Research ArticleMATERIALS SCIENCE

Directed aging, memory, and nature’s greed

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Science Advances  20 Dec 2019:
Vol. 5, no. 12, eaax4215
DOI: 10.1126/sciadv.aax4215
  • Fig. 1 Change in Poisson’s ratio, ν, by aging.

    (A) Sample systems of each kind that were trained. Left to right: a jammed packing of discs, a network based on jamming, a disordered holey sheet, and a random network based on triangular lattice. Scale bars, 50 mm. (B) Effect of aging as a function of time (seconds). When foam networks are aged under constant strain, the Poisson’s ratio drops until it saturates to a final value. Different curves represent different aging strains. The horizontal line at 0.38 shows the initial ν of the networks. (C) Four different systems: jammed packings (black circles), jammed networks (blue diamonds), holey sheets (red squares), and random triangular networks (green crosses) show a drop in ν when aged under uniform compression. (D) Role of geometry: Networks prepared by aging under different strains (blue diamonds) compared to unaged networks cut out to have the same geometry (green squares). The deviation suggests that both geometry and the material properties change during aging. (E) Results from numerical simulation of a system aged at 2.5% compressive strain. The Poisson’s ratio within linear response decreases as a function of time. Inset: The Poisson’s ratio in the nonlinear regime as a function of the measuring strain for unaged networks and networks aged at τ/τ0 = 103. Note that the system is not auxetic within linear response but auxetic when compressed to larger strains.

  • Fig. 2 The Poisson’s ratio of experimental foam networks aged under pure shear strain.

    The abscissa is the strain at which Poisson’s ratio is measured when the network is compressed along a given direction. The unaged (circles) networks do not show a substantial dependence on measurement strain along either axis. The aged networks (triangles) show a marked change in behavior when ν is measured by compressing along the pulling axis (blue) or along the axis that was compressed (red) during aging.

  • Fig. 3 A holey sheet with an array of holes in a square lattice pattern.

    Initially, the sheet is auxetic under compression along one of the major axes. If the sheet is aged while fixing four holes (shown in red) while the sheet is under compression, the sheet now responds in a scalloped pattern when compressed in the vertical direction. Scale bar, 100 mm.

  • Fig. 4 Characterization of EVA foam used.

    (A) Stripes of foam aged under uniaxial compression relax back over time. Relaxation rate depends on how long the sample was aged and the strips do not go back to their original lengths. (B) The Poisson’s ratio of solid sheets of foam aged under compression. These sheets do not age the same way as other networks. Instead of a decrease, we see a slight increase in the Poisson’s ratio as a function of aging strain.

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • Evolution of the bulk and shear modulus as a function of time
    • Fig. S1. The evolution of the bulk and shear modulus as a function of time in simulations.

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