Abstract
The onion-like multilamellar structure is ubiquitous in various soft materials, such as surfactant solutions, biological membranes, and block copolymers, as well as hard materials such as carbon. Onions appear to be perfectly spherical, giving the impression that it has a point-symmetric, seamless, and defect-free internal structure. Here, we focus on surfactant onions formed in the sponge phase and experimentally study whether the interior of surfactant onions is defect-free and isolated from the surrounding matrix or not. By directly observing the coalescence process between onions and a planar lamellar domain with optical microscopy, we have demonstrated that the onions do not have a completely closed structure but have a linear array of defects along the radial direction formed upon onion formation. In other words, the onion structure is symmetric axially but not spherically, like the plant-onion structure. This discovery of the hidden breakdown of spherical symmetry sheds fresh light on the onion's internal topology and formation mechanism.
Highlights
IntroductionAmphiphilic molecules such as surfactants are known to spontaneously form various types of self-organized structures in aqueous solutions, such as the lamellar (lyotropic smectic), onion (or multilamellar vesicle), and sponge structures [1]
Amphiphilic molecules such as surfactants are known to spontaneously form various types of self-organized structures in aqueous solutions, such as the lamellar, onion, and sponge structures [1]
It was shown that R ∝ γ −1 when onions are formed from a phase-separating lamellar-sponge mixture. We found another route of onion formation via spontaneous nucleation of a lamellar domain from the isotropic sponge phase [39]
Summary
Amphiphilic molecules such as surfactants are known to spontaneously form various types of self-organized structures in aqueous solutions, such as the lamellar (lyotropic smectic), onion (or multilamellar vesicle), and sponge structures [1]. The onion-like multilamellar vesicle has long been known to be formed from the lamellar phase of lipid or surfactant systems either spontaneously or by shaking them [14,15,16,17]. It was found that the onion structure is formed from the lamellar phase of surfactant solutions by shear-induced mechanical instability [18]. Such shear-induced onion formation can occur directly from the planar lamellar phase or through
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