Micrometer-sized gold-silica Janus particles as particulate emulsifiers

Thermocapillary migration of a drop with a thermally conducting stagnant cap

Hypothesis The thermocapillary migration of a spherical drop with a stagnant cap in the presence of a constant applied temperature gradient can be strongly affected by the finite thermal conductivity of the stagnant cap. Numerics The heat conduction of the stagnant cap is analytically modeled. The effects of the additional interfacial stresses generated by the disturbances to the local temperature field due to the presence of the cap at the fluid-fluid interface and the corresponding velocity of migration of the drop are evaluated by solving for the temperature and hydrodynamic field equations in and around the drop. An asymptotic model is derived to predict the terminal velocity in the presence of an infinitely conducting stagnant cap. Findings The effects of the surface conductivity and size of the stagnation region alongside the bulk thermal conductivities and viscosities of the drop and surrounding media are evaluated. The terminal velocity of the drop is shown to have a monotonic dependence on the conductivity of the stagnant cap. The bounds to the terminal velocity increment due to the stagnant cap are derived. These bounds can be of significance to multiphysics problems involving particle laden drops, Pickering emulsions and other multi-phase technologies where the conductivity of the surface adsorbents is non-negligible.

Asymmetric imaging through engineered Janus particle obscurants using a Monte Carlo approach for highly asymmetric scattering media

The advancement of imaging systems has significantly ameliorated various technologies, including Intelligence Surveillance Reconnaissance Systems and Guidance Systems, by enhancing target detection, recognition, identification, positioning, and tracking capabilities. These systems can be countered by deploying obscurants like smoke, dust, or fog to hinder visibility and communication. However, these counter-systems affect the visibility of both sides of the cloud. In this sense, this manuscript introduces a new concept of a smoke cloud composed of engineered Janus particles to conceal the target image on one side while providing clear vision from the other. The proposed method exploits the unique scattering properties of Janus particles, which selectively interact with photons from different directions to open up the possibility of asymmetric imaging. This approach employs a model that combines a genetic algorithm with Discrete Dipole Approximation to optimize the Janus particles' geometrical parameters for the desired scattering properties. Moreover, we propose a Monte Carlo-based approach to calculate the image formed as photons pass through the cloud, considering highly asymmetric particles, such as Janus particles. The effectiveness of the cloud in disguising a target is evaluated by calculating the Probability of Detection (PD) and the Probability of Identification (PID) based on the constructed image. The optimized Janus particles can produce a cloud where it is possible to identify a target more than 50% of the time from one side (PID > 50%) while the target is not detected more than 50% of the time from the other side (PD < 50%). The results demonstrate that the Janus particle-engineered smoke enables asymmetric imaging with simultaneous concealment from one side and clear visualization from the other. This research opens intriguing possibilities for modern obscurant design and imaging systems through highly asymmetric and inhomogeneous particles besides target detection and identification capabilities in challenging environments.

A Robust Switchable Oil-In-Water Emulsion Stabilized by Electrostatic Repulsions between Surfactant and Similarly Charged Carbon Dots

How to control the stability of oil-in-water (O/W) emulsions is one of the main topics for scientists working in colloidal systems. Recently, carbon dots (CDs) have received great interest as smart materials because of their excellent physicochemical properties and versatile applications. Herein, for the first time, advanced and switchable O/W emulsions are presented that are stabilized by the synergistic effect of cationic surfactant cetyltrimethylammonium bromide CTAB (emulsifier) and similarly charged CDs (stabilizer). In the formulated emulsion, the cationic surfactant molecules are adsorbed at the oil and water interface to decrease the interfacial tension and enrich the drops with a positive charge to ensure intensive electrostatic repulsions among them. On the contrary, cationic CDs are distributed in the water phase among the droplets to reduce the water secretion and prevent flocculation and droplet coalescence. The stabilizing effect is found to be universal for emulsions of a range of oil phases. Furthermore, the formulated emulsion is found to be switchable between "stable" and "unstable" modes by adding an equivalent of anionic surfactant sodium dodecyl benzene sulphonate (SDBS). The stabilized and switchable O/W emulsions are believed to have wide practical applications in water purification, pharmaceuticals, protein recognition, as well as catalysis.

