arXiv subjects

Stefano Gonella

Publications and source records attributed to Stefano Gonella.

At least 19 recordsLinked to original sources

arXiv2026

Centipede-Like Metastrip Enables On-Demand Programmable Droplet Motion

We demonstrate programmable, frequency-tunable and size-selective drop motion on an elastic metastrip substrate with a centipede-like array of cantilever resonators. Under harmonic excitation, the strip experiences simultaneously an in-plane (IP) collective motion and out-of-plane (OOP) deformation, thus establishing a frequency-dependent IP-OOP phase landscape. This prescribes the direction of motion to the drops on the strip based on their position, causing the emergence of clustering and rarefaction regions. By tuning the tip masses of the resonators, we open reconfigurable OOP bandgaps that modify the IP-OOP phase makeup and locally suppress drop motion, contributing an additional layer of spatial selectivity. Using multi-frequency excitations, we selectively actuate drops based on their resonances, effectively filtering them by volume and position.

cond-mat.soft↗

arXiv2026

Giant Flat Band Amplification via Inertial Anchors

In electronic materials, flat bands are associated with compact electron localization, with implications for superconductivity, ferromagnetism and strongly correlated systems. The physical significance of their counterparts in elastic media is far less charted. Here we report a strategy to achieve elastic flat bands through an inertial retrofitting of classical lattice architectures. The idea is to alter the cell geometry to realize a network of inertial anchors, effectively partitioning the lattice into an array of weakly coupled emergent resonators, whose resonances appear as flat bands in the phonon spectrum. We demonstrate flat-band conditions that combine localized and extended state attributes and induce a giant response that is spatially and temporally persistent. Laser vibrometry experiments reveal three signatures of this mechanism: amplification up to two orders of magnitude compared to pass band and band gap conditions, multi-cell activation that is agnostic to the source location, and a persistent transient response even after several excitation cycles.

cond-mat.mtrl-sci↗

arXiv2026

Boosting lattice polarization Mixing the perspectives of geometry optimization and cell-augmentation

Topologically polarized mechanical metamaterials enjoy a special built-in asymmetry that manifests as a preferred localization of edge states on selected edges. While this property has been shown for a few ideal Maxwell lattices, we currently lack systematic criteria to design families of structural systems exhibiting polarization. Here, we propose a framework to design polarized structural configurations enabled by topology optimization (TO), using both band and mode morphological properties as drivers of the optimization algorithm. Through the lens of TO, we are able to tap into a vast design space, unlocking geometric freedom far beyond what is achievable with canonical lattice architectures. At the same time, we elucidate important criteria that need to be satisfied, beyond the optimization outcome, to ensure robustness of the achieved polarization against perturbations of the edge morphology. These results provide the inspiration to loop back into the realm of ideal lattices in search of new configurations characterized by extreme polarization. The peculiar shape and connectivity of the TO-generated lattice offer a blueprint for identifying a new family of Maxwell trusses based on augmented kagome geometry. We demonstrate the achievement of strong polarization signatures up to a three-count edge state mismatch. For all the cases studied, we show agreement between theory, simulations, and experiments, which include laser vibrometry wave measurements on a waterjet-cut specimen and static tests on a 3D-printed prototype.

cond-mat.mtrl-sci↗

arXiv2025

Drops on architected elastic substrates: A repertoire of regimes at the turn of a knob

Drops on a vibrating substrate can experience a variety of motion regimes, including directional motion and climbing. The key ingredient to elicit these regimes is simultaneously activating the in-plane and out-of-plane degrees of freedom of the substrate with the proper phase difference. This is typically achieved by using a rigid substrate and two independent actuators. However, this framework is unable to establish different motion conditions in different regions of the substrate, achieving spatial variability and selectivity, since this would violate the rigid-body assumption and require a proliferation of actuation channels. Challenging this paradigm, we leverage the inherent elasticity of the substrate to provide the modal and spatial diversity required to establish the desired regimes. To this end, we design deformable substrates exhibiting a rich landscape of deformation modes, and we exploit their multi-modal response to switch between drop motion regimes and select desired spatial patterns, using the excitation frequency as our tuning parameter.

