Structural Coloration by Internal Reflection and Interference in Hydrogel Microbubbles and Their Precursors

Abstract

Compound microbubbles with engineered shells, fabricated by microfluidic techniques, have drawn considerable interest for interdisciplinary research, including ultrasound imaging agents, chemical micromotors, and optical microcavities. Meanwhile, though vastly investigated, dynamic shell thickness variations of microbubbles can be hardly calculated via conventional techniques. Here the fabrication of colorful microbubbles encapsulated in hydrogel precursor using glass‐capillary microfluidic methods is demonstrated. The proposed optical geometrical model elucidating the structural coloration is used for thickness assessment. Thin‐film interference occurring upon illumination of the microbubble shell is found to be responsible for the coloration. The interfered lights travel along with the shell interfaces through total internal reflection in microbubbles with buoyancy‐induced asymmetric shell. A concise thickness evaluation methodology provided by the established model predicts the thickness evolution of microbubbles. These results demonstrate tunable optical properties of shin‐shell compound microbubbles for potential applications in displays, sensing, and anti‐counterfeiting materials.

Recent progress in low‐swellable polymer‐based smart photonic crystal sensors

“…To this end, we also expect more specific stimuliswelling materials and micro-nano structure-modulated color-changing mechanisms to be applied to the construction of SPCSs. A large number of stimuli-responsive micronano structures will play important roles in SPCSs, possibly including 2D nano-to-micro structures, [37,294] surface wrinkles, [295,296] photonic porous microparticles, [279,285] quasiamorphous structures, [289,297] diffraction gratings, [298,299] bilayer photonic structures, [26,82] photonic microcapsules, [208,275] stratified porous structures, [300] particle-nested nanostructures, [271,301] periodic wavy structures, [302] woodpile PCs, [303] micro-sized bowl-array, [39] core-shell particles, [304] microbubbles, [305] polyhedron particle-arrays, [306] and so on. Moreover, some novel sensing-material systems, such as cholesteric liquid crystal [307][308][309] and blue-phase liquid crystal, [310][311][312] also show extraordinary sensing properties.…”

Dynamic Interference Colors in Electrospun Microfibrous Mats

“…Contrary to the more‐commonly explored colors that arise due to Bragg reflection, [ 17–20 ] or thin‐film interference, [ 21,22 ] interference colors in LCs result from wavelength‐dependent phase differences between orthogonally polarized rays. [ 23 ] Between crossed polarizers, the transmitted color spectrum is the result of each individual wavelength intensity, I , in the visible range (400–800 nm) being governed by the relation $$\[ \begin{array}{*{20}{c}}{I\left( \lambda \right) = {{\sin }^2}\frac{{\Gamma \left( \lambda \right)}}{2}}\end{array} \]$$In the case of a material with no, or negligible, optical dispersion, we can apply Equation () to () to calculate the colors over an LC‐relevant scale.…”

“…where Δn is the birefringence, d is the thickness, and λ is the wavelength of light. [14][15][16] Contrary to the more-commonly explored colors that arise due to Bragg reflection, [17][18][19][20] or thin-film interference, [21,22] interference colors in LCs result from wavelength-dependent phase differences between orthogonally polarized rays. [23] Between crossed polarizers, the transmitted color spectrum is the result of each individual wavelength intensity, I, in the visible range (400-800 nm) being governed by the relation…”

Tunable Structural Coloration in Eccentric Water-in-Oil-in-Water Droplets

“…29,30 Subsequently, similar microscale concave interfaces have been constructed to generate structural colors, such as dome-shaped microdroplets and particles, 31−34 Janus microdroplets of chiral liquid crystals and silicone oils, 35,36 and crescent-shaped microparticles. 37,38 However, these interfaces are mostly oil−oil or liquid−solid interfaces, which hinders the applications of such techniques in many fields based on water droplets, such as microsensors and medical diagnostics that generally use water as the reaction medium.…”

“…In nature, structural coloration often forms through scattering, interference, diffraction, and absorption effects of visible light at microscopically structured surfaces. Such surfaces mainly either contain fine parallel lines formed of one or more parallel thin layers, − or are composed of periodic architectures at the nanoscale. − Different from chemical pigments, structural colors show diverse advantages such as fade resistance, eco-friendliness, iridescence and high saturation, therefore have been extensively studied and utilized in optics, − displays, − colloidal inks − and sensors. − Recently, a new type of structural coloration via total internal reflection (TIR) and interference at microscale concave interfaces has been thoughtfully demonstrated in biphasic organic droplets dispersed in water (partially dewetted oil-in-oil-in-water (O/O/W) droplets) (Figure S1, Supporting Information (SI)). , Subsequently, similar microscale concave interfaces have been constructed to generate structural colors, such as dome-shaped microdroplets and particles, − Janus microdroplets of chiral liquid crystals and silicone oils, , and crescent-shaped microparticles. , However, these interfaces are mostly oil–oil or liquid–solid interfaces, which hinders the applications of such techniques in many fields based on water droplets, such as microsensors and medical diagnostics that generally use water as the reaction medium.…”