Application of Janus Particles in Point-of-Care Testing

Janus particles (JPs), named after the two-faced Roman god, are asymmetric particles with different chemical properties or polarities. JPs have been widely used in the biomedical field in recent years, including as drug carriers for targeted controlled drug release and as biosensors for biological imaging and biomarker detection, which is crucial in the early detection and treatment of diseases. In this review, we highlight the most recent advancements made with regard to Janus particles in point-of-care testing (POCT). Firstly, we introduce several commonly used methods for preparing Janus particles. Secondly, we present biomarker detection using JPs based on various detection methods to achieve the goal of POCT. Finally, we discuss the challenges and opportunities for developing Janus particles in POCT. This review will facilitate the development of POCT biosensing devices based on the unique properties of Janus particles.

Recent Progress on Asymmetric Carbon- and Silica-Based Nanomaterials: From Synthetic Strategies to Their Applications

HIGHLIGHTS

The synthetic strategies and fundamental mechanisms of various asymmetric carbon- and silica-based nanomaterials were systematically summarized. The advantages of asymmetric structure on their related applications were clarified by some representative applications of asymmetric carbon- and silica-based nanomaterials. The future development prospects and challenges of asymmetric carbon- and silica-based nanomaterials were proposed.

ABSTRACT

Carbon- and silica-based nanomaterials possess a set of merits including large surface area, good structural stability, diversified morphology, adjustable structure, and biocompatibility. These outstanding features make them widely applied in different fields. However, limited by the surface free energy effect, the current studies mainly focus on the symmetric structures, such as nanospheres, nanoflowers, nanowires, nanosheets, and core–shell structured composites. By comparison, the asymmetric structure with ingenious adjustability not only exhibits a larger effective surface area accompanied with more active sites, but also enables each component to work independently or corporately to harness their own merits, thus showing the unusual performances in some specific applications. The current review mainly focuses on the recent progress of design principles and synthesis methods of asymmetric carbon- and silica-based nanomaterials, and their applications in energy storage, catalysis, and biomedicine. Particularly, we provide some deep insights into their unique advantages in related fields from the perspective of materials’ structure–performance relationship. Furthermore, the challenges and development prospects on the synthesis and applications of asymmetric carbon- and silica-based nanomaterials are also presented and highlighted. [Image: see text]

Size-Controlled Formation of Polymer Janus Discs

A straightforward method is presented for the preparation of nano- to micrometer-sized Janus discs with controlled shape, size, and aspect ratio. The method relies on cross-linkable ABC triblock terpolymers and involves first the preparation of prolate ellipsoidal microparticles by combining Shirasu porous glass (SPG) membrane emulsification with evaporation-induced confinement assembly (EICA). By varying the pore diameter of the SPG membrane, we produce Janus discs with controlled size distributions centered around hundreds of nanometers to several microns. We further transferred the discs to water by mild sulfonation of PS to polystyrene sulfonic acid (PSS) and verified the Janus character by subsequent labelling with cationic nanoparticles. Finally, we show that the sulfonated Janus discs are amphiphilic and can be used as efficient colloidal stabilizers for oil-in-water (O/W) emulsions.

Janus Particles at Fluid Interfaces: Stability and Interfacial Rheology

The use of the Janus motif in colloidal particles, i.e., anisotropic surface properties on opposite faces, has gained significant attention in the bottom-up assembly of novel functional structures, design of active nanomotors, biological sensing and imaging, and polymer blend compatibilization. This review is focused on the behavior of Janus particles in interfacial systems, such as particle-stabilized (i.e., Pickering) emulsions and foams, where stabilization is achieved through the binding of particles to fluid interfaces. In many such applications, the interface could be subjected to deformations, producing compression and shear stresses. Besides the physicochemical properties of the particle, their behavior under flow will also impact the performance of the resulting system. This review article provides a synopsis of interfacial stability and rheology in particle-laden interfaces to highlight the role of the Janus motif, and how particle anisotropy affects interfacial mechanics.

