary forces in the near-field limit between arbitrarily shaped, closely spaced objects. Operating similarly to an atomic force microscope, the method inserts into the acoustic cavity a suitably shaped probe tip at the end of a long, flexible cantilever and optically detects its deflection. This makes it possible to measure forces with a resolution better than 50 nN, and also to apply stress or strain in a controlled manner to manipulate levitated objects. We demonstrate this by extracting the acoustic potential present in a levitation cavity, directly measuring the acoustic scattering force between two objects, and applying tension to a levitated granular raft of acoustically-bound particles in order to obtain the force-displacement curve for its deformation.

cond-mat.soft↗

arXiv2024

Shear thickening in suspensions of particles with dynamic brush layers

Control of frictional interactions among liquid-suspended particles has led to tunable, strikingly non-Newtonian rheology via the formation of strong flow constraints as particles come into close proximity under shear. Typically, these frictional interactions have been in the form of physical contact, controllable via particle shape and surface roughness. We investigate a different route, where molecular bridging between nearby particle surfaces generates a controllable "sticky" friction. This is achieved with surface-functionalized colloidal particles capable of forming dynamic covalent bonds with telechelic polymers that comprise the suspending fluid. At low shear stress this results in particles coated with a uniform polymer brush layer. Beyond an onset stress the telechelic polymers become capable of bridging and generate shear thickening. Over the size range investigated, we find that the dynamic brush layer leads to dependence of the onset stress on particle diameter that closely follows a power law with exponent -1.76. In the shear thickening regime, we observe an enhanced dilation in measurements of the first normal stress difference and reduction in the extrapolated volume fraction required for jamming, both consistent with an effective particle friction that increases with decreasing particle diameter. These results are discussed in light of predictions for suspensions of hard spheres and of polymer-grafted particles.

cond-mat.soft↗

arXiv2024

Rheology of bidisperse non-Brownian suspensions

We study the rheology of bidisperse non-Brownian suspensions using particle-based simulation, mapping the viscosity as a function of the size ratio of the species, their relative abundance, and the overall solid content. The variation of the viscosity with applied stress exhibits shear thickening phenomenology irrespective of composition, though the stress-dependent limiting solids fraction governing the viscosity and its divergence point are non-monotonic in the mixing ratio. Contact force data demonstrate an asymmetric exchange in dominant stress contribution from large-large to small-small particle contacts as the mixing ratio of the species evolves. Combining a prior model for shear thickening with one for composition-dependent jamming, we obtain a full description of the rheology of bidisperse non-Brownian suspensions capable of predicting effects such as the viscosity reduction observed upon adding small particle fines to a suspension of large particles.

cond-mat.soft↗

arXiv2024

Training physical matter to matter

Biological systems offer a great many examples of how sophisticated, highly adapted behavior can emerge from training. Here we discuss how training might be used to impart similarly adaptive properties in physical matter. As a special form of materials processing, training differs in important ways from standard approaches of obtaining sought after material properties. In particular, rather than designing or programming the local configurations and interactions of constituents, training uses externally applied stimuli to evolve material properties. This makes it possible to obtain different functionalities from the same starting material (pluripotency). Furthermore, training evolves a material in-situ or under conditions similar to those during the intended use; thus, material performance can improve rather than degrade over time. We discuss requirements for trainability, outline recently developed training strategies for creating soft materials with multiple, targeted and adaptable functionalities, and provide examples where the concept of training has been applied to materials on length scales from the molecular to the macroscopic.

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