The properties of foams, an important class of cellular solids, are most sensitive to the volume fraction and openness of its elementary compartments; size, shape, orientation, and the interconnectedness of the cells are other important design attributes. Control of these morphological traits would allow the tailored fabrication of useful materials including highly porous solids, anisotropic heat conductors, tough composites, among others. While approaches like ice templating has produced foams with elongated cells, there is a need for rapid, versatile, and energy efficient methods that also control the local order and macroscopic alignment of cellular elements. Here we describe a fast and convenient method to obtain anisotropic structural foams using frontal polymerization. We fabricated foams by curing mixtures of dicyclopentadiene and a physical blowing agent via frontal ring opening metathesis polymerization (FROMP). The materials were characterized using micro-computed tomography and an image analysis protocol to quantify morphological characteristics including volume fraction and anisotropy. The cellular structure, porosity, and hardness of the foams changed with blowing agent, concentration, and resin viscosity. Moreover, we used a full factorial combination of variables to correlate each parameter with the structure of the obtained foams. We found a strong correlation between the resin viscosity and the foam's cellular structure. Furthermore, a specific combination of input parameters controlled the transitions from (i) isotropic to anisotropic cellular structures, (ii) porous to non-porous, and (iii) soft to hard foams. Our results demonstrate the controlled production of foams with specific morphologies using the simple and efficient method of frontal polymerization. This work shows promise for creating foams with aligned cellular structures that allow anisotropic mass and energy transport properties in high performance structural solids.

to survive throughout extreme hot dry or extremely humid weathers equally. [3] A truly fascinating example of stretchable camouflaging texture morphing skin is seen in cephalopods-the underwater invertebrates known as the masters of camouflage. These marine creatures control their skin morphology by stimulating cutaneous muscles known as papillae. These muscles generate complex texture patterns by pushing the overlying epidermal tissue upward and away from the mantle surface during their contraction. [4] Thanks to the large number of high-resolution texture muscles, cephalopods undergo complex morphology change. Overall, the skin of the cephalopod is a 3D display, where the papillae muscles control each voxel's extension on-demand by several millimeters out of the skin plane, create hierarchical textures, and collectively change the overall skin pattern. The generation of complex 3D shapes not only facilitates camouflage by pattern matching but also could enhance the swimming efficiency by controlling the hydrodynamic drag.The material systems required to achieve such morphological changes are extremely heterogenous and complex. The flexible and stretchable skin tissue of cephalopods is coupled to dermal shape-changing mechanisms, seamlessly embedded muscles, and integrated neurological sensing and control. A few texture and morphology change technologies inspired by cephalopods' papillae have been recently proposed. Wang et al. [5] produce on-demand fluorescent patterns using electroactive and mechanoresponsive elastomers, where high electric voltage (>50 kV mm −1 ) induces surface roughness of a millimeter. Current material developments are underway to reduce the required voltage and increase the materials voltage breakdown strength of these materials. Pikul et al. [6] use pneumatically actuated elastomeric membranes coupled to rigid mesh to achieve programmable 3D texture morphing. This material produces complex preprogrammed morphological camouflage, yet their broad applicability is limited by the need for heavy, rigid, and noisy air compressor.In this study, we produce a new type of stretchable skin based on electromechanical digital texture voxels (DTVs) to emulate the 3D morphing display of cephalopods papillae. The DTVs provide surface roughness, which actively changes its amplitude from the sub-millimeter to more than a centimeter and requires only 0.02 V mm −1 for actuation. These surface actuators undergo giant and reversible extensions exceeding 2000% strain within a few seconds. The DTVs are made from Smart skins capable of on-demand dynamic texture morphing are attractive for several applications, ranging from haptic feedback devices [1] to drag control in aerial or underwater vehicles. [2] There are countless inspiring examples of biological creatures, which intelligently morph their skin patterns to achieve multifunctionality. For example, the leaves of the silver tree (Leucadendron argenteum) morph their hair-like texture

The properties of foams, an important class of cellular solids, are most sensitive to the volume fraction and openness of its elementary compartments; size, shape, orientation, and the interconnectedness of the cells are other important design attributes. Control of these morphological traits would allow the tailored fabrication of useful materials including highly porous solids, anisotropic heat conductors, tough composites, among others. While approaches like ice templating has produced foams with elongated cells, there is a need for rapid, versatile, and energy efficient methods that also control the local order and macroscopic alignment of cellular elements. Here we describe a fast and convenient method to obtain anisotropic structural foams using frontal polymerization. We fabricated foams by curing mixtures of dicyclopentadiene and a physical blowing agent via frontal ring opening metathesis polymerization (FROMP). The materials were characterized using micro-computed tomography and an image analysis protocol to quantify morphological characteristics including volume fraction and anisotropy. The cellular structure, porosity, and hardness of the foams changed with blowing agent, concentration, and resin viscosity. Moreover, we used a full factorial combination of variables to correlate each parameter with the structure of the obtained foams. We found a strong correlation between the resin viscosity and the foam's cellular structure. Furthermore, a specific combination of input parameters controlled the transitions from (i) isotropic to anisotropic cellular structures, (ii) porous to non-porous, and (iii) soft to hard foams. Our results demonstrate the controlled production of foams with specific morphologies using the simple and efficient method of frontal polymerization. This work shows promise for creating foams with aligned cellular structures that allow anisotropic mass and energy transport properties in high performance structural solids.

The properties of foams, an important class of cellular solids, are most sensitive to the volume fraction and openness of its elementary compartments; size, shape, orientation, and the interconnectedness of the cells are other important design attributes. Control of these morphological traits would allow the tailored fabrication of useful materials including highly porous solids, anisotropic heat conductors, tough composites, among others. While approaches like ice templating has produced foams with elongated cells, there is a need for rapid, versatile, and energy efficient methods that also control the local order and macroscopic alignment of cellular elements. Here we describe a fast and convenient method to obtain anisotropic structural foams using frontal polymerization. We fabricated foams by curing mixtures of dicyclopentadiene and a physical blowing agent via frontal ring opening metathesis polymerization (FROMP). The materials were characterized using micro-computed tomography and an image analysis protocol to quantify morphological characteristics including volume fraction and anisotropy. The cellular structure, porosity, and hardness of the foams changed with blowing agent, concentration, and resin viscosity. Moreover, we used a full factorial combination of variables to correlate each parameter with the structure of the obtained foams. We found a strong correlation between the resin viscosity and the foam’s cellular structure. Furthermore, a specific combination of input parameters controlled the transitions from (i) isotropic to anisotropic cellular structures, (ii) porous to non-porous, and (iii) soft to hard foams. Our results demonstrate the controlled production of foams with specific morphologies using the simple and efficient method of frontal polymerization. This work shows promise for creating foams with aligned cellular structures that allow anisotropic mass and energy transport properties in high performance structural solids.

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