Session U28: Focus Session: Tunable Materials

Bukliball and Beyond: 3-D Soft Auxetic Metamaterials

We present a new class of 3-D soft metamaterials whose microstructure can be dramatically changed in response to mechanical loading. Patterned spherical shells, the Buckliballs (PNAS 109(16):5978) which undergo undergoing a buckling-induced structural transformation under pressure, are employed as building blocks, and are assembled to construct 3-D super-structures. We present procedures to guide the selection of both the building blocks and their arrangement, and design materials with tunable 3-D auxetic behavior that exploit buckling as the actuation mechanism. The validity of the proposed material design is demonstrated through both experiments and finite element simulations. This pattern transformation induced by a mechanical instability opens the possibility for fabrication of 3-D auxetic materials/structures over a wide range of length scales.   

Grayscale gel lithography: From umlti-strips to responsive origami

Non-uniform swelling of hydrogel sheets with two-dimensional (2D) patterns of crosslink density has the potential to yield a rich array of three-dimensional (3D) structures, yet many of the design rules remain poorly understood. Here, we study the geometrically simple case of ``multi-strips'', consisting of alternating parallel strips of high and low crosslink density. These materials are patterned using sequential UV exposure of a photo-crosslinkable polymer film through two photomasks. We show that these materials deform by rolling around the axis perpendicular to the interface between the regions, with a characteristic dimension that depends on the strip width and sheet thickness. However, beyond a critical minimum strip width, the material remains flat, instead forming an anisotropically swelled state that provides fruitful information on the contrast in modulus between the two regions. We also consider the deformation of sheets patterned with multiple regions that define geometrically incompatible rolling axis. Finally, we discuss the formation of hinges based on symmetric tri-strips that can be used to defined fold patterns, yielding responsive gel origami structures.   

Materials with Tailored Thermal Expansion Coefficient

Designing materials with tailored coefficient of thermal expansion (CTE) has applications in a number of fields, including biomedical and mechanical engineering and solar energy. It is particularly important to combine a desired (usually low) CTE with mechanical robustness. Most of previous work has been focused on designing low-CTE materials by modifying compounds at the chemical level. It is also possible to design materials with tailored CTE by using specific topologies of different materials to achieve overall properties outside the range of the constituent materials. Here, we exploit buckling in laminated periodic structures to design materials whose coefficient of thermal expansion can be tuned (from positive to negative) by varying the unit cell geometry.   

Buckligami: Actuation of soft structures through mechanical instabilities

We present a novel mechanism for actuating soft structures, that is triggered through buckling. Our elastomeric samples are rapid-prototyped using digital fabrication and comprise of a cylindrical shell patterned with an array of voids, each of which is covered by a thin membrane. Decreasing the internal pressure of the structure induces local buckling of the ligaments of the pattern, resulting in controllable folding of the global structure. Using rigid inclusions to plug the voids in specific geometric arrangements allows us to excite a variety of different fundamental motions of the cylindrical shell, including flexure and twist. We refer to this new mechanism of buckling-induced folding as ``buckligami.'' Given that geometry, elasticity and buckling are the underlying ingredients of this local folding mechanism, the global actuation is scalable, reversible and repeatable. Characterization and rationalization of our experiments provide crucial fundamental understanding to aid the design of new scale-independent actuators, with potential implications in the field of soft robotics.   

Shaping and morphing three dimensional structures using thin film stress

The spatial patterning and stimuli-responsive manipulation of mechanical stresses within thin films can be used to self-assemble static and reconfigurable materials and devices. I will discuss the utilization of stresses associated with the minimization of surface tension, the relaxation of polycrystalline films, and the differential cross-linking of polymers and hydrogels to realize assembly and reversible actuation of functional structures of importance in electronics, optics and medicine.   

Tunable phononic crystals through dielectric elastomers

Phononic crystals are periodic materials that display phononic band gaps -- frequency ranges in which elastic waves are prohibited. Through deformation of the periodic structure the frequency ranges of band gaps may be adjusted or new band gaps may be created. Phononic materials made from elastomers enable large reversible deformation and, as a result, significant tunability of the phononic properties. Dielectric elastomers may be used in phononic crystals, in which deformation is actuated through the application of an electrical voltage, opening the door for easily tunable phononic crystals. In order to realize these exciting capabilities, robust simulation and design tools are needed. We have developed finite-element technology to address this problem and have applied these tools to designing phononic crystals with band gaps tuned through the application of voltage. The key ingredients of our finite-element tools are (i) the incorporation of electro-mechanical coupling, (ii) large-deformation capability, and (iii) an accounting for inertial effects. We present an application of our simulation capability to the design of a phononic crystal consisting of a square array of circular-cross-section threads embedded in a dielectric elastomeric matrix.   