Pt-SiO2 Janus Particles and the Water/Oil Interface: A Competition between Motility and Thermodynamics

Various aspects of the behavior of Janus particles near liquid/liquid interfaces have been studied through different experimental and theoretical realizations, but the effect of motility on the behavior of Janus particles near liquid/liquid interfaces has not been investigated, yet. Here, we demonstrate the ability to engineer the behavior of highly interfacial active Janus particles near a water/oil interface by introducing motility to the system. Passive, i.e., nonmotile, platinum-capped 8 μm silica (Pt-SiO₂) Janus particles exhibit a strong tendency to attach to water/oil interfaces with the Pt-cap facing the oil and the SiO₂ side facing the water phase. In contrast, we show that active, i.e., motile, 8 μm Pt-SiO₂ Janus particles approach the interface, orient in a sideways fashion with the Janus boundary perpendicular to the interface, and then swim in the vicinity of the interface similar to observations reported near solid/liquid interfaces. Active Pt-SiO₂ Janus particles near the water/oil interface show motility as a result of adding H₂O₂ to the particle solution. The decomposition of H₂O₂ into O₂ and H₂O creates a nonuniform gradient of O₂ around the particle that hydrodynamically interacts with the water/decalin boundary. The interaction enables rotation of the particle within the swimming plane that is parallel to the interface but restricts rotation in and out of the swimming plane, thereby preventing adsorption to the liquid/liquid interface.

Temperature and pH Responsive Janus Silica Nanoplates Prepared by the Sol-Gel Process and Postmodification

During the process of emulsifying and hydrolyzing, reactive poly(3-(triethoxysilyl)propyl methacrylate)-b-polystyrene (PTEPM-b-PS) diblock copolymers can self-assemble and become cross-linked to form hollow spheres in situ with polystyrene on their inner surfaces. The addition of tetraethoxysilane (TEOS), which was hydrolyzed and condensed together with PTEPM block, can make those spheres as soft foldable capsules or hard hollow spheres depending on the amount of added TESO. Then postmodification, the surface-initiated Atom Transfer Radical Polymerization (ATRP) was applied to afford stimuli-responsive spheres, and the corresponding responsive Janus nanoplates (RJPs) were finally obtained by crushing those responsive hollow spheres (HSs) showing smart tunable emulsifiability and great potential in oily water purification. This facile method to fabricate HSs and RJPs could be used for preparing different Janus polymer-inorganic capsules and nanoplates with varied functions by changing the chemical composition of copolymer surfactants as well as the postmodification process.

Janus particles: design, preparation, and biomedical applications

Janus particles with an anisotropic structure have emerged as a focus of intensive research due to their diverse composition and surface chemistry, which show excellent performance in various fields, especially in biomedical applications. In this review, we briefly introduce the structures, composition, and properties of Janus particles, followed by a summary of their biomedical applications. Then we review several design strategies including morphology, particle size, composition, and surface modification, that will affect the performance of Janus particles. Subsequently, we explore the synthetic methodologies of Janus particles, with an emphasis on the most prevalent synthetic method (surface nucleation and seeded growth). Following this, we highlight Janus particles in biomedical applications, especially in drug delivery, bio-imaging, and bio-sensing. Finally, we will consider the current challenges the materials face with perspectives in the future directions.

Switchable Oil-in-Water Emulsions Stabilized by Like-Charged Surfactants and Particles at Very Low Concentrations

A novel CO₂/N₂ switchable n-decane-in-water emulsion was prepared, which is stabilized by a CO₂/N₂ switchable surfactant [ N'-dodecyl- N, N-dimethylacetamidine (DDMA)] in cationic form in combination with positively charged alumina nanoparticles at concentrations as low as 0.01 mM and 0.001 wt %, respectively. The particles do not adsorb at the oil-water interface but remain dispersed in the aqueous phase between surfactant-coated droplets. A critical zeta potential of the particles of ca. +18 mV is necessary for the stabilization of the novel emulsions, suggesting that the electrical double-layer repulsions between particles and between particles and oil droplets are responsible for their stability. By bubbling N₂ into the emulsions, demulsification occurs following transformation of DDMA molecules from the surface-active cationic form to the surface-inactive neutral form and desorption from the oil-water interface. Bubbling CO₂ into the demulsified mixtures, cationic DDMA molecules are re-formed, which adsorb to the droplet interfaces, ensuring stable emulsions after homogenization. Compared with Pickering emulsions and traditional emulsions, the amount of switchable surfactant and number of like-charged particles required for stabilization are significantly reduced, which is economically and environmentally benign for practical applications.

