Transforming material topology with a drop of liquid

(Nanowerk News) Networks of cells in nature have inspired researchers to develop their own materials made of interconnected microscopic circles, squares, triangles, and other shapes. The way cells in these materials are connected and arranged leads to novel energy transport and chemical reaction capabilities.
Biological materials constantly adapt by merging, fusing, and redefining the boundaries of their cells. In synthetic materials, existing methods can stretch or squash the cells, but researchers previously thought nothing could change the basic features of the cells without breaking it apart and rebuilding it from scratch.
This study (Nature, "Liquid-induced topological transformations of cellular microstructures") introduces a way to manipulate a material’s arrangement of cells at the microscale using a simple liquid.
triangular and hexagonal lattice
A drop of liquid applied to the center of a stiff triangular lattice softens and zips together the walls to transform the material into a hexagonal lattice. (Image: Joanna Aizenberg, Harvard University)
Researchers previously considered synthetic materials topology to be unchangeable. Topology explores how a material retains certain geometric features as it is continuously stretched, twisted, crumpled, and bent.
For example, the rules of topology mean that a donut shape will still have a hole even if it changes its shape into a cube, unless its topology is broken. This study shows how researchers can seamlessly change a wide range of microcellular materials to new topologies – and back – with precise control and timing.
This opens new possibilities for innovative materials. For example, it could lead to energy-efficient coatings with tunable mechanical and acoustic properties. It could also lead to new ways to control catalytic reactions. It also offers a new foundation for systematically exploring the relationship between material topology and energy transport.
Living systems continuously transform the connectivity of their cellular networks by fusing, splitting, and redefining boundaries to adapt to changing physical and chemical demands. However, synthetic analogs of microcellular materials face a formidable barrier to these transformations because they require intricate forces that can fold, bend, and repack walls.
In this study, scientists show how applying a drop of volatile liquid can overcome this challenge. First, a drop of liquid infiltrates a triangular lattice made of stiff polymer and temporarily softens the material. As the liquid evaporates, the softened structure aids weak local forces to zip together the walls of the triangles.
The result is a hexagonal lattice that is robust even under harsh conditions. It can be switched back to a triangular lattice by applying a drop containing a mix of two liquids that pry apart the zipped walls, allowing the lattice to relax back and stiffen into its original topology. The key to controlling these dynamics is to choose appropriate liquids for particular geometries and compositions.
The researchers developed a generalized theoretical model to expand this strategy to a variety of complex combinations of compartment shapes, sizes, and responsive materials. Using tiny drops enabled precisely controlled tuning of several key properties as well as trapping and release of gas bubbles.
Source: U.S. DOE Office of Science
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