Non-Aqueous Polyhedral Liquid Marbles Stabilized with Polymer Plates Having Surface Roughness.

Hydrophobic polymer plates with smooth and rough surfaces are used as a stabilizer for cubic liquid marbles (LMs) to study the effect of surface roughness on their formation. The smooth and rough polymer plates can stabilize LMs using liquids with surface tensions of 72.8-26.6 and 72.8-22.9 mN m-1, respectively. It is clarified that the higher the surface roughness, the lower the surface tension of the liquids are stabilized to form the LMs. These results indicated that the introduction of surface roughness improves the hydrophobicity of the polymer plates and the rough polymer plates can stabilize LMs using liquids with a wider surface tension range. Electron microscopy studies and numerical analyses confirmed that the LMs can be formed, when the Cassie-Baxter wetting state, where θY>90° (θY: the contact angle on smooth surfaces) and θR>90° (θR: the contact angle on rough surfaces), and the metastable Cassie-Baxter wetting state, where θY<90° and θR>90°, are realized. Finally, the synthesis of cubic polymer particles are succeeded by free radical polymerization of the cubic LMs containing a hydrophobic vinyl monomer (dodecyl acrylate) in a solvent-free manner.

High-Sensitive Spatiotemporal Distribution Imaging of Compression Stresses Based on Time-Evolutional Responsiveness.

Mechanoresponsive materials have been studied to visualize and measure stresses in various fields. However, the high-sensitive and spatiotemporal imaging remain a challenging issue. In particular, the time evolutional responsiveness is not easily integrated in mechanoresponsive materials. In the present study, high-sensitive spatiotemporal imaging of weak compression stresses is achieved by time-evolutional controlled diffusion processes using conjugated polymer, capsule, and sponge. Stimuli-responsive polydiacetylene (PDA) is coated inside a sponge. A mechanoresponsive capsule is set on the top face of the sponge. When compression stresses in the range of 6.67-533 kPa are applied to the device, the blue color of PDA is changed to red by the diffusion of the interior liquid containing a guest polymer flowed out of the disrupted capsule. The applied strength (F/N), time (t/s), and impulse (F·t/N s) are visualized and quantified by the red-color intensity. When a guest metal ion is intercalated in the layered structure of PDA to tune the responsivity, the device visualizes the elapsed time (τ/min) after unloading the stresses. PDA, capsule, and sponge play the important roles to achieve the time evolutional responsiveness for the high-sensitive spatiotemporal distribution imaging through the controlled diffusion processes.

pH- and Photoresponsive Liquid Plasticine.

Liquid marbles (LMs) can be prepared by adsorption of hydrophobic particles at the air-liquid interface of a water droplet. LMs have been studied for their application as microreaction vessels. However, their opaqueness poses challenges for internal observation. Liquid plasticines (LPs), akin to LMs, can be prepared by the adsorption of hydrophobic particles with a diameter of 50 nm or less, at the air-liquid interface of a water droplet. Unlike LMs, LPs are transparent, allowing for internal observation, thus presenting promising applications as reactors and culture vessels on a microliter scale. In this study, the surface of silica particles, approximately 20 nm in diameter, was rendered hydrophobic to prepare hydrophobic silica particles (SD0). A small amount of poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) was then grafted onto the surface of SD0, yielding SD1. SD0 particles exhibited consistent hydrophobicity irrespective of the environmental pH atmosphere. Under acidic conditions, SD1 became hydrophilic due to the protonation of pendant tertiary amines in the grafted PDPA chains. However, SD1 alone was unsuitable for LP preparation due to its high surface wettability regardless of atmospheric pH, attributable to the presence of PDPA-grafted chains. Therefore, to prepare pH-responsive LP, SD1 and SD0 were mixed (SD1/SD0 = 3/7). Upon exposure to HCl gas, these LPs ruptured, with the leaked water from the LPs being absorbed by adjacent paper. Moreover, clear LPs, prepared using an aqueous solution containing a water-soluble photoacid generator (PAG), disintegrated upon exposure to light as PAG generated acid, leading to LP breakdown. In summary, pH-responsive LPs, capable of disintegration under acidic conditions and upon light irradiation, were successfully prepared in this study.

Nitrogen-Containing Carbon Tubes Fabricated by Light Irradiation.

