Despite its applications, metal-assisted chemical etching (MACE) is still not well understood. To elucidate its underlying mechanism, MACE with micron-sized gold mesh was systematically carried out across a wide range of volume ratio R of HF acid to H₂O₂ solution. Slant wires were observed at room temperature over a broad range of parameters, which contradicts the well-established viewpoint that slantwise etching occurs only at elevated temperatures. Vertical wires form only in a narrow parameter space featuring a low R. The wide variation in observed slant angles indicates that the associated MACE processes do not exhibit a preferred etching direction. The observed R-dependent change of oxygen content in wires and of bubbling phenomena was explained by two competing mechanisms associated with direct and indirect Si dissolution. The high and low R regime is dominated by direct and indirect mechanism, respectively, whereas in the intermediate regime both mechanisms play an indispensable role. Slant wire formation is caused directly by the slantwise mesh movement. Although this movement is closely correlated with the direct mechanism, it cannot be fully explained by it. We hypothesize that hydrogen bubbling-induced flow has the potential to hydrodynamically drive the mesh slantwise and hence can be responsible directly for slant wire formation. To the best of our knowledge, this work not only reports a relatively novel phenomenon—slant wire formation by MACE at room temperature—but also provides, for the first time, a comprehensive understanding of its underlying complex mechanism.

This study investigates the morphological and biomechanical characteristics of Nephrotoma appendiculata (NA) wings, identifying a sophisticated, multifunctional fractal-like venation system optimized for balancing flight stability and impact resilience. Unlike conventional studies that focus on common models such as dragonflies or bees, this work highlights NA as an exceptional biomimetic model due to its superior yaw control, low flapping frequency (30–50 Hz) for stealth, and high energy efficiency. Using optical microscopy, scanning electron microscopy (SEM), mass measurements, and mechanical testing, we characterized the gender-specific morphology, geometry, mass distribution, and other key features of NA wings, and analyzed the anisotropic properties of their tapered veins and corrugated membranes. Furthermore, both analytical and finite element analyses were performed to examine the passive torsional behavior of tapered versus untapered veins. The results indicate that tapered veins significantly reduce twist angles and bending displacements under micro-Newton scale loads. Unlike previous studies focusing on frontal impacts, this research highlights a defense mechanism against dorsal and ventral disturbances, where tapering minimizes displacement transmission to the critical wing root (radial stop region). Our strain-energy models indicate that tapered veins significantly reduce twist angles and restrict the concentration of strain energy near the proximal root under micro-Newton loads, suggesting a mechanism to mitigate impact-induced disturbance transmission. These findings imply that the tapered venation system may represent an evolutionary trade-off, using limited material to create a structure that is rigid enough for stable flight yet resilient enough to absorb damaging impacts—an optimization strategy that could contribute to survival.

We report the temporal variation of slant angles across three distinct segments within a single wire at a fixed HF‐to‐H ₂ O ₂ volume ratio R during metal‐mesh‐assisted chemical etching (MACE). This intrawire change in slant angle at a constant R differs fundamentally, in both phenomenon and mechanism, from the wire‐to‐wire variation in slant angles observed under varying R values. MACE of (100) Si substrate over extended durations typically yields tilted wires when using a medium‐to‐high R . Interestingly, while these structures appear slanted macroscopically, electron microscopy reveals a microscopic vertical segment at the uppermost portion of the wires. The underlying mechanism remains poorly understood. In this work, the existence of bubble‐induced two‐phase flow was confirmed indirectly by comparing stirred and unstirred MACE. The hydrogen produced by MACE initially remains dissolved, and after some time, bubbles start to nucleate and grow until they detach from the etching front. This characteristic three‐stage gas evolution behavior was found to align well, in chronological order, with the vertical, curved, and slanted segments observed in the etched wires. To the best of our knowledge, this study offers the first interpretation linking gas evolution phases to intrawire morphology of macroscopically oblique wires produced by MACE.