Micrometric Monodisperse Solid Foams as Complete Photonic Bandgap Materials

Abstract

Solid foams with micrometric pores are used in different fields (filtering, 3D cell culture, etc.), but today, controlling their foam geometry at the pore level, their internal structure, and the monodispersity, along with their mechanical properties, is still a challenge. Existing attempts to create such foams suffer either from slow speed or size limitations (above 80 μm). In this work, by using a temperature-regulated microfluidic process, 3D solid foams with highly monodisperse open pores (PDI lower than 5%), with sizes ranging from 5 to 400 μm and stiffnesses spanning 2 orders of magnitude, are created for the first time. These features open the way for exciting applications, in cell culture, filtering, optics, etc. Here, the focus is set on photonics. Numerically, these foams are shown to open a 3D complete photonic bandgap, with a critical index of 2.80, thus compatible with the use of rutile TiO2. In the field of photonics, such structures represent the first physically realizable self-assembled FCC (face-centered cubic) structure that possesses this functionality.

Light in correlated disordered media

“…The main challenge to overcome at this stage is the fabrication of 3D connected structures with finely-tuned correlated morphologies with sufficiently high refractive indices (ideally offering an index contrast above 3) and sufficiently large thicknesses (L t ). The steady progress on bottom-up approaches such as bio-templating (Galusha et al, 2010), DNAorigami (Zhang and Yan, 2017) and microfluidic-based foam processing (Maimouni et al, 2020) gives hope for first successful realizations in the next few years. As a longer-term objective, the design and fabrication of 3D stealthy hyperuniform media would be an outstanding result.…”

“…Near-field interaction and light polarization considerably complexify theoretical modelling (Escalante and Skipetrov, 2017;Vynck et al, 2016), explaining the widespread use of full-wave numerical methods to address this issue, alongside phenomenological models (Naraghi et al, 2015). The so-called hyperuniform disordered structures (Torquato and Stillinger, 2003) have received considerable attention in this context, leading to a wider exploration of their optical properties (Bigourdan et al, 2019;Leseur et al, 2016;Sheremet et al, 2020) and advances on top-down and bottom-up fabrication techniques (Maimouni et al, 2020;Muller et al, 2017;Ricouvier et al, 2017;Weijs et al, 2015).…”

Spontaneous Wrinkle Formation on Hydrogel Surfaces Using Photoinitiator Diffusion from Oil–Water Interface

“…Finally, we demonstrate a unique technique for wrinkle formation in emulsion-templated porous materials where close-packed emulsion droplets are embedded in a cross-linked polymer (Figure e). Their porosity and surface area may be tuned for a variety of applications such as cell scaffolds. − In this study, we prepare oil-in-water emulsions that the aqueous hydrogel solution acts as the continuous phase containing photoinitiator-dissolved oil droplets. Photopolymerization of the entire oil-in-water solution will generate wrinkles on the concave inner surface of the cross-linked hydrogel and the oil droplets can be washed off with ethanol and water.…”

Investigating Pore‐Opening of Hydrogel Foams at the Scale of Freestanding Thin Films

“…[ 1,2 ] As shown in Figure a, neighboring pores can either be separated by thin films (often called “membranes” in the solid foam), leading to what is commonly called a “closed‐cell foam,” or they can be interconnected after rupture of the films, leading to a (partially) “open‐cell foam.” The percentage of interconnected pores is referred to as the “pore connectivity” [ 3,4 ] or “pore interconnectivity” [ 5–7 ] and has a crucial impact on most of the final foam properties, including liquid/gas uptake, foam mechanics, acoustic/thermal insulation, or its capacity to host bacteria and cell colonies for biomedical applications. [ 8–14 ] Despite the diversity of existing applications and the potential for innovation in key areas such as biomedicine, sustainability, and energy savings, the influence of the formulation and foaming procedure on the pore‐opening process is still poorly understood. The optimization of the pore connectivity therefore relies on tedious trial‐and‐error procedures driven by specific industrial needs.…”