Emulsion Droplets Stabilized by Close-Packed Janus Regular Polygonal Particles

In Pickering-Ramsden emulsions, the packing structure of the colloidal particles at the liquid-liquid (or liquid-gas) interface significantly affects the structure and behavior of the emulsion. Here, using a series of platelike particles with regular polygonal shapes and Janus amphiphilicity, we created emulsion droplets stabilized by close-packed polygonal particles at the interface. The systematic variation of the particle morphology shows that the geometrical features of the regular polygons in (curved) planar packing dominate over the self-assembled structures. The structures are tessellations of triangular, square, and hexagonal particles at the surface for large droplets and regular tetrahedral, cubic, and dodecahedral particle shells of triangular, square, and pentagonal particles for small droplets, respectively. This work creates the possibility of geometrically designing the structure and functionality of emulsions.

pH-Responsive Aqueous Bubbles Stabilized With Polymer Particles Carrying Poly(4-vinylpyridine) Colloidal Stabilizer

Free radical dispersion polymerization was conducted to synthesize near-monodispersed, micrometer-sized polystyrene (PS) particles carrying pH-responsive poly(4-vinylpyridine) (P4VP) colloidal stabilizer (P4VP-PS particles). The P4VP-PS particles were extensively characterized in terms of morphology, size, size distribution, chemical composition, surface chemistry, and pH-response using optical and scanning electron microscopies, elemental microanalysis, X-ray photoelectron spectroscopy, laser diffraction particle size analysis, and zeta potential measurement. The P4VP-PS particles can work as a pH-responsive stabilizer of aqueous bubbles by adsorption at the air-water interface. At and above pH 4.0, where the particles have partially protonated/non-protonated P4VP stabilizer with relatively hydrophobic character, particle-stabilized bubbles were formed. Optical and scanning electron microscopy studies confirmed that the P4VP-PS particles were adsorbed at the air-water interface of the bubbles in aqueous media. At and below pH 3.0, where the particles have cationic P4VP stabilizer with water-soluble character, no bubble was formed. Rapid disruption of the bubbles can be induced by decreasing the pH; the addition of acid caused the in situ protonation of pyridine groups in P4VP, which impart water-soluble character to the P4VP stabilizer, and the P4VP-PS particles were desorbed from the air-water interface. The bubble stabilization/destabilization cycles could be repeated at least five times.

Gas Bubbles Stabilized by Janus Particles with Varying Hydrophilic-Hydrophobic Surface Characteristics

Micrometer-sized polymer-grafted gold-silica (Au-SiO₂) Janus particles were fabricated by vacuum evaporation followed by polymer grafting. The Janus particle diameter, diameter distribution, morphology, surface chemistry, and water wettability were characterized by optical microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and contact angle measurements. The optical microscopy results showed that the polystyrene (PS)-grafted Au-SiO₂ Janus particles exhibited monolayer adsorption at the air-water interface and could stabilize bubbles, preventing their coalescence for more than 1 month. The hydrophobic PS-grafted Au and hydrophilic SiO₂ surfaces were exposed to the air and water phases, respectively. Bare Au-SiO₂ and poly(2-(perfluorobutyl)ethyl methacrylate) (PPFBEM)-grafted Au-SiO₂ Janus particles could also stabilize bubbles for up to 2 weeks. By contrast, bare silica particles did not stabilize bubbles and were dispersed in water. The bubbles that formed in the PS-grafted Janus particle system were more stable than those formed in the bare Au-SiO₂ Janus particles, PPFBEM-grafted Au-SiO₂ Janus particles, and SiO₂ particle systems because of the high adsorption energy of the PS-grafted particles at the air-water interface.