physics.flu-dyn↗

arXiv2025

Lattice Materials with Topological States Optimized On-Demand

Topological states of matter, first discovered in quantum systems, have opened new avenues for wave manipulation beyond the quantum realm. In elastic media, realizing these topological effects requires identifying lattices that support the corresponding topological bands. However, among the vast number of theoretically predicted topological states, only a small fraction has been physically realized. To close this gap, we present a new strategy capable of systematically and efficiently discovering metamaterials with any desired topological state. Our approach builds on topological quantum chemistry (TQC), which systematically classifies topological states by analyzing symmetry properties at selected wavevectors. Because this method condenses the topological character into mathematical information at a small set of wavevectors, it encodes a clear and computationally efficient objective for topology optimization algorithms. We demonstrate that, for certain lattice symmetries, this classification can be further reduced to intuitive morphological features of the phonon band structure. By incorporating these band morphology constraints into topology optimization algorithms and further fabricating obtained designs, we enable the automated discovery and physical realization of metamaterials with targeted topological properties. This methodology establishes a new paradigm for engineering topological elastic lattices on demand, addressing the bottleneck in material realization and paving the way for a comprehensive database of topological metamaterial configurations.

cond-mat.mtrl-sci↗

arXiv2025

Robustness of stress focusing in soft lattices under topology-switching deformation

Recent developments in topological mechanics have demonstrated the ability of Maxwell lattices to effectively focus stress along domain walls between differently polarized domains. The focusing ability can be exploited to protect the lattice bulk from accidental stress concentration -- and eventually onset and propagation of fracture -- at structural hot spots such as defects and cracks. A recent study has revisited the problem for structural lattices featuring non-ideal hinges, showing that the focusing remains robust, albeit diluted in strength. Realizing that the problem of domain wall localization has been traditionally framed in the context of linear elasticity, in this work we extend the study to the realm of soft structures undergoing nonlinear finite deformation. Through experiments performed on silicone hyperelastic prototypes, we assess and quantify the robustness of the phenomenon against the macroscopic shape changes induced by large deformation, with special attention to deformation levels that alter the topology of the bulk, lifting the topological protection. Furthermore, we identify a simple geometric indicator for this transition.

cond-mat.soft↗

arXiv2025

Non-local twist sequences in floppy kagome chains

The twisted kagome family comprises a spectrum of configurations that can be realized through the sweep of a single configurational degree of freedom known as a twist angle. Recently, it was shown that certain pairs of configurations along this sweep were linked by duality transformations and displayed matching phonon spectra. In this work, we introduce an intercell-connection system that spreads the lattice in the dimension orthogonal to the tessellation plane. The resulting three-dimensional character of the lattice allows us to sweep the entirety of the twist-angle spectrum, including all the compact configurations featuring overlapping triangles that, in a strictly two-dimensional space, are forbidden. Duality provides precious guidance for interpreting the availability of floppy mechanisms arising in the compact configurations through the one-to-one correspondence with their expanded counterparts. Our focus is on the compact configuration corresponding to a null twist angle, where the lattice degenerates to a chain. From the perspective of the chain, several of the local connections between neighboring lattice cells play the role of nonlocal long-range interactions between cells of the chain. We demonstrate experimentally some peculiar behavior that results from such nonlocality, including a selective activation of floppy sequences that is informed by the direction of loading.