Cotton-core/polypyrrole (PPy)-sheath fibers (cotton/PPy fibers) were synthesized by aqueous chemical oxidative seeded polymerization and were utilized as precursors for nitrogen-containing carbon (NCC) tubes. Irradiation of the cotton/PPy fibers with a near-infrared (NIR) laser heated them to approximately 300 °C due to light-to-heat photothermal conversion by the PPy, and the cotton core was thermally decomposed and vaporized. Scanning electron microscopy studies revealed the formation of tubes with monodispersed diameters, and elemental microanalysis, Fourier transform infrared spectroscopy, and Raman spectroscopy confirmed that the PPy sheath was converted into NCC. Furthermore, sunlight also worked as the light source in fabricating the NCC tubes. The thicknesses of the tubes were controlled between 410 nm and 2.30 µm by tuning the PPy sheath thickness. The method developed in this study can be extended to other polymeric fibers, including acrylic and wool fibers. The shapes of the cross sections and surface nanomorphologies of the NCC tubes can be reflected in those of the polymer/PPy fibers.

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.

Aqueous Bubbles Stabilized with Millimeter-Sized Polymer Plates.

(Sub)millimeter-sized hexagonal polymer plates that were monodisperse in shape and size were utilized as stabilizers for aqueous bubbles, and the effects of the hydrophilic-hydrophobic property, size, and solid concentration of the plates on the formability, stability, and shape and structure of aqueous bubbles were investigated. The formability and stability of the bubbles were improved by increasing the hydrophobicity of the plate surface, decreasing the plate size, and increasing the solid concentration of the plates. For plates with suitable water wettability, three-dimensional bubbles with nearly spherical and polyhedral shapes were formed by the adsorption of plates to the bare air bubbles introduced into the continuous water phase by air-water mixing. On the contrary, two-dimensional bubbles with accordion-type structures consisting of alternating layers of plates and entrapped air bubbles were formed by the transfer of multiple plates with poor wettability from the air phase to the water phase by air-water mixing. Furthermore, a correlation was found between the bubble/stabilizer size ratio and bubble shape for plates with the suitable wettability: bubbles with nearly spherical shapes were formed when the bubble/plate size ratios were >2, bubbles with hexahedral, pentahedral, and tetrahedral shapes were formed when the size ratios were approximately 1, and bubbles with triangular and sandwich shapes were formed when the size ratios were <0.8. Additionally, bubbles with similar shapes were formed when the bubble/plate size ratios were close, even when the sizes of the plates and bubbles were different.

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.

Electrostatic Adsorption Behaviors of Polymer Plates to a Droplet.

Electrostatic transfer and adsorption of electrically conductive polymer-coated poly(ethylene terephthalate) plates from a particle bed to a water droplet were studied, with the influence of plate thickness and shape observed. After synthesis and confirmation of the particles' properties using stereo and scanning electron microscopies, elemental microanalysis, and water contact angle measurement, the electric field strength and droplet-bed separation distance required for transfer were measured. An electrometer and high-speed video footage were used to measure the charge transferred by each particle, and its orientation and adsorption behavior during transfer and at the droplet interface. The use of plates of consistent square cross section allowed the impact of contact-area-dependent particle cohesion and gravity on the electrostatic transfer of particles to be decoupled for the first time. The electrostatic force required to extract a plate was directly proportional to the plate mass (thickness), a trend very different from that previously observed for spherical particles of varied diameter (mass). This reflected the different relationship between mass, surface area, and cohesive forces for spherical and plate-shaped particles of different sizes. Thicker plates transferred more charge to the droplet, probably due to their remaining at the bed at higher field strengths. The impact of plate cross-sectional geometry was also assessed. Differences in the ease of transfer of square, hexagonal, and circular plates seemed to depend only on their mass, while other aspects of their comparative behavior are attributed to the more concentrated charge distribution present on particles with sharper vertices.

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.

Colloidally Stable Polypyrrole Nanoparticles Synthesized by Surfactant-Free Coupling Polymerization.

