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Re-Entrant Foam

  • Example of how processing affects properties of a material.

  • Material: furnace filter foam

  • Foam will constrict or bulge when stretched, depending upon how it was processed.

  • "Imploded" pores prepared by compressing with heat. When placed under tension in one direction, the pore expands in all directions.

  • When stretched in one direction, the sample expands in perpendicular directions as well.

  • This is an example of negative Poisson ratio: material expands in cross section as it is stretched.

  • Rubber band exhibits positive Poisson ratio: shrinks in cross section as it is stretched.