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John O | October 2017

Researchers create material that expands when heated to fill space

scientists from the university of pennsylvania (philadelphia) and the korea institute of science and technology (kist) in seoul have developed microbombs, materials that expand when heated to form microclusters that fill the space in which they are physically confined.


microbombs expanding to fill a physical space.
(korea institute of science and technology)


according to a report from the university, the expanding microclusters are lightweight and have “soft and adaptable boundaries.” the material could be used for thermal and acoustic insulation, electromagnetic interference (emi) shielding, and for particulate jamming used in robotics.


“the researchers prepared microwells from a rigid material that could not be deformed against the expansion of microbombs,” the report explained. “they then carefully heated the microbombs, causing them to expand, thinning the shell around the ‘bubble’ without fracturing it.”


the article continued, “using this strategy, the researchers were able to create microclusters with a wide variety of shapes, such as circles, triangles, squares, pentagons and hexagons, and partitions (from single to multiple units per cluster), edge profiles (from round to sharp corners) and hierarchy. they were able to transfer micro-nanopatterns onto the surface of the microclusters.”


scientists demonstrated that this process allows them to mold the pattern of the wall into the beads. future study of this material will include understanding the volumetric explosion of microbombs, how they react to different conditions, and how the atoms interact to better enhance jamming.


“by inscribing patterns on the microclusters, they hope to mimic complex structures in nature, such as the sahara ant’s hairs, which are hollow but triangular,” the article said. “the surface has corrugations on the top facets of the triangular prism and a flat bottom facing the ant’s body. the complex design allows the ant’s hair to effectively reflect the infrared light and keep the ant’s body cool in the hot desert sand.”


the research was recently published in nature communications. the abstract read:


“designing topographic clusters is of significant interest, yet it remains challenging as they often lack mobility or deformability. here we exploit the huge volumetric expansion (up to 3000%) of a new type of building block, thermally expandable microbombs.


“they consist of a viscoelastic polymeric shell and a volatile gas core, which, within structural confinement, create micro-clusters via inverse jamming and topographical close-packing. upon heating, microbombs anchored in rigid confinement underwent balloon-like blowing up, allowing for dense clusters via soft interplay between viscoelastic shells.


“importantly, the confinement is unyielding against the internal pressure of the microbombs, thereby enabling self-assembled clusters, which can be coupled with topographic inscription to introduce structural hierarchy on the clusters.


“our strategy provides densely packed yet ultralight clusters with a variety of complex shapes, cleavages, curvatures, and hierarchy. in turn, these clusters will enrich our ability to explore the assemblies of the ever-increasing range of microparticle systems.”

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