Session G25: Mechanical Metamaterials III / Physics of Liquids I

Inflatable Kirigami Structures

Kirigami inspired metamaterials have shown increasing potential in science and engineering due to their ability to achieve large deformations and morphology changes. Here, by embedding a thin kirigami shell into a soft silicon rubber, we present a novel, programmable and inflatable kirigami structure that can deform into a desired shape or trajectory at a given pressure. Moreover, we can easily assemble different actuators together to obtain very complex deformations via a modular design. Sequencing can also be achieved by introducing pressure drops in between the different modules, and feeding a pressure profile as a single input to the actuators. This could potentially benefit future designs for climbing robots and robotic arms, where disposing of multiple outputs given a single input is often necessary. Our study provides a simple, yet powerful strategy to design unprecedented kirigami balloons that will enrich the functionalities of inflatable structures.   

Capillarity-driven Transformation of Microscopic Cellular Structures

Cellular structures are attracting increasing interest because of their unique mechanical, thermal, electrical, and acoustic properties, which largely depend on their size, shape and topology. As such, systems with tunable functionality can be constructed by tuning the geometry of the structures. Here we report a capillarity-driven transformation on 2D onsite microcellular structures that enables us to change their topology on demand and reversibly. To trigger such transformation both the cellular structure and the material have to be carefully designed, as it requires the formation of capillary menisci upon solvent evaporation at the unit-cell scale as well as the softening and stiffening of the material at the molecular scale. Finally, we show that our strategy can be applied to realize surface with tunable properties including adhesion, friction, hardness, and particle trapping.   

Multiscale frequency conversion through input-independent dynamics of bistable lattices

In this study, we extend the input-independent dynamics to higher-dimensional metastructures. A metabeam is constructed by integrating a bistable lattice along a beam-like outer frame. We obtain frequency response diagrams showing output frequency spectra for each input frequency and observe nonlinear out-of-plane behavior as long as transition waves are triggered along the in-plane bistable lattice direction. Similar to the observations in its one-dimensional counterpart, the transverse output frequencies remain coherent around a single dominant frequency regardless of the input frequency that triggers the transition waves. This result shows transfer of energy between two different length scales – localized in-plane waves and global out-of-plane deflections – and implies that such metabeams display efficient multiscale frequency conversion. Also identified is two qualitatively different routes to frequency conversion depending on the discreteness of the bistable lattice, enabling a greater design freedom. Furthermore, the unit cell design can be easily tuned to alter the metabeam properties, especially in terms of the operating frequency range and output frequency, allowing for a broad range of engineering applications.   

Tunable vibro-acoustic metamaterials

Phononic crystals and acoustic metamaterials are architected lattices designed to control the propagation of acoustic or elastic waves. In these materials, the dispersion properties and the energy transfer are controlled by selecting the geometry of the lattices and their constitutive material properties. Most designs, however, only affect one mode of energy propagation, transmitted either as acoustic airborne sound or as elastic structural vibrations. Here, we present a design methodology to attenuate both acoustic and elastic waves simultaneously in all polarizations. We experimentally realize a three-dimensional load-bearing architected lattice, composed of a single material, that responds in a broadband frequency range in all directions and polarizations for airborne sound and elastic vibrations simultaneously. In addition, we show how we can tune the vibro-acoustic response of the metamaterial through external stimuli.   

Exotic Soft Modes in 2D Mechanical Metamaterials Yield Powerful New Analytic Prediction Methods

Maximally Auxetic behavior, where Poisson’s ratio is the most negative, has been explored and identified in 2D perforated elastic sheets in which rigid square elements are connected at the corners by comparatively flexible elastic “hinges”. While these metamaterials are designed to emulate a uniform zero-energy motion of the free hinge material (mechanism), experiments have revealed qualitatively different non-uniform mechanical response. To understand this, we utilize a coarse graining approach, combined with highly detailed finite element simulations and experiments, to reveal that the perforated elastic sheet mechanics is controlled by a novel set of soft modes that correspond precisely to the well-studied planar Conformal Maps. We exploit this very convenient result to demonstrate new and highly accurate methods of analytically solving for linear and non-linear deformations of real materials. This includes a powerful holographic approach, in which large non-linear deformations may be predictably controlled by simple actuation at the boundary. Finally, we introduce a more general methodology for identifying and controlling the soft modes associated with a broad class of 2D mechanisms including the Miura and Eggbox origami patterns.   

Asymmetrical Reflection in Passive Non-Hermitian Structure

Alternating a lossy with a near-lossless material within a structure can lead to remarkable asymmetry in wave propagation. The field of passive (without control of gain and loss by external stimuli) non-Hermitian acoustics is quickly emerging. Here, we propose a simple system comprised of a single pair of lossless (metallic) and lossy (polymeric) resonators in the form of circular ~0.1 mm thin sheets – that demonstrates close to 100 % asymmetry in the acoustic reflection coefficient, while at the same time the transmission coefficients are identical. Experimental measurements and theoretical modeling over a wide range of audible frequencies (reflection and transmission coefficients from 1 kHz up to 6.3 kHz) reveal a nearly ideal sonic unidirectional reflector. Such asymmetric acoustic properties are valuable, for example, for concealing objects from sonars, while allowing communication.   