Pickering Membranes Stabilized by Saturn Particles

We report on a novel method to synthesize particles-called by us Saturn particles-having two hydrophobic caps that oppose each other and are separated from each other by a hydrophilic belt that encircles the particle. Mixtures of these particles with water and air, without the usage of low molar mass surfactants, easily form Pickering foams and Pickering membranes that are stable for days. These Pickering membranes are composed of a thin film of water into which the particles are embedded in such a way that the belt is surrounded by the water and the caps protrude out of the water into the air at the top and bottom side of the water film. As expected for a liquid membrane, these Pickering membranes are permeable for gases, with the permeance being proportional to the solubility and diffusion coefficient of the gas considered. Experimentally obtained permeance values agree reasonably well with theoretical calculations.

Synthesis of gold nanoparticles supported on functionalized nanosilica using deep eutectic solvent for an electrochemical enzymatic glucose biosensor

Engineering of nanoparticle (NP) surfaces offers an effective approach for the development of enzymatic biosensors or microbial fuel cells with a greatly enhanced direct electron transport process. However, lack of control over the surface functionalization process and the operational instability of the immobilized enzymes are serious issues. Herein, we demonstrate a facile and green deep eutectic solvent (DES)-mediated synthetic strategy for efficient amine-surface functionalization of silicon dioxide and to integrate small gold nanoparticles (AuNPs) for a glucose biosensor. Owing to the higher viscosity of the DES, it provides uniform surface functionalization and further coupling of the AuNPs on the SiO₂ support with improved stability and dispersion. The amine groups of the functionalized Au-SiO₂NPs were covalently linked to the FAD-center of glucose oxidase (GOx) through glutaraldehyde as a bifunctional cross-linker, which promotes formation of "electrical wiring" with the immobilized enzymes. The Au-SiO₂NP/GOx/GC electrode exhibits direct electron transfer (DET) for sensing of glucose with a sensitivity of 9.69 μA mM^-1, a wide linear range from 0.2 to 7 mM and excellent stability. The present green DES-mediated synthetic approach expands the possibilities to support different metal NPs on SiO₂ as a potential platform for biosensor applications.

Stimuli-Responsive Bubbles and Foams Stabilized with Solid Particles

Particle-stabilized bubbles and foams have been observed and used in a wide range of industrial sectors and have been exploited as a technology platform for the production of advanced functional materials. The stability, structure, shape, and movement of these bubbles and foams can be controlled by external stimuli such as the pH, temperature, magnetic fields, ultrasonication, mechanical stress, surfactants, and organic solvents. Stimuli-responsive modes can be categorized into three classes: (i) bubbles/foams whose stability can be controlled by the adsorption/desorption/dissolution of solid particles to/from/at gas-liquid interfaces, (ii) bubbles/foams that can move, and (iii) bubbles/foams that can change their shapes and structures. The stimuli-responsive characteristics of bubbles and foams offer potential applications in the areas of controlled encapsulation, delivery, and release.

Influence of hydrophobicity distribution of particle mixtures on emulsion stabilization

Whilst emulsions stabilized by uniform particles are well established, the emulsification behavior of heterogeneous mixtures of particles with varying hydrophobicity is rarely examined. Consequently, the influence of the distribution of particle hydrophobicity on oil-water emulsion stabilization is poorly understood. In the present work, the wettability of the bitumen froth fine solids from Alberta oil sands was studied by film flotation and toluene-water emulsification tests, before and after a hydrothermal treatment at 300-420°C. This approach provided a series of populations of particles with different distributions of hydrophobicity. The initial fine particles in the bitumen froth had a critical surface tension ranging from 26 to 56mN/m, with a mean value of 39mN/m. Hydrothermal treatment at 300-420°C progressively shifted the hydrophobicity distribution of the fine particles, resulting in a lower mean critical surface tension and a narrower critical surface tension range. The emulsifying capacity of the fine particle mixtures, as indicated by the volume of the produced toluene-water emulsions, was unrelated to the mean critical surface tension. Instead, emulsification depended on the proportion of a specific sub-fraction of particles with a critical surface tension of 27-30mN/m. This sub-fraction of particles, with intermediate hydrophobicity, dominated the emulsification behavior of the particle mixtures.

