Photo/Thermo Dual Stimulus-Responsive Liquid Marbles Stabilized with Polypyrrole-Coated Stearic Acid Particles

Cascading responses of stimuli-responsive materials

Responsiveness to stimuli is important in daily life: natural biological activity is governed by continuous stimulus responsiveness. The design of stimuli-responsive materials is required for the development of advanced sensing systems. Although fully controlled stimuli-responsive systems have been constructed in nature, artificial systems remain a challenge. Conventional stimuli-responsive materials show direct responsiveness to an applied stimulus (Stimulus 1), with structural changes in their molecules and organized states. This feature article focuses on cascading responses as a new concept for integrating stimuli-responsive material design. In cascading responses, an original stimulus (Stimulus 1) is converted into other stimuli (Stimulus 2, 3, …, N) through successive conversions. Stimulus N provides the eventual output response. Integration of multiple stimuli-responsive materials is required to achieve cascading responses. Although cascade, domino, and tandem chemical reactions have been reported at the molecular level, they are not used for materials with higher organized structures. In this article, we introduce functional carriers and sensors based on cascading responses as model cases. The concept of cascading responses enables the achievement of transscale responsivity and sensitivity, which are not directly induced by the original stimulus or its responsive material, for the development of advanced dynamic functional materials.

Polyhedral Vinyl Polymer Particles Synthesized Via Solvent-Free Radical Polymerization

Liquid marbles (LMs) with a cubic shape are created by using various vinyl monomers as an inner liquid and polymer plates with mm size as a stabilizer. The relationship between the surface tension of the vinyl monomers and formability of the LMs is investigated. LMs can be fabricated using vinyl monomers with surface tensions of 42.7-40.3 mN m-1. The cubic polymer particles are successively synthesized via free-radical polymerizations by irradiation of the cubic LMs with UV light in a solvent-free manner. In addition, controlling the number of polymer plates per one LM, the shape of the plate or the coalescence of the LMs can lead to production of polymer particles with desired forms (e.g., Platonic and rectangular solids) that correspond to the shapes of the original LMs.

Capillarity in Interfacial Liquids and Marbles: Mechanisms, Properties, and Applications

The mechanics of capillary force in biological systems have critical roles in the formation of the intra- and inter-cellular structures, which may mediate the organization, morphogenesis, and homeostasis of biomolecular condensates. Current techniques may not allow direct and precise measurements of the capillary forces at the intra- and inter-cellular scales. By preserving liquid droplets at the liquid-liquid interface, we have discovered and studied ideal models, i.e., interfacial liquids and marbles, for understanding general capillary mechanics that existed in liquid-in-liquid systems, e.g., biomolecular condensates. The unexpectedly long coalescence time of the interfacial liquids revealed that the Stokes equation does not hold as the radius of the liquid bridge approaches zero, evidencing the existence of a third inertially limited viscous regime. Moreover, liquid transport from a liquid droplet to a liquid reservoir can be prohibited by coating the droplet surface with hydrophobic or amphiphilic particles, forming interfacial liquid marbles. Unique characteristics, including high stability, transparency, gas permeability, and self-assembly, are observed for the interfacial liquid marbles. Phase transition and separation induced by the formation of nanostructured materials can be directly observed within the interfacial liquid marbles without the need for surfactants and agitation, making them useful tools to research the interfacial mechanics.

Axisymmetric Compression of a Circular Particle Raft Driven by the Diffusion of Surfactant

Particle rafts are a new kind of soft matter formed by self-organization on the interface, which possesses mechanical properties between fluid and solid, and they have been widely used in many industrial fields. In the present study, the compression experiment of a circular particle raft is first performed, where an SDS (sodium dodecyl sulfate)-coated metal ring is placed around its periphery. When the surfactant diffuses, the particle raft shrinks, and its shrinkage ratio increases with the increase in the surfactant concentration, where the experimental results are consistent with the numerical simulation. Next, the relationship between the initial surface tension difference of SDS and the radius shrinkage of the particle raft is obtained by dimensional analysis. In what follows, the diffusion model is built to quantify the diffusion process of SDS at the liquid-gas interface, and then the analytical concentration solution of the concentration of SDS at the periphery of particle raft is given. The particle raft is viewed as an elastic circular plate under the action of the radial pressure, which originates from the surface tension difference, which has been verified by the experimental result. These explorations cast a new light on how to apply loads to measure mechanical properties of soft matter, which also provide some inspirations on the design of microsensors and microfluidics.

