Quasi-superhydrophobic microscale two-dimensional phononic crystals of stainless steel 304

Fabrication of metallic phononic and photonic crystals of characteristic size between 10 and 1000μm remains a challenge in precision using the conventional machining processes or too tedious for the cleanroom-based processes. We report the fabrication and elastodynamic bandgaps of two-dimensional phononic crystals (PhCs) machined on stainless steel 304 (SS304) substrates using the wire-electrochemical micromachining (wire-ECMM) process. Square arrays of pillars of length 400μm and cross section either 350×350μm2 or 250×250μm2 with periods 650 and 550μm, respectively, were micromachined on an SS304 homogeneous substrate. Based on these arrays, three types of PhCs were considered: air-SS304, water-SS304, and epoxy-SS304, where air, water, and epoxy are the hosts and SS304 pillars are the scatterers. We found that texturing the surface increased the contact angle of a 5-μl-water-droplet from 97.9° for an untextured SS304 substrate to a maximum of 145° for SS304 PhCs, making the latter quasi-superhydrophobic. Dispersion relations evaluated using the finite-element method revealed the presence of partial bandgaps in the 0.1–2.7 MHz for all PhCs and a complete bandgap for the epoxy-SS304 PhCs. Transmittance spectrums for incident plane waves also provided evidence for the occurrence of bandgaps. Furthermore, the buckling analysis indicated that these pillars do not undergo buckling until yield—making them mechanically robust.