Dependence of the Internal Structure on Water/Particle Volume Ratio in an Amphiphilic Janus Particle-Water-Oil Ternary System: From Micelle-like Clusters to Emulsions of Spherical Droplets

Amphiphilic Janus particles (AJP), composed of hydrophilic and hydrophobic hemispheres, are one of the simplest anisotropic colloids, and they exhibit higher surface activities than particles with homogeneous surface properties. Consequently, a ternary system of AJP, water, and oil can form extremely stable Pickering emulsions, with internal structures that depend on the Janus structure of the particles and the system composition. However, the detail of these structures has not been fully explored, especially for the composition range where the amount of the minority liquid phase and AJP are comparable, where one would expect the Janus characteristics to be directly reflected. In this study, we varied the volume ratio of the particles and the minority liquid phase, water, by 2 orders of magnitude around the comparable composition range, and observed the resultant structures at the resolution of the individual particle dimensions by optical microscopy. When the volume ratio of water is smaller than that of the Janus particles, capillary interactions between the hydrophilic hemispheres of the particles induce micelle-like clusters in which the hydrophilic sides of the particles face inward. With increasing water content, these clusters grow into a rodlike morphology. When the water volume exceeds that of the particles, the structure transforms into an emulsion state composed of spherical droplets, colloidosomes, because of the surface activity of particles at the liquid-liquid interface. Thus, we found that a change in volume fraction alters the mechanism of structure formation in the ternary system, and large resulting morphological changes in the self-assembled structures reflect the anisotropy of the particles. The self-assembly shows essential commonalities with that in microemulsions of surfactant molecules, however the AJP system is stabilized only kinetically. Analysis of the dependence of the emulsion droplet size on composition shows that almost all the particles are adsorbed at the water-oil interface; i.e., the particles show ideal surface activity.

Fast swelling strategy for flower-like micro-sized colloidal surfactants with controllable patches by regulating the Tg of seed particles

We are reporting an efficient fast swelling procedure by regulating the glass transition temperature T_g of poly(glycidyl methacrylate) (PGMA) seed particles via copolymerization with n-butyl acrylate (nBA).

Tuning Amphiphilicity of Particles for Controllable Pickering Emulsion

Pickering emulsions with the use of particles as emulsifiers have been extensively used in scientific research and industrial production due to their edge in biocompatibility and stability compared with traditional emulsions. The control over Pickering emulsion stability and type plays a significant role in these applications. Among the present methods to build controllable Pickering emulsions, tuning the amphiphilicity of particles is comparatively effective and has attracted enormous attention. In this review, we highlight some recent advances in tuning the amphiphilicity of particles for controlling the stability and type of Pickering emulsions. The amphiphilicity of three types of particles including rigid particles, soft particles, and Janus particles are tailored by means of different mechanisms and discussed here in detail. The stabilization-destabilization interconversion and phase inversion of Pickering emulsions have been successfully achieved by changing the surface properties of these particles. This article provides a comprehensive review of controllable Pickering emulsions, which is expected to stimulate inspiration for designing and preparing novel Pickering emulsions, and ultimately directing the preparation of functional materials.

Surface activity of Janus particles adsorbed at fluid-fluid interfaces: Theoretical and experimental aspects

Since de Gennes coined in 1992 the term Janus particle (JP), there has been a continued effort to develop this field. The purpose of this review is to present the most relevant theoretical and experimental results obtained so far on the surface activity of amphiphilic JPs at fluid interfaces. The surface activity of JPs at fluid-fluid interfaces can be experimentally determined using two different methods: the classical Langmuir balance or the pendant drop tensiometry. The second method requires much less amount of sample than the first one, but it has also some experimental limitations. In all cases collected here the JPs exhibited a higher surface or interfacial activity than the corresponding homogeneous particles. This reveals the significant advantage of JPs for the stabilization of emulsions and foams.