Surfactant-free polypyrrole (PPy) nanoparticles, which were colloidally stable in aqueous medium, were successfully synthesized by coupling polymerization of pyrrole using Fe(NO3)3 solids in the absence of any colloidal stabilizer. The pyrrole monomers were gradually supplied from the vapor phase, and the coupling reaction of the monomers could proceed to generate PPy in a water medium. The resulting PPy nanoparticles were extensively characterized in terms of diameter, bulk chemical composition, surface chemistry, and colloidal stability by dynamic light scattering, electron microscopy, elemental microanalysis, Fourier transform infrared spectroscopy, Raman spectroscopy, electrophoresis, and X-ray photoelectron spectroscopy. The characterization results indicated that the PPy nanoparticles can be colloidally stable based on the electrostatic stabilization mechanism due to cationic charges generated on the PPy molecules by doping during the polymerization. General chemical oxidative polymerization in aqueous medium using the Fe(NO3)3 oxidant without a colloidal stabilizer as a control experiment resulted in generation of atypical PPy aggregates with over a micrometer size, indicating that the polymerization at low ionic strength is essential for colloidal particle formation. Finally, it was demonstrated that the PPy nanoparticles worked as a surfactant-free black-colored particulate emulsifier by adsorption at the oil-water interface to stabilize Pickering-type oil-in-water emulsions.

Controlled Deformation of Soft Nanogel Particles Generates Artificial Biominerals with Ordered Internal Structure.

Biominerals can exhibit exceptional mechanical properties owing to their hierarchically-ordered organic/inorganic nanocomposite structure. However, synthetic routes to oriented artificial biominerals of comparable complexity remain a formidable technical challenge. Herein we design a series of soft, deformable nanogels that are employed as particulate additives to prepare nanogel@calcite nanocomposite crystals. Remarkably, such nanogels undergo a significant morphological change-from spherical to pseudo-hemispherical-depending on their degree of cross-linking. This deformation occurs normal to the growth direction of the (104) face of the calcite and the underlying occlusion mechanism is revealed by in situ atomic force microscopy studies. This model system provides new mechanistic insights regarding the formation of oriented structures during biomineralization and offers new avenues for the design of synthetic nanocomposites comprising aligned anisotropic nanoparticles.

Liquid Marble as an Amphibious Carrier for the Controlled Delivery and Release of Substances.

The motion control of small objects has received significant interest in the research field of soft active matter. Controlling the release of substances from small objects has also attracted attention in other fields, such as the agrochemical and biomaterial fields. Until now, these two research objectives have been conducted independently in most cases but have the same ultimate goal: to transport small objects loaded with functional substances in a controlled manner and to release the substances at a desired time within the same system. This Perspective aims to highlight the challenges for realizing both the amphibious motion control of objects and the controlled release of substances by the application of external stimuli within the same system, using liquid marbles, which are particle-layer-coated small droplets, as a carrier.

"Foam Marble" Stabilized with One Type of Polymer Particle.

There has been increasing interest in colloidal particles adsorbed at the air-water interface, which lead to stabilization of aqueous foams and liquid marbles. The wettability of the particles at the interface is known to play an important role in determining the type of air/water dispersed system. Foams are preferably formed using relatively hydrophilic particles, and liquid marbles tend to be formed using relatively hydrophobic particles. In this study, submicrometer-sized polystyrene particles carrying poly(N,N-diethylaminoethyl methacrylate) hairs (PDEA-PS particles), which are synthesized by dispersion polymerization, are demonstrated to work as a particulate stabilizer for both aqueous foams and liquid marbles. A key point for the hydrophilic PDEA-PS particles to stabilize both aqueous foams and liquid marbles, which have been generally stabilized with hydrophilic and hydrophobic particles, respectively, is the wetting mode of the particles with respect to water. The flocculates of PDEA-PS particles adsorb to the air-water interface from the aqueous phase to stabilize foam in a Wenzel mode, and the dried PDEA-PS particles adsorb to the interface as aggregates from the air phase to stabilize liquid marbles in a metastable Cassie-Baxter mode. On the basis of the difference in the wetting mode, stabilization of an air-in-water-in-air multiple gas-liquid dispersed system, named "foam marble", is realized. After the evaporation of water from the foam marble, a porous sphere is successfully obtained with pore sizes of a few tens of micrometers (reflecting the bubble sizes) and a few tens of nanometers (reflecting the gap sizes among the PDEA-PS particles).

Interparticle Repulsion of Microparticles Delivered to a Pendent Drop by an Electric Field.

