Micromachined polymeric honeycomb membranes having conventional and re-entrant cell geometries have been fabricated using femtosecond laser ablation. Mechanical properties characterization confirms that the re-entrant membrane is auxetic (possesses negative Poisson's ratios:  ν xy = −1.82 ± 0.05 and ν yx = −0.51 ± 0.01) whereas the conventional membrane possesses positive Poisson's ratios (ν xy = +0.86 ± 0.06 and ν yx = +0.6 ± 0.1). Comparison with honeycomb theory confirms that the dominant deformation mechanism is flexure of the honeycomb ribs. The auxetic membrane has been challenged with single-sized glass chromatography beads such that the beads were initially resting on the re-entrant cells. Subsequent tensile loading of the membrane showed the auxetic cells opening during deformation, enabling the beads to pass through the membrane. We have modeled the pore-opening properties of both types of membranes, and the observed behavior for the auxetic membrane is consistent with the model. This is a clear proof-of-concept demonstration of the potential of auxetic materials and structures in filter defouling or cleaning operations. This paper, therefore, demonstrates the successful design and fabrication of a micromachined auxetic structure having specifically tailored mechanical properties that show enhanced functional performance over the conventional filter structure.

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An auxetic foam structure is described which has the property of widening when stretched (negative Poisson's ratio behaviour), in contrast to the situation with most materials which narrow when stretched. The fabrication of this material involves a combination of heat and compressive treatments of a conventional commercial air-filtration foam. The resulting microstructure is examined by scanning electron microscopy (SEM). The pertinent mechanical properties are described in terms of the changes in specimen dimensions and associated strains. From these quantities, Poisson's ratios are determined for the large strain regime involved. The property of being able to open pores by stretching the material is a useful feature in various types of filtration application. In particular, where the long-term efficiency of the process is hindered by ingress of particles into the system which block the active pore structure, the marriage of permeability and mechanical deformation via a negative Poisson's ratio offers an alternative to other methods of clearance. This notion is illustrated by the use of some very simple tests in which the foam is challenged by glass beads of uniform diameter. Simple air pressure-drop tests are also performed on auxetic and conventional foams to demonstrate that the change in permeability due to mechanical deformation enables a means of varying the pressure-drop acting across an auxetic foam to a far greater degree than is possible for the conventional foams.

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