We present the results of an experimental investigation on the crystallography of the dimpled patterns obtained through wrinkling of a curved elastic system. Our macroscopic samples comprise a thin hemispherical shell bound to an equally curved compliant substrate. Under compression, a crystalline pattern of dimples selforganizes on the surface of the shell. Stresses are relaxed by both out-of-surface buckling and the emergence of defects in the quasihexagonal pattern. Three-dimensional scanning is used to digitize the topography. Regarding the dimples as point-like packing units produces spherical Voronoi tessellations with cells that are polydisperse and distorted, away from their regular shapes. We analyze the structure of crystalline defects, as a function of system size. Disclinations are observed and, above a threshold value, dislocations proliferate rapidly with system size. Our samples exhibit striking similarities with other curved crystals of charged particles and colloids. Differences are also found and attributed to the far-fromequilibrium nature of our patterns due to the random and initially frozen material imperfections which act as nucleation points, the presence of a physical boundary which represents an additional source of stress, and the inability of dimples to rearrange during crystallization. Even if we do not have access to the exact form of the interdimple interaction, our experiments suggest a broader generality of previous results of curved crystallography and their robustness on the details of the interaction potential. Furthermore, our findings open the door to future studies on curved crystals far from equilibrium. mechanical instabilities | curved surfaces | pattern formation | packing | defects T he classic design of a soccer ball, with its 20 hexagonal (white) patches interspersed with 12 (black) pentagons, the buckminsterfullerene C 60 (1), virus capsules (2), colloidosomes (3), and geodesic architectural domes (4) are all examples of crystalline packings on spherical surfaces. In contrast with crystals on flat surfaces, these structures cannot be constructed from a tiling of hexagons alone. Instead, disclinations--nonhexagonal elements such as the 12 pentagons on a soccer ball--are required by topology (5, 6), which constrains how the crystal order must comply with the geometry of the underlying surface. For example, seeding a hexagonal crystal with a pentagon (fivefold disclination) disrupts the perfect hexagonal symmetry and introduces a localized stress concentrator, which can be relaxed through out-ofplane deformation with positive Gaussian curvature (7,8). Likewise, a heptagon (sevenfold disclination) induces a disturbance with negative Gaussian curvature.An example of a physical realization of curved crystals is found in experiments on colloidal emulsions, where equally charged particles self-organize at the curved interface of two immiscible liquids (3, 9-11). These experiments build upon a wealth of previous theoretical and numerical investigations, as reviewed by Bowick...
Smart Morphable Surfaces enable switchable and tunable aerodynamic drag reduction of bluff bodies. Their topography, resembling the morphology of golf balls, can be custom-generated through a wrinkling instability on a curved surface. Pneumatic actuation of these patterns results in the control of the drag coefficient of spherical samples by up to a factor of two, over a range of flow conditions.
The elastocaloric cooling, utilizing latent heat associated with martensitic transformation in shape-memory alloys, is being considered in the recent years as one of the most promising alternatives to vapour compression cooling technology. It can be more efficient and completely harmless to the environment and people. In the first part of this work, the basics of the elastocaloric effect (eCE) and the state-of-the-art in the field of elastocaloric materials and devices are presented. In the second part, we are addressing crucial challenges in designing active elastocaloric regenerators, which are currently showing the largest potential for utilization of eCE in practical devices. Another key component of elastocaloric technology is a driver mechanism that needs to provide loading for active elastocaloric regenerators in an efficient way and recover the released energy during their unloading. Different driver mechanisms are reviewed and the work recovery potential is discussed in the third part of this work.
Structural fatigue is the major obstacle that prevents practical applications of the elastocaloric effect (eCE) in cooling or heat-pumping devices. Here, the eCE and fatigue behaviour of Ni-Ti sheets are systematically investigated in order to define the material's fatigue strain limit and the associated eCE. Initially, the eCE was evaluated by measuring adiabatic temperature changes at different strain amplitudes and different mean strains along the loading and unloading transformation plateaus. By comparing the eCE with and without pre-strain conditions, the advantages of cycling an elastocaloric material at the mean strain around the middle of the transformation plateau were demonstrated. In the second part of this work, we evaluated the fatigue life at the mean strain of 2.25% within the loading plateau and at the unloading plateau after initial pre-straining up to 6% and 10%, respectively. It is shown that on polished samples, durable operation of 10 5 cycles can be reached with a strain amplitude of 0.50% at the loading plateau, which corresponds to adiabatic temperature changes of approximately 5 K. At the unloading plateau (after initial pre-strain of 10%), durable 1 operation was reached at a strain amplitude of 1.00%, corresponding to adiabatic temperature changes of approximately 8 K. The functional fatigue was analysed after the cycling and it is shown that once the sample has been stabilized there is no further degradation of the eCE, even after 10 5 cycles. These results present guidelines for the design and operation of efficient and durable elastocaloric devices in the future.
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