We report an unusually large spacing observed between microparticles after delivery to the surface of a pendent water droplet using a DC nonuniform electrostatic field, primarily via dielectrophoresis. The influence of particle properties was investigated using core particles, which were either coated or surface-modified to alter their wettability and conductivity. Particles that exhibited this spacing were both hydrophobic and possessed some dielectric material exposed to the external field, such as a coating or exposed dielectric core. The origin of this behavior is proposed to be the induced dipole-dipole repulsion between particles, which increases with particle size and decreases when the magnitude of the electric field is reduced. When the particles were no longer subjected to an external field, this large interparticle repulsion ceased and the particles settled to the bottom of the droplet under the force of gravity. We derive a simple model to predict this spacing, with the dipole-dipole repulsion balanced against particle weight. The external electric field was calculated using the existing electric field models. The spacing was found to be dependent on particle density and the induced dipole moment as well as the number of particles present on the droplet interface. As the number of particles increased, a decrease in interparticle spacing was observed.

How Liquid Marbles Break Down: Direct Evidence for Two Breakage Scenarios.

Liquid marbles are nonsticking droplets wrapped with hydrophobic nano- to micrometer particles and are expected to be useful for various applications, especially in industrial and biomedical fields. However, the practical use of liquid marbles is limited by their fragility. In this study, the dynamics of particle monolayer-stabilized liquid marble breakage upon impacting a solid surface are monitored in situ by high-speed interfacial microscopy. The experiments show that the breakage of liquid marbles can be induced by either i) cracking or ii) water penetration depending on the impact energy. The applicable scenario is determined by whether a jamming transition of the wrapping particles occurs during impact. The breakage mechanisms provide insights on how to improve the robustness of liquid marbles in accordance with these scenarios.

Multimotion of Marangoni Propulsion Ships Controlled by Two-Wavelength Near-Infrared Light.

Multimode motion of Marangoni propulsion ships on a water surface as per a near-infrared, two-wavelength selective response is achieved for the first time. The ships are rhombus- or propeller-shaped polyacrylamide or siloxane resin-based gels in which Nd2O3 and Yb2O3 nanoparticles are incorporated separately for photothermal conversion at 808 and 980 nm, respectively. The rhombus geometry is for straight locomotion, and the propeller geometry is for rotation. On/off remote control of the forward and backward locomotion of a rhombus-shaped ship and of the clockwise and counterclockwise rotations of a propeller-shaped ship via irradiation with 808 or 980 nm near-infrared light is demonstrated. The nanoparticles are incorporated into the desired locations of the gels, enabling selective local heating of the gels without focusing the light. The temperature gradient of the ships by local heating, based on a photothermal conversion, generates a Marangoni propulsion force to move the ship in the desired direction.

Locomotion of a Nonaqueous Liquid Marble Induced by Near-Infrared-Light Irradiation.

Micrometer-sized hydrophobic polyaniline (PANI) grains were synthesized via an aqueous chemical oxidative polymerization protocol in the presence of dopant carrying perfluoroalkyl or alkyl groups. The critical surface tensions of the PANIs synthesized in the presence of heptadecafluorooctanesulfonic acid and sodium dodecyl sulfate dopants were lower than that of PANI synthesized in the absence of dopant, indicating the presence of hydrophobic dopant on the grain surfaces. The PANI grains could adsorb to air-liquid interfaces, and aqueous and nonaqueous liquid marbles (LMs) were successfully fabricated using liquids with surface tensions ranging between 72.8 and 42.9 mN/m. Thermography studies confirmed that the surface temperature of the LMs increased by near-infrared light irradiation thanks to the photothermal property of the PANI, and the maximum temperatures measured for nonaqueous LMs were higher than that measured for aqueous LM. We demonstrated that transport of the LMs on a planar water surface can be achieved via Marangoni flow generated by the near-infrared light-induced temperature gradient. Numerical analyses indicated that the LMs containing liquids with lower specific heat and thermal conductivity and higher density showed longer path length per one light irradiation shot and longer decay time. This is because generated heat could efficiently transfer from the LMs to the water surface and larger inertial force could work on the LMs. The LMs could also move over the solid substrate thanks to their near-spherical shapes. Furthermore, it was also demonstrated that the inner liquids of the LMs could be released on site by an external stimulus.

Controllable Positive/Negative Phototaxis of Millimeter-Sized Objects with Sensing Function.