cond-mat.mtrl-sci↗

arXiv2025

Omnidirectional domain wall modes protected by fragile topological states

So-called fragile topological states of matter challenge our conventional notion of topology by lacking the robustness typically associated with topological protection, thereby displaying elusive manifestations that are difficult to harness for wave control. In this Letter, we leverage the recent discovery of fragile topological states in special classes of structural kagome lattices to document the availability of domain wall elastic wave modes that are directly traceable to fragile topology and, yet, exhibit remarkably strong signatures that support omni-directionality. We design twisted kagome bi-domains comprising two topologically distinct sublattices - one trivial and the other fragile topological - sharing a common bandgap and meeting at a domain wall. The two phases are achieved via carefully engineered surface cut patterns that modify the band landscape of the underlying lattices in complementary fashions, leading to dichotomous irreps landscapes. Under these circumstances, a domain wall-bound mode emerges within the shared bandgap and displays remarkable stability against domain wall orientation and introduced defects. We corroborate these findings via $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian and Jackiw-Rabbi analysis and validate them experimentally through laser vibrometry tests on a prototype endowed with water jet-induced perforation patterns.

cond-mat.mtrl-sci↗

arXiv2025

Dynamics of self-dual kagome metamaterials and the emergence of fragile topology

Recent years have seen the discovery of systems featuring fragile topological states. These states of matter lack certain protection attributes typically associated with topology and are therefore characterized by weaker signatures that make them elusive to observe. Moreover, they are typically confined to special symmetry classes and, in general, rarely studied in the context of phononic media. In this Letter, we theoretically predict the emergence of fragile topological bands in the spectrum of a twisted kagome elastic lattice with three-fold rotational symmetry, in the so-called self-dual configuration. A necessary requirement is that the lattice is a structural metamaterial, in which the role of the hinges is played by elastic finite-thickness ligaments. The interplay between the edge modes appearing in the bandgaps bounding the fragile topological states is also responsible for the emergence of corner modes at selected corners of a finite hexagonal domain, which qualifies the lattice as a second-order topological insulator. We demonstrate our findings through a series of experiments via 3D Scanning Laser Doppler Vibrometry conducted on a physical prototype. The selected configuration stands out for its remarkable geometric simplicity and ease of physical implementation in the panorama of dynamical systems exhibiting fragile topology.

cond-mat.mtrl-sci↗

arXiv2024

Edge-selective reconfiguration in polarized lattices with magnet-enabled bistability

The signature topological feature of Maxwell lattices is their polarization, which manifests as an unbalance in stiffness between opposite edges of a finite domain. The manifestation of this asymmetry is especially dramatic in the case of soft lattices undergoing large nonlinear deformation under concentrated loads, where the excess of softness at the soft edge can result in the activation of sharp indentations. This study explores how this mechanical dichotomy between edges can be tuned and possibly extremized by working with soft magneto-mechanical metamaterials. The magneto-mechanical coupling is obtained by endowing the lattice sites with permanent magnets, which activate a network of magnetic forces that can interact with (either augmenting or competing with) the elasticity of the material. Specifically, under sufficiently large deformation that macroscopically alters the equilibrium positions of the sites, the attractive forces between the magnets can trigger bistable reconfiguration mechanisms. The strength of such mechanisms depends on the landscapes of elastic reaction forces exhibited by the edges, which are different due to the polarization, and is therefore inherently edge-selective. We show that, on the soft edge, the addition of magnets simply enhances the softness of the edge. In contrast, on the stiff edge, the magnets activate snapping mechanisms that locally reconfigure the cells and produce a lattice response reminiscent of plasticity, characterized by residual deformation that persists upon unloading.

cond-mat.mtrl-sci↗

arXiv2024

Programming droplet motion using metamaterials

Motion control of droplets has generated much attention for its applications to microfluidics, where precise control of small fluid volumes is an imperative requirement. Mechanical vibrations have been shown to be effective at inducing controllable depinning, and activation of different drop motion regimes. However, existing vibration-based strategies involve establishing homogeneous rigid-body dynamics on the substrate, and therefore lack any form of spatial heterogeneity and tuning. Addressing this limitation, metamaterials provide an ideal platform to achieve spectrally and spatially selective drop motion control, which leverages their ability to attenuate vibrations in selected frequency bands and in selected regions of a substrate. In this work, we illustrate the potential of metamaterials-based drop control by experimentally demonstrating a variety of drop motion capabilities on the surface of metaplates endowed with locally resonant stubs. The experiments leverage the design versatility of a LEGO component-enabled reconfigurable design platform and laser vibrometry measurements with high spatial resolution.