Foams stabilized with solid particles carrying stimuli-responsive polymer hairs

Submicrometer-sized polystyrene (PS) particles carrying stimuli-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) hairs with degrees of polymerization of 30, 60 and 90 were synthesized by dispersion polymerization and used as a particulate foam stabilizer. The effects of the composition of these PDEA-PS particles and foam formation conditions on foamability, foam stability and foam microstructures were extensively investigated. The hairy particles were found to work as an effective stabilizer of aqueous foams in basic media, in which the PDEA hairs are not protonated and thus the particle surfaces exhibit suitable wettability at the air-water interface. In contrast, little to no foam or unstable foams were formed in acidic aqueous media, in which the hairs are protonated and are therefore water soluble. Particles carrying longer hairs resulted in greater foamability and more highly stabilized foams that were capable of persisting for more than one month. Foams were found to form in a narrower pH range when using PS particles with longer hairs, due to both entropic and image charge effects. Data obtained from the touch mixer mixing method showed that both foamability and foam stability increased with increases in the concentration of polymer particles up to 10 wt%, because higher concentrations allowed greater air-water interfacial areas to be stabilized. Conversely, only minimal foam was obtained at and above 15 wt% because of the high viscosity of the resulting aqueous particle dispersion. Trials using the homogenizer mixing method showed that foam with a cream-like texture could be formed even at 40 wt% particle concentration as a result of the improved mixing efficiency. Defoamation could be induced by exposing the foams to HCl vapor. Exposure to acidic vapor led to in situ protonation of the 2-(diethylamino)ethyl methacrylate residues, rendering the PDEA hairs hydrophilic and water soluble, and desorption of the PDEA-PS particles from the air-water interface.

Shape-Controlled Metal-Metal and Metal-Polymer Janus Structures by Thermoplastic Embossing

We report the fabrication of metal-metal and metal-polymer Janus structures by embossing of thermoplastic metallic glasses and polymers. Hybrid structures with controllable shapes and interfaces are synthesized by template-assisted embossing. Different manufacturing strategies such as co-embossing and additive embossing are demonstrated for joining the materials with diverse compositions and functionalities. Structures with distinct combinations of properties such as hydrophobic-hydrophilic, opaque-transparent, insulator-conductor, and nonmagnetic-ferromagnetic are produced using this approach. These anisotropic properties are further utilized for selective functionalization of Janus structures.

Introducing asymmetric functionality into MOFs via the generation of metallic Janus MOF particles

Herein we report a versatile methodology for engineering metallic Janus MOF particles based on desymmetrization at interfaces, whereby each MOF particle is partially coated with a desired metal. We demonstrate that it enables the fabrication of homogeneous Janus MOF particles according to the MOF (ZIF-8, UiO-66 or UiO-66-SH), the metal (Au, Co or Pt), the MOF particle size (from the micrometer to the submicrometer regime) and the metal-film thickness (from 5 nm to 50 nm) employed. We anticipate that our strategy could be applied to impart new functionalities to MOFs, including asymmetric functionalization, magnetic-guidance and motorization.

Double-phase-functionalized magnetic Janus polymer microparticles containing TiO2 and Fe2O3 nanoparticles encapsulated in mussel-inspired amphiphilic polymers

Recently, anisotropic colloidal polymeric materials including Janus microparticles, which have two distinct aspects on their surfaces or interiors, have garnered much interest due to their anisotropic alignment and rotational orientation with respect to external electric or magnetic fields. Janus microparticles are also good candidates for pigments in "twisting ball type" electronic paper, which is considered promising for next-generation flexible display devices. We demonstrate here a universal strategy to encapsulate inorganic nanoparticles and to introduce different such inorganic nanoparticles into distinct polymer phases in Janus microparticles. TiO2 and Fe2O3 nanoparticles were separately encapsulated in two different mussel-inspired amphiphilic copolymers, and then organic-inorganic composite Janus microparticles were prepared by simple evaporation of solvent from the dispersion containing the polymer and nanoparticle. These Janus microparticles were observed to rotate quickly in response to applied magnetic fields.