Phototaxis, which is the directional motion toward or away from light, is common in nature and inspires development of artificial light-steered active objects. Most of the light-steered objects developed so far exhibit either positive or negative phototaxis, and there are few examples of research on objects that exhibit both positive and negative phototaxis. Herein, small objects showing both positive and negative phototaxis on the water surface upon near-infrared (NIR) light irradiation, with the direction controlled by the position of light irradiation, are reported. The millimeter-sized tetrahedral liquid marble containing gelled water coated by one polymer plate with light-to-heat photothermal characteristic, which adsorbs onto the bottom of the liquid marble, and three polymer plates with highly transparent characteristic, which adsorb onto the upper part of the liquid marble, is utilized as a model small object. Light irradiation on the front side of the object induces negative phototaxis and that on the other side induces positive phototaxis, and the motion can be controlled to 360° arbitrary direction by precise control of the light irradiation position. Thermographic studies confirm that the motions are realized through Marangoni flow generated around the liquid marble, which is induced by position-selective NIR light irradiation. The object can move centimeter distances, and numerical analysis indicates that average velocity and acceleration are approximately 12 mm/s and 71 mm/s2, respectively, which are independent of the direction of motions. The generated force is estimated to be approximately 0.4 µN based on Newton's equation. Furthermore, functional cargo can be loaded into the inner phase of the small objects, which can be delivered and released on demand and endows them with environmental sensing ability.

Monodispersed Nitrogen-Containing Carbon Capsules Fabricated from Conjugated Polymer-Coated Particles via Light Irradiation.

Near-infrared (NIR) light irradiation induced the transformation of polypyrrole (PPy) to nitrogen-containing carbon (NCC) material due to its light-to-heat photothermal property. The temperature of the PPy increased over 700 °C within a few seconds by the NIR laser irradiation, and elemental microanalysis confirmed the decreases of hydrogen and chloride contents and increases of carbon and nitrogen contents. Monodispersed polystyrene (PS)-core/PPy shell particles (PS/PPy particles) synthesized by aqueous chemical oxidative seeded polymerization were utilized as a precursor toward monodispersed NCC capsules. When the NIR laser was irradiated to the PS/PPy particles, the temperature rose to approximately 300 °C and smoke was generated, indicating that the PS component forming the core was thermally decomposed and vaporized. Scanning electron microscopy studies revealed the successful formation of spherical and highly monodispersed capsules, and Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy studies confirmed the capsules consisted of NCC materials. Furthermore, sunlight was also demonstrated to work as a light source to fabricate NCC capsules. The size and thickness of the capsules can be controlled between 1 and 80 µm and 146 and 231 nm, respectively, by tuning the size of the original PS/PPy particles and PPy shell thickness.

Particle Monolayer-Stabilized Light-Sensitive Liquid Marbles from Polypyrrole-Coated Microparticles.

Liquid marbles are water droplets coated with solid particles that prevent coalescence and allow storage, transport, and handling of liquids in the form of a powder. Here, we report on the formation of liquid marbles that are stabilized entirely by a single monolayer of solid particles and thus minimize the amount of required solid material. As stabilizing particles, we synthesize relatively monodisperse, 80 µm-sized polystyrene (PS) particles coated with heptadecafluorooctanesulfonic acid-doped polypyrrole (PPy-C8F) shell (PS/PPy-C8F particles) by aqueous chemical oxidative seeded polymerization of pyrrole using FeCl3 as an oxidant and heptadecafluorooctanesulfonic acid as a hydrophobic dopant. We characterize the physicochemical properties of the particles as a function of the PPy-C8F loading. Laser diffraction particle size analyses of dilute aqueous suspensions indicate that the polymer particles disperse stably in water medium before and after coating with the PPy-C8F shell. X-ray photoelectron spectroscopy studies indicate the formation of a PPy-C8F shell around the PS seed particles, which was further supported by deflated morphologies observed by scanning electron microscopy after extraction of the PS component from the PS/PPy-C8F particles. We investigate the performance of the dried PS/PPy-C8F particles to stabilize liquid marbles. Stereo- and laser microscope observations, as well as gravimetric studies, confirm that the PS/PPy-C8F particles adsorb to the water droplet surface in the form of a particle monolayer with the characteristic hexagonal close-packed structure expected for spherical (colloidal) particles. Mechanical integrity of the liquid marble increases with an increase of PPy-C8F loading of the PS/PPy-C8F particles. The water contact angle of the PS/PPy-C8F particles at air-water interface increases from 82 ± 12° to 102 ± 17° with an increase of PPy-C8F loading. This increase in water contact angle directly correlates with the shape of the LM, with higher contact angles giving more spherical LMs. Furthermore, the broadband light absorption properties of the PPy coating was used to control evaporation rate of the enclosed water phase on demand by irradiation with a near-infrared laser. The evaporation rate could be finely controlled by the thickness of the PPy-C8F shell of the particles stabilizing the liquid marbles.