Gaoxiang Wu

Spray coating of superhydrophobic and angle-independent coloured films

Angle-independent coloured films with superhydrophobicity were fabricated from quasi-amorphous arrays of monodispersed fluorinated silica nanoparticles via one-step spray coating. The film exhibited a high contact angle (>150°) and a low roll-off angle (~2°) and the colour could be tuned to blue, green and moccasin by varying the size of the nanoparticles.

Angle-independent colours from spray coated quasi-amorphous arrays of nanoparticles: combination of constructive interference and Rayleigh scattering

Angle-independent colours have enormous potential in buildings, displays and sensors. Here, we have demonstrated angle-independent full-colour films by spray-coating monodispersed silica nanoparticles (NPs) of different sizes (100–340 nm in diameter). Ethanol and isopropanol with low surface tension and medium volatility were selected as solvents to spray-coat nanoparticles, forming quasi-amorphous arrays on various substrates. The angle-resolved reflection and scattering measurements showed that the real sample colour matched the colour from interference of reflected light at the specular angle but did not match the scattering peak position. Further study and colour fitting suggests that the observed colour of the spray-coated nanoparticle assembly arose from a combination of constructive interference and Rayleigh scattering. Moreover, we attempted to enhance the colour saturation by replacing air with water between the black background and the coating, in order to suppress reflection from the background.

Directing the Deformation Paths of Soft Metamaterials with Prescribed Asymmetric Units

By prescribing asymmetric ligaments with different arrangements in elastomeric porous membranes of pre-twisted kagome lattices, the buckling instability is avoided, allowing for smooth and homogenous structural reconfiguration in a deterministic fashion. The stress-strain behaviors and negative Poissons ratios can be tuned by the pre-twisting angles.

Production of Structural Colors with High Contrast and Wide Viewing Angles from Assemblies of Polypyrrole Black Coated Polystyrene Nanoparticles

Structural color with wide viewing angles has enormous potential applications in pigment, ink formulation, displays, and sensors. However, colors obtained from colloidal assemblies with low refractive index contrast or without black additives typically appear pale. Here, we prepare polypyrrole (PPy) black coated polystyrene (PS) nanoparticles and demonstrate well-defined colors with high color contrast and wide viewing angles under ambient light. Depending on the loading of pyrrole during polymerization, PPy nanogranules of different sizes and coverages are grafted to the surface of PS nanoparticles. The bumpy particles can self-assemble into quasi-amorphous arrays, resulting in low angle dependent structure colors under ambient light. The color can be tuned by the size of the PS nanoparticles, and the presence of the PPy black on PS nanoparticles enhances the color contrast by suppressing incoherent and multiple scattering.

Capillarity induced instability in responsive hydrogel membranes with periodic hole array

We report capillary force induced instability from drying water swollen poly(2-hydroxyethyl methacrylate) (PHEMA) based hydrogel membranes with micron-sized holes in a square array. When the PHEMA membrane was exposed to deionized-water, the size of the holes became smaller but retained the shape, so-called breathing mode instability. However, during the drying process, the square pore array buckled into a diamond plate pattern. The deformed pattern could be recovered upon re-exposure to water. The instability mechanism was confirmed by comparing the observations from optical and scanning electron microscopy (SEM) images with theoretical prediction. When thermoresponsive poly(N-isopropylacrylamide) was introduced to the PHEMA gel, the poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide) (PHEMA-co-PNIPAAm) membrane underwent pattern transformation only if dried below the lower critical solution temperature of PNIPAAm. Along the pattern transformation, we observed a dramatic change of the optical property of the film, from colourful reflection to transparent window.

Varying and unchanging whiteness on the wings of dusk-active and shade-inhabiting Carystoides escalantei butterflies

Significance Whiteness, although frequently apparent on the wings, legs, antennae, or bodies of many species of moths and butterflies, has often escaped our attention. Here, we investigate the nanostructure and microstructure of white spots on the wings of Carystoides escalantei, a dusk-active and shade-inhabiting Costa Rican rain forest butterfly (Hesperiidae). We identify two types of whiteness: angle dependent and angle independent. We speculate that the biological functions and evolution of Carystoides spot patterns, scale structures, and their varying whiteness are adaptations to the butterfly’s low light habitat and to airflow experienced on the wing base vs. wing tip during flight. Sex and species differences in the location of angle-dependent white spots on the wings may function in both intraspecific and interspecific communication. Whiteness, although frequently apparent on the wings, legs, antennae, or bodies of many species of moths and butterflies, along with other colors and shades, has often escaped our attention. Here, we investigate the nanostructure and microstructure of white spots on the wings of Carystoides escalantei, a dusk-active and shade-inhabiting Costa Rican rain forest butterfly (Hesperiidae). On both males and females, two types of whiteness occur: angle dependent (dull or bright) and angle independent, which differ in the microstructure, orientation, and associated properties of their scales. Some spots on the male wings are absent from the female wings. Whether the angle-dependent whiteness is bright or dull depends on the observation directions. The angle-dependent scales also show enhanced retro-reflection. We speculate that the biological functions and evolution of Carystoides spot patterns, scale structures, and their varying whiteness are adaptations to butterfly’s low light habitat and to airflow experienced on the wing base vs. wing tip.

Cuts Guided Deterministic Buckling in Arrays of Soft Parallel Plates for Multifunctionality

Harnessing buckling instability in soft materials offers an effective strategy to achieve multifunctionality. Despite great efforts in controlling the wrinkling behaviors of film-based systems and buckling of periodic structures, the benefits of classical plate buckling in soft materials remain largely unexplored. The challenge lies in the intrinsic indeterminate characteristics of buckling, leading to geometric frustration and random orientations. Here, we report the controllable global order in constrained buckling of arrays of parallel plates made of hydrogels and elastomers on rigid substrates. By introducing patterned cuts on the plates, the randomly phase-shifted buckling in the array of parallel plates transits to a prescribed and ordered buckling with controllable phases. The design principle for cut-directed deterministic buckling in plates is validated by both mechanics model and finite element simulation. By controlling the contacts and interactions between the buckled parallel plates, we demonstrate on-demand reconfigurable electrical and optical pathways, and the potential application in design of mechanical logic gates. By varying the local stimulus within the plates, we demonstrate that microscopic pathways can be written, visualized, erased, and rewritten macroscopically into a completely new one for potential applications such as soft reconfigurable circuits and logic devices.