We study numerically and experimentally the dynamics and control of viscous fingering patterns in a circular Hele-Shaw cell. The nonlocality and nonlinearity of the system, especially interactions among developing fingers, make the emergent pattern difficult to predict and control. By controlling the injection rate of the less viscous fluid, we can precisely suppress the evolving interfacial instabilities. There exist denumerable attractive, self-similarly evolving symmetric, universal shapes. Experiments confirm the feasibility of the control strategy, which is summarized in a morphology diagram.
The index of refraction governs the flow of light through materials. At visible and near infrared wavelengths the real part of the refractive index is limited to less than 3 for naturally occurring transparent materials, fundamentally restricting applications. Here, we carried out experiments to study the upper limit of the effective refractive index of self-assembled metasurfaces at visible and near-infrared wavelengths. The centimeter-scale metasurfaces were made of a hexagonally close packed (hcp) monolayer of gold nanospheres coated with tunable alkanethiol ligand shells, controlling the interparticle gap from 2.8 to 0.45 nm. In contrast to isolated dimer studies, the macro-scale areas allow for billions of gaps to be simultaneously probed and the hcp symmetry leads to large wavelength shifts in the resonance mode, enabling subnanometer length scale mechanisms to be reproducibly measured in the far-field. We demonstrate for subnanometer gaps, that the optical response of the metasurfaces agrees well with a classical (local) model, with minor nonlocal effects and no clear evidence of ligand-mediated charge transfer at optical frequencies. We determine the effective real part of the refractive index for the metasurfaces has a minimum of 1.0 for green-yellow colors, then quickly reaches a maximum of 5.0 in the reds and remains larger than 3.5 far into the near infrared. We further show changing the terminal group and conjugation of the ligands in the metasurfaces has little effect on the optical properties. These results establish a pragmatic upper bound on the confinement of visible and near infrared light, potentially leading to unique dispersion engineered coatings. * jake.fontana@nrl.navy.mil
Information display utilizing plasmonic color generation has recently emerged as an alternative paradigm to traditional printing and display technologies. However, many implementations so far have either presented static pixels with a single display state or rely on relatively slow switching mechanisms such as chemical transformations or liquid crystal transitions. Here, we demonstrate spatial, spectral, and temporal control of light using dynamic plasmonic pixels that function through the electric-field-induced alignment of plasmonic nanorods in organic suspensions. By tailoring the geometry and composition (Au and Au@Ag) of the nanorods, we illustrate light modulation across a significant portion of the visible and infrared spectrum (600–2400 nm). The fast (∼30 μs), reversible nanorod alignment is manifested as distinct color changes, characterized by shifts of observed chromaticity and luminance. Integration into larger device architectures is showcased by the fabrication of a seven-segment numerical indicator. The control of light on demand achieved in these dynamic plasmonic pixels establishes a favorable platform for engineering high-performance optical devices.
A simple method for the fabrication of nanocomposite materials using thiol click chemistry is reported. The thiol click nanocomposite materials produced each displayed distinctive colors which were found to be dependent on both the ligand used to functionalize the nanoparticles and the concentration of nanoparticles in the materials. Functionalized metallic nanospheres were combined with thiol click solutions forming viscous prepolymer solutions which were then polymerized upon UV light exposure. Films were fabricated in a custom-built film mold, and microfibers were fabricated using hydrodynamic focusing in a microfluidic channel. For this study, three unique thiolated ligandsincluding a newly synthesized ligandwere used to functionalize the nanospheres, thus assisting in the facile incorporation and stability of the nanospheres within the polymers. In comparison to a previously reported method in which thiol−ene nanocomposite films were fabricated, the method reported herein reduces the fabrication time from weeks to minutes. Furthermore, the method in this report is expanded to also include fabrication of thiol−yne nanocomposites. Young's moduli and glass transition temperatures were determined for the materials, while UV−vis spectroscopy, transmission electron microscopy, and optical analyses were also performed in order to characterize the nanocomposites.
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