physics.flu-dyn↗

arXiv2023

Stress control in non-ideal topological Maxwell lattices via geometry

Topological mechanical metamaterials have demonstrated exotic and robust mechanical properties which led to promising engineering applications. One of such properties is the focusing of stress at the interface connecting domains of topological Maxwell lattices of opposite topological polarizations, which protects the bulk of the material against fracturing. Here we generalize this theory to non-ideal Maxwell lattices, incorporate real material features that leads to interactions beyond previous ideal models. By quantitative analysis of stress distributions of topological Maxwell lattices with self-stress interfaces theoretically and computationally, we propose a design rule that minimizes stress in the bulk of the material. This design rule can guide the realization of stress focusing and fracturing protection in real materials.

cond-mat.soft↗

arXiv2023

Cell augmentation framework for topological lattices

Maxwell lattices are characterized by an equal number of degrees of freedom and constraints. A subset of them, dubbed topological lattices, are capable of localizing stress and deformation on opposing edges, displaying a polarized mechanical response protected by the reciprocal-space topology of their band structure. In two dimensions, the opportunities for topological polarization have been largely restricted to the kagome and square lattice benchmark configurations, due to the non-triviality of generating arbitrary geometries that abide by Maxwell conditions. In this work, we introduce a generalized family of augmented topological lattices that display full in-plane topological polarization. We explore the robustness of such polarization upon selection of different augmentation criteria, with special emphasis on augmented configurations that display dichotomous behavior with respect to their primitive counterparts. We corroborate our results via intuitive table-top experiments conducted on a lattice prototype assembled from 3D-printed mechanical links.

cond-mat.mtrl-sci↗

arXiv2022

Stress focusing and damage protection in topological Maxwell metamaterials

Advances in the field of topological mechanics have highlighted a number of special mechanical properties of Maxwell lattices, including the ability to focus zero-energy floppy modes and states of self-stress (SSS) at their edges and interfaces. Due to their topological character, these phenomena are protected against perturbations in the lattice geometry and material properties, which makes them robust against the emergence of structural non-idealities, defects, and damage. Recent computational work has shown that the ability of Maxwell lattices to focus stress along prescribed SSS domain walls can be harnessed for the purpose of protecting other regions in the bulk of the lattice from detrimental stress concentration and, potentially, inhibiting the onset of fracture mechanisms at stress hot spots such as holes and cracks. This property provides a powerful, geometry-based tool for the design of lattice configurations that are robust against damage and fracture. In this work, we provide a comprehensive experiment-driven exploration of this idea in the context of realistic structural lattices characterized by non-ideal, finite-thickness hinges. Our experiments document the onset of pronounced domain wall stress focusing, indicating a remarkable robustness of the polarization even in the presence of the dilutive effects of the structural hinges. We also demonstrate that the polarization protects the lattice against potential failure from defected hinges and cracks in the bulk. Finally, we illustrate numerically the superiority of SSS domain walls compared to other trivial forms of reinforcements.

cond-mat.mtrl-sci↗

arXiv2022

Omnimodal topological polarization of bilayer networks: analysis in the Maxwell limit and experiments on a 3D-printed prototype

