Global Performance Indices for Dynamic Crystals as Organic Thermal Actuators

Abstract

same basic thermodynamic principles of energy transduction and are collectively referred to as actuators. [1][2][3][4][5][6][7][8][9][10] While complex natural biomechanical systems have evolved to aid functions such as dispersal and survival, the artificially manufactured actuators are specifically designed for integration in mechatronic or adaptronic systems that range from miniature devices such as navigation and positioning systems in flying machines, to complex systems that can perform tasks in specific environments, such as humanoid robots and spacecraft. The current applications of actuators are many and diverse; they include sensors, [11,12] artificial muscles, [13] soft robotics, [14] and energy-harvesting materials. [15] Simple or complex actuating motions can be induced by heat, light, electricity, pressure, and moisture, among other stimuli, and can be realized with shape-memory polymers, [16] conductive fibers, [17] liquid crystal elastomers, [18] piezoelectric, [19] magnetostrictive, [20] and electromagnetic materials. [21] The energy supplied to the artificial actuators can vary from fluid pressure or flow in the pneumatics, to current, charge or voltage in the electromagnetic, piezoelectric and magnetostrictive actuators, to heat in shape memory and thermal expansion actuators.The active media in actuators include fluids, hard solids such as metals, and even complex soft matter such as tissuesensembles of cells with similar structure that act together to perform a specific function. While molecular crystals do not immediately appeal as working actuating medium, their dynamic properties have recently transpired as an alternative to other materials and they are thought to hold an untapped potential for miniature, soft actuating devices, quickly shaping up into a new research field, crystal adaptronics. [22,23] Smallscale demonstrations of the usefulness of molecular crystals as microscale devices, including cantilevers, [24,25] ratchets, [26] optical waveguides, [27][28][29][30][31][32][33][34][35][36] and other "crystal gears" [37] that can move or reshape are occasionally reported for dynamic crystals. Although being very illustrative, these demonstrations have thus far remained limited to a laboratory scale and have not been implemented in actual, functional devices. Some of the obstacles toward wider applications are difficulties with reproducible growth of crystals of predictable size without the use Crystal adaptronics is an emergent materials science discipline at the intersection of solid-state chemistry and mechanical engineering that explores the dynamic nature of mechanically reconfigurable, motile, and explosive crystals. Adaptive molecular crystals bring to materials science a qualitatively new set of properties that associate long-range structural order with softness and mechanical compliance. However, the full potential of this class of materials remains underexplored and they have not been considered as materials of choice in an engineer's toolbox. A set of general performance metric...

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