A universal identity for the Poisson ratios of oblique Miura-ori

Certain origami and origami-like tessellations, such as the Miura-ori and the eggbox pattern, remarkably exhibit equal and opposite in-plane and out-of-plane Poisson ratios. In this talk, we propose and prove a generalization of this identity to all tessellations that can be obtained as the translation surface of one zigzag along another. These include in particular the entire family of oblique, i.e., non-orthotropic, Miura-ori. The proof is based on a perturbative scheme and makes some typical assumptions of rigid folding kinematics; it remains valid for small and large deformations whether the underlying tessellation is symmetrical, developable, flat-foldable, or not.   

Toward a microscopic understanding of the dynamics of simple glass-forming liquids

The dynamical arrest predicted by mode-coupling theory and the entropy crisis at the random first-order transition are both exact descriptions of simple glass-forming liquids, albeit only in abstract, infinite-dimensional systems. What survives of these features and what other processes come into play in three-dimensional glass formers are questions that remain largely unanswered. In this talk, I present our recent advances toward a microscopic understanding of the finite-dimensional echo of the infinite-dimensional transitions, and of some of the activated processes that affect the dynamical slowdown of simple yet realistic glass formers.   

Unifying the percolation and mean-field description of the random Lorentz gas

The random Lorentz gas (RLG) is a minimal off-lattice model of transport in porous media. It is also a minimal model of structural glasses. The exact mean-field, infinite-dimensional limit solution of the RLG indeed predicts a discontinuous dynamical caging transition akin to that of simple liquid glass formers. The RLG, however, is also in the percolation universality class, in which the dynamical caging transition is continuous. These two descriptions are thus fundamentally contradictory. To resolve this paradox, we study the caging regime of the RLG as a function of dimension. We find that the static cage size quantitatively matches the mean-field predictions, and that the percolation transition and the (finite-dimensional echo of the) dynamical transition are clearly distinguishable in all dimensions. As dimension increases, however, the system dynamics grows increasingly controlled by the dynamical transition. In fact, the escape time from the dynamical cage grows exponentially quickly with d, thus resolving the paradox. More significantly, cage escape events are similar to hopping processes, which had mostly eluded theoretical description in standard supercooled liquids.   

Unveiling the connection between liquid water and its amorphous/glass states

Water is unique in many ways. One of the most remarkable peculiarities of water is its polyamorphism, i.e., its capability to acquire more than one amorphous state. The very nature of amorphous ices is still highly debated, but even less is known about their connection to the liquid state. The possibility of a continuous thermodynamic link between the supercooled liquid(s) and the amorphous ices is highly debated. On the other hand, probing such connection is a challenging task, mostly because of the lack of theoretical and/or experimental tools able to account for the high degeneracy of local configurations in statistically isotropic materials.
In this work, we bypass these difficulties by proposing a new strategy that explores structural similarity between the different disordered configurations. We combine GPU-accelerated molecular dynamics simulations with a neural network approach that allows us to clarify the connections between equilibrium liquid water and its amorphous/glass states. Moreover, our results also confirm that liquid water can be interpreted as a two state system, an hypothesis that was always considered at the theoretical level and that is at the heart of water anomalies.   

Dynamical theory predicted correlation between activated relaxation and thermodynamics in glass-forming liquids

The microscopic Elastically Collective Nonlinear Langevin Equation (ECNLE) theory of glassy dynamics, in conjunction with an a priori mapping of thermal liquids to an effective hard sphere fluid, captures the structural relaxation time of nonpolar molecular liquids over 14 decades. We re-visit this theory for monodisperse hard spheres using the modified-Verlet integral equation theory closure as equilibrium input. Comparison with simulation shows the equation-of-state, static correlation lengths, radial distribution function, and structure factor are remarkably well captured up to very high volume fractions. Numerical ECNLE theory calculations then reveal that the logarithm of the alpha time scales behaves as an inverse power law of the dimensionless compressibility which is a thermodynamic property that quantifies the amplitude of long wavelength density fluctuations. The scaling is linear (cubic) in the low (high) barrier regime, establishing an operational link between glassy relaxation and thermodynamics. The predicted connection is directly tested using solely experimental data, and is well verified for molecular liquids. By introducing one adjustable parameter to capture the low to high barrier crossover, experimental data over 14 decades can be linearized.   

How does the character of the Sastry transition depend on the range of interatomic interactions?

The mean pressure <P_IS(ρ)> within the inherent structures (IS) of liquids maintained at density ρ and temperature T >> Τ_melt is minimized at the “Sastry” density ρ_S. For ρ < ρ_S (ρ > ρ_S), these IS are typically cavitated (homogenous). Using molecular dynamics simulations of Mie liquids [U_n(r) = ε(r^-2n - 2r^-n) with 4 ≤ n ≤ 12], we determine how the character of the transition between cavitated and homogeneous IS depends on the range of the interatomic pair potential. We find three principal results: (i) As n increases with T held fixed, ρ_S approaches and then exceeds the crystallization density ρ_X; (ii) as a consequence, the portion of systems’ phase diagrams (in terms of P and T) where systems are liquids with predominantly cavitated IS decreases with increasing n; (iii) For ρ > ρ_S, the pair correlation functions g(r) of the liquids’ IS are nearly conformal, i.e. the g(r) for different n nearly map to a common reduced pair correlation function G(ρ^1/3r) = ρ^-1/3g(r). We discuss the implications of these results for condensation and cavitation of simple atomic liquids.