Periodic networks on the verge of mechanical instability, called Maxwell lattices, are known to exhibit zero-frequency modes localized to their boundaries. Topologically polarized Maxwell lattices, in particular, focus these zero modes to one of their boundaries in a manner that is protected against disorder by the reciprocal-space topology of the lattice's band structure. Here, we introduce a class of mechanical bilayers as a model system for designing topologically protected edge modes that couple in-plane dilational and shearing modes to out-of-plane flexural modes, a paradigm that we refer to as omnimodal polarization. While these structures exhibit a high-dimensional design space that makes it difficult to predict the topological polarization of generic geometries, we are able to identify a family of mirror-symmetric bilayers that inherit the in-plane modal localization of their constitutive monolayers whose topological polarization can be determined analytically. Importantly, the coupling between the layers results in the emergence of omnimodal polarization, whereby in-plane and out-of-plane edge modes localize on the same edge. We demonstrate these theoretical results by fabricating a mirror-symmetric, topologically polarized kagome bilayer consisting of a network of elastic beams via additive manufacturing and confirm this finite-frequency polarization via finite element analysis and laser-vibrometry experiments.

cond-mat.soft↗

arXiv2022

Soft mechanical metamaterials with transformable topology protected by stress caching

Maxwell lattice metamaterials possess a rich phase space with distinct topological states featuring mechanically polarized edge behaviors and strongly asymmetric acoustic responses. Until now, demonstrations of non-trivial topological behaviors from Maxwell lattices have been limited to either monoliths with locked configurations or reconfigurable mechanical linkages. This work introduces a transformable topological mechanical metamaterial (TTMM) made from a shape memory polymer and based on a generalized kagome lattice. It is capable of reversibly exploring topologically distinct phases of the non-trivial phase space via a kinematic strategy that converts sparse mechanical inputs at free edge pairs into a biaxial, global transformation that switches its topological state. Thanks to the shape memory effect, all configurations are stable even in the absence of confinement or a continuous mechanical input. Topologically-protected mechanical behaviors, while robust against structural (with broken hinges) or conformational defects (up to ~55% mis-rotations), are shown to be vulnerable to the adverse effects of stored elastic energy from prior transformations (up to a ~70% reduction in edge stiffness ratios, depending on hinge width). Interestingly, we show that shape memory polymer's intrinsic phase transitions that modulate chain mobility can effectively shield a dynamic metamaterial's topological response (with a 100% recovery) from its own kinematic stress history, an effect we refer to as "stress caching".

cond-mat.mtrl-sci↗

arXiv2021

Topological Flexural Modes in Polarized Bilayer Lattices

Topological lattices have recently generated a great deal of interest based on the unique mechanical properties rooted in their topological polarization, including the ability to support localized modes at certain floppy edges. The study of these systems has been predominantly restricted to the realm of in-plane mechanics, to which many topological effects are germane. In this study, we stretch this paradigm by exploring the possibility to export certain topological attributes to the flexural wave behavior of thin lattice sheets. To couple the topological modes to the out-of-plane response, we assemble a bilayer lattice by stacking a thick topological kagome layer onto a thin twisted kagome lattice. The band diagram reveals the existence of modes whose out-of-plane character is controlled by the edge modes of the topological layer, a behavior elucidated via simulations and confirmed via laser vibrometer experiments on a bilayer prototype specimen. These results open an alternative direction for topological mechanics whereby flexural waves are controlled by the in-plane topology, leading to potential applications for flexural wave devices with engineered polarized response.

physics.app-ph↗

arXiv2021

Bandgap tuning in kerfed metastrips under extreme deformation

The process of kerfing enables planar structures with the ability to undergo dramatic out-of-plane deformation in response to static loads. Starting from flat and stiff sheets, kerfing allows for the formation of a wide variety of unconventional free-form shapes, making the process especially attractive for architectural applications. In this work, we investigate numerically and experimentally the bandgap behavior of densely cut kerfed strips. Our study reveals a rich landscape of bandgaps that is predominantly ascribable to the activation of resonant sub-units within the kerf unit cells. We also document how the extreme deformability of the strips under twisting and bending loads, enhanced by the meandering cut pattern, can serve as a powerful bandgap tuning mechanism.

physics.app-ph↗