Nonspherical Epoxy Resin Polymer Particles Synthesized via Solvent-Free Polyaddition Reactions

Cubic liquid marbles (LMs) were fabricated by using various epoxy monomers as internal liquids and millimeter-sized polymer plates as stabilizers. Successively, cubic polymer particles were synthesized via solvent-free polyaddition reactions by exposing the cubic LMs to NH₃ vapor used as a curing agent. The effect of the solubility parameters (SPs) for the epoxy monomers on the formation of the cubic polymer particles was investigated. As a result, we succeeded in fabricating cubic polymer particles reflecting the shapes of the original LMs by using epoxy monomers with SP values of 23.70-21.66 (MPa)^1/2. Furthermore, the shapes of the LMs could be controlled on demand (e.g., pentahedral and rectangular) by control of the number of polymer plates per LM and/or coalescence of the LMs, resulting in fabrication of polymer particles with shapes reflecting those of the LMs.

Robust cellulose-based hydrogel marbles with excellent stability for gas sensing

Liquid marbles, as particle-armored droplets, have potential applications in microreactors, biomedicine, controlled release and gas detection. To improve the stability and biocompatibility of marble, biocompatible cellulose acetate particles and 3-allyloxy-2-hydroxy-propyl-cellulose (AHP-cellulose) were used to fabricate robust cellulose-based liquid marbles with excellent stability. Liquid marble was gelled into hydrogel marble via blue-light-irradiated polymerization of AHP-cellulose. The mechanical properties of cellulose-based hydrogel marble are superior to those of liquid marble. The rupture height of liquid marble is 10.5 m, which is 420 times greater than that of water marble (0.025 m). Surprisingly, the hydrogel marble with a 3 % AHP-cellulose concentration remained intact even after being dropped from a height of 50 m, which is comparable with the ability of a leather ball to withstand larger impact. When released from a height of 60 mm, hydrogel marble bounced to approximately 25.5 mm, 881 % higher than liquid marble (2.6 mm). Hydrogel marble exhibited long-lasting stability and was capable of monitoring ammonia with a detection limit of 365.2 mg/m³. The biocompatible cellulose-based hydrogel marble with excellent mechanical stability and reusability detection has great potential in chemical and environmental engineering as gas sensors.

Photo- and Thermoresponsive Liquid Marbles Based on Fatty Acid as Phase Change Material Coated by Polypyrrole: From Design to Applications

Responsive liquid marbles (LMs), which can change their shape, stability, and motion by the application of stimuli, attract a growing interest due to their wide range of applications. Our approach to design photo- and thermoresponsive LMs is based on the use of micrometer-sized fatty acid (FA) particles as phase change material covered with polypyrrole (PPy) overlayers with photothermal property. The core-shell particles were synthesized by aqueous chemical oxidative seeded dispersion polymerization. First, we investigated the effect of the alkyl chain length of FA on the resulting FA/PPy core-shell particles by characterizing their size and its distribution, shape, morphology, chemical composition, and photothermal behavior. Then LMs were fabricated by rolling water droplets on the dried FA/PPy particle powder bed and their light and temperature dual stimuli-responsive nature was studied as a function of the FA alkyl chain length. For all FAs studied, LMs disrupted in a domino manner by light irradiation as the first trigger: the temperature of the FA/PPy particles on the LM surface increased by light irradiation, followed by phase change of FA core of the particles from solid to liquid, resulting in disruption of the LM and release of the encapsulated water. The disruption time was closely correlated to the melting point of FA linked to the alkyl chain length and light irradiation power, and it could be controlled and tuned easily between quasi instantaneous and approximately 10 s. Finally, we showed potential applications of the LMs as a carrier for controlled delivery and release of substances and a sensor.