Tunable Mechanical Response in Biholar Elastic Media

We probe the mechanics of 2D and 3D elastic media that are structured with arrays of holes of two different sizes. Hole size ratio plays a crucial role for the mechanical response - allowing to tune the Poisson ratio and qualitative response of the material under uniaxial loading. Biaxial and triaxial loading of these biholar structures leads to a wealth of new phenomena, including mechanically switchable hysteresis and memory effects.   

Coupling geometrical frustration with mechanical instabilities to design surfaces with three dynamically changing states

The interplay between mechanical instabilities and non-linear deformation in soft, porous structures give us the exciting opportunities to design materials that can suddenly change from one shape to another in response to an external stimulus. Based on this approach, there have been an increasing number of studies demonstrating reversible pattern formation between two states. Inspired by triple-shape-memory polymers [1], here we show a new mechanism to generate three-state ordered pattern formation using athermal process by exploiting buckling and geometrical frustration of cellular structures. Our new approach allows dynamical switching among three successive states simply by varying the external stimuli. Moreover, our scale-independent mechanism based on geometry and mechanical instability can provide a unique opportunity for studying dynamics of complex pattern formation with tunable surface properties. Reference: [1] I. Belin, S. Kelch, R. Langer, and A. Lendlein, Proc. Natl. Acad. Sci. USA, 103, 18043-18047 (2006).   

Shape transformations in liquid crystal elastomers with complex microstructure

Recent experimental and theoretical studies have reported thermal-induced shape transformations in nematic liquid crystal elastomer (LCE) sheets with a complex director field. Director twist across the film thickness induces formation of twisted or curled structures whose chiral sense switches with temperature [1]. Using finite element simulations, we explore more complex director geometries that produce a variety of different actuation behaviors. We explore films containing a $+$1 topological defect with radial or azimuthal director alignment; and stripes and checkerboard patterns of twisted domains. We compare our results with recent experimental studies by D. Broer and coworkers and theoretical work by Modes and Warner. These results demonstrate the potential for application of LCE materials as mechanical actuators. [1] Y. Sawa, F. Ye, K. Urayama,~ T. Takigawa, V. Gimenez-Pinto, R. L. B. Selinger, and J. V. Selinger, PNAS 108, 6364 (2011).   

Soft 3-D Phononic Crystals: Design and engineering of the band-gap and propagation directionality

We present a new class of 3-D bi-continuous soft phononic crystals. Different solid-fluid inter-penetrating periodic micro-structures are proposed for the geometric configurations. Buckling and large deformation of the meta-material is intentionally exploited as a novel and very simple approach to tune and transform the phononic band gaps as well as the preferential propagation directions of acoustic and elastic waves. The nonlinear effects of both geometry and material behavior during the deformation are investigated. The dispersion relations of deformed phononic crystals are calculated by using frequency domain numerical simulations on the unit cell of spatial periodicity. The characteristics of soft phononic crystals are demonstrated with tunable band-gaps, adjustable directionality and adaptive refractive index. This study provides us with a deeper understanding of the design parameters and engineering guidelines for various potential applications, including sound filters in noise-cancelling devices, wave guides, acoustic imaging equipment and vibration isolators.   

Multifunctional Applications of Nanostructured Mechanical Metamaterials

Mechanical metamaterials have been shown to possess extraordinary properties, and thus have been of great interest to mathematicians, physical scientists, material scientists, and biologists. A large part of the study of materials science is to obtain new structure-property-function relationships needed for achieving optimized mechanical properties. Here, we demonstrate the potential to design and fabricate periodically ordered structures. These structures are shown to have a unique combination of stiffness, strength, and energy absorption, as well as damage tolerance. The results provide guidelines to advance the digital design (materials by design) and manufacturing concepts (advanced manufacturing) into the realm of engineered materials with desired properties and further to create multifunctional materials. For example, the periodic nature of the structures enables mechanically tunable band gap (phononic or photonic) materials, and tunable sensors in tissue engineering.   

Fluid-structure Interactions for the Design of Adaptive Acoustic Metamaterials

The present research focuses on the analysis of fluid-structure interactions as a new paradigm for the design of adaptive phononic crystals and acoustic metamaterials. Whereas in conventional design procedures couplings between structures and fluids represent a source of concern due to the possible onset of catastrophic instabilities, in this research such interactions are exploited as the enabling mechanism for mechanical adaptation. Analytical and numerical models illustrate how such interactions can be exploited for the design of periodic structures with wave propagation properties that can be controlled by the surrounding fluid environment. Analysis of the dispersion relations computed for one-dimensional phononic crystals and acoustic metamaterials show that the location of frequency bandgaps is directly correlated to the conditions of the external fluid flow. Direct simulations of assemblies of finite size and preliminary experimental results are presented to further illustrate the concept.