Comparative study of the self-assembly of gold and silver nanoparticles onto thiophene oil

Nanoparticle self-assembly is fundamentally important for bottom-up functional device fabrication. Currently, most nanoparticle self-assembly has been achieved with gold nanoparticles (AuNPs) functionalized with surfactants, polymeric materials, or cross-linkers. Reported herein is a facile synthesis of gold and silver nanoparticle (AgNP) films assembled onto thiophene oil by simply vortex mixing neat thiophene with colloidal AuNPs or AgNPs for ∼1 min. The AuNP film can be made using every type of colloidal AuNPs we have explored, including sodium borohydride-reduced AuNPs with a diameter of ∼5 nm, tannic acid-reduced AuNPs of ∼10 nm diameter, and citrate-reduced AuNPs with particle sizes of ∼13 and ∼30 nm diameter. The AuNP film has excellent stability and it is extremely flexible. It can be stretched, shrunken, and deformed accordingly by changing the volume or shape of the enclosed thiophene oil. However, the AgNP film is unstable, and it can be rapidly discolored and disintegrated into small flakes that float on the thiophene surface. The AuNP and AgNP films prepared in the glass vials can be readily transferred to glass slides and metal substrates for surface-enhanced Raman spectral acquisition.

Comparison of the interfacial activity between homogeneous and Janus gold nanoparticles by pendant drop tensiometry

The interfacial activity of 3.5 nm homogeneous (HPs) and amphiphilic Janus gold nanoparticles (JPs) was characterized by pendant drop tensiometry for water/air and water/decane interfaces. This technique requires a smaller quantity of nanoparticles than the traditional Langmuir balance technique. The direct deposition at the interface of the nanoparticles dispersed in a spreading solvent also requires smaller quantities of sample than does adsorption from the bulk. From the growing and shrinking of the pendant drops, the interfacial activity of the nanoparticles can be evaluated and compared within a wide range of area per particle. In this work, the JPs exhibited a higher interfacial activity than did the HPs in all cases. A hard disk model fits the piecewise compression isotherm of the HPs, yet this model underestimates the interactions between the JPs adsorbed at the interface.

Thermodynamically stable emulsions using Janus dumbbells as colloid surfactants

One of the most important properties of emulsions is their stability. Most emulsions stabilized with molecular surfactants tend to lose their stability over time via different mechanisms. Although the stability of emulsions stabilized with homogeneous particles have been shown to be superior to that of surfactant-stabilized emulsions, these Pickering emulsions nevertheless are only kinetically stable and thus can undergo destabilization. Janus particles that have two opposite wetting surfaces have shown promise in imparting emulsions with long-term stability because of their strong attachment to the oil-water interface. In this theoretical study, we consider thermodynamics of emulsion stabilization using amphiphilic Janus dumbbells, which are nonspherical particles made of two partially fused spherical particles of opposite wettability. These amphiphilic dumbbells are attractive candidates as colloid surfactants for emulsion stabilization because highly uniform Janus dumbbells can be synthesized in large quantities; thus, their application in emulsion stabilization can become practical. Our theoretical calculation demonstrates that Janus dumbbells can indeed generate thermodynamically stable Pickering emulsions. In addition, we also find that there exists a total oil-water interfacial area that results in the lowest energy state in the system, which occurs when Janus dumbbells available in the system are completely consumed to fully cover the droplet interfaces. We show that the geometry of dumbbells as well as the composition of the emulsion mixtures has significant influences on the average size of dumbbell-stabilized emulsions. We also investigate the effect of asymmetry of Janus dumbbells on the average droplet radius. Our results clearly show that amphiphilic Janus dumbbells provide unique opportunities in stabilizing emulsions for various applications.

Self-assembly of Janus ellipsoids II: Janus prolate spheroids

In self-assembly, the anisotropy of the building blocks and their formation of complex structures have been the subject of considerable recent research. Extending recent research on Janus particles and completing the study of Janus spheroids, we conduct a systematic investigation on the self-assembly of Janus prolate spheroids based on a primitive model that we proposed. Janus prolate spheroids are particles that have a prolate spheroidal body and two hemi-surfaces along the major axis coded with different chemical properties. Using Monte Carlo simulations, we investigate the effects of the aspect ratio on the self-assembly process. In contrast to the vesicle-like aggregates for Janus oblate spheroids, we obtain various ordered cluster structures for Janus prolate spheroids through self-assembly. With an increasing aspect ratio, we find a transition of cluster morphology, from vesicles to tubular micelles and micelles. In particular, a relatively small change in the aspect ratio leads to a rather significant change in morphology. We apply a cluster analysis to understand the mechanism associated with such a transition.