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Incorporating unmodified silica nanoparticles onto polymer latexes to fabricate aqueous polymer dispersions without relying on electrostatic attraction during the Pickering emulsion polymerization process still faces challenges. For negatively charged silica nanoparticles to successfully adsorb onto polymer latexes, particularly in an anionic initiator emulsion polymerization system, they have remained elusive without the use of auxiliary monomers and cationic initiators. This study investigates various experimental parameters, such as emulsion polymerization temperature, monomer solubility, salt concentration, and cation type, to elucidate the factors influencing the adsorption of unmodified silica nanoparticles in Pickering emulsion polymerization. While poly(methyl methacrylate) (PMMA)/SiO2 hybrid latexes can be obtained under pH conditions of 5-6 and at temperatures of 65 °C or below, the loading rate of silica nanoparticles decreases as the reaction temperature increases, resulting in bare PMMA latexes without silica nanoparticle adsorption at temperatures exceeding 70 °C. Introducing styrene (St) into the monomer mixture with methyl methacrylate in a ratio of up to 10 wt % leads to a gradual decrease in silica nanoparticle loading rate, from 27.3 to 8.2 wt %, attributed to the low solubility of St in water. Furthermore, the presence of sodium ions (Na+) is found to be crucial for silica nanoparticle adsorption onto PMMA latexes, as the sodium ions have a stabilizing effect on both the silica nanoparticles and the silica nanoparticle-armored latexes. These findings highlight the complex nature of Pickering emulsion polymerization in the presence of unmodified silica nanoparticles, demonstrating that the loading rate of silica nanoparticles onto polymer latexes is influenced by various factors. These insights pave the way for developing aqueous polymer dispersions with high silica nanoparticle loading rates onto polymer latexes, which is a desirable trait in the coating industry.

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In this work, a series of polymer microcapsules based on UV-curable prepolymers are prepared by combining an emulsion template and photopolymerization. The modulation of the shell structure is achieved by employing UV-curable prepolymers with different chemical structures (polyurethane acrylates, polyester acrylates, and epoxy acrylates) and functionalities (di-, tetra-, and hex-). The relationships between the shell structure and the microcapsule properties are investigated in detail. The results show that the properties of the microcapsules can be effectively regulated by adjusting the composition and cross-linking density of the shell. Epoxy acrylate-based microcapsules exhibit higher impermeability, solvent resistance, and barrier and mechanical properties than polyurethane acrylate and polyester acrylate-based microcapsules. Using UV-curable prepolymer with high functionality as a shell-forming material could effectively improve the impermeability, solvent resistance, and barrier and mechanical properties of microcapsules. In addition, the dispersion of microcapsules in the coating matrix tends to follow the "similar component, better compatibility" principle, i.e., a uniform dispersion of the microcapsule in the coating matrix is more easily achieved when the compositions of the microcapsule shell and coating are similar in structure. The convenient adjustment of the shell structure and the investigation of the "structure-property" relationship provide guidance for the further controlled design of microcapsules.

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Phase inversion of Pickering emulsions from water-in-oil (W/O) to oil-in-water (O/W) is achieved by the formation of an interfacial particle bilayer using negatively charged and positively charged particles dispersed in water and oil, respectively, before emulsification. A mechanism based on electrostatic attraction across the toluene-water interface is proposed and verified by systematic investigation of the parameters that affect the surface charge of negatively charged particles such as pH and salt concentration. Cationic silica-FITC particles (600 nm) can be dispersed in toluene and stabilize W/O emulsions alone; phase inversion of this emulsion can be induced by the addition of anionic silica-RB particles in the aqueous phase at a concentration of 1.0 wt % or above. It is revealed that silica-RB particles of a smaller size (100 nm) can induce emulsion phase inversion at a much lower concentration (0.4 wt %) and an interfacial particle bilayer is clearly revealed by CLSM and SEM images. By tuning the surface charge density of silica-RB particles, the electrostatic attraction mechanism leading to the formation of the interfacial particle bilayer is confirmed and emulsion stability can be tuned as demonstrated by osmotic pressure enhancement results obtained from centrifugation.

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The contact angle of colloidal particles at an oil-water interface plays a crucial role in determining Pickering emulsion stability and emulsion type, but the contact angle cannot be directly determined using conventional methods. In this work, a Pickering emulsion was prepared with photocurable resin as the internal phase containing silica nanoparticle stabilizers. Particles adsorbed at the oil-water interface were then fixed through UV curing, allowing for the investigation of various parameters that influence the contact angle of colloidal particles at the interface. After curing, the contact angle can then be observed using scanning electron microscopy and subsequently measured. The contact angle of interfacial adsorbed silica nanoparticles gradually decreases as the size increases due to the line tension at the three-phase contact line, but, more importantly, we found that the surface chemistry of the silica nanoparticles plays the most important role in determining the contact angle. The fast fixation of solid nanoparticles at emulsion interfaces facilitates accurate measurements of the partition of particles between oil and water, providing a new method for studying the factors that affect Pickering emulsion stability.

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This work presented a new concept for designing dual-chamber self-healing microcapsules, which encapsulated both healing and curing species within a single microcapsule via Pickering emulsion photopolymerization. In our strategy, robust SiO2 spheres encapsulating curing agent were firstly synthesized and used as Pickering emulsifiers to prepare emulsions loaded with self-healing agent and photo-curable monomer. Upon exposure to UV light, the photo-curable monomer underwent photo-crosslinking and converted into microcapsules wall. In the meanwhile, the SiO2 spheres encapsulating curing agent were trapped in the microcapsule wall. The dual-chamber microcapsule which loaded the healing agent in its core and curing agent within its shell, was thus prepared. The presence of both the encapsulated healing and curing agent within a single capsule was demonstrated by infrared spectrometry and thermogravimetric analysis. Upon fracture, the healing agent and curing agent are released simultaneously from the dual-chamber microcapsule, which facilitates the interaction of the two agents, and enhances the healing efficiency. Up to 85% healing efficiency of the epoxy resin was achieved in 1 h, which was much higher than that of the traditional double microcapsules self-healing system (65%), demonstrating the excellent self-healing performance of the dual-chamber microcapsules. It has been demonstrated that the coating based on dual-chamber microcapsule presented reliable and outstanding self-healing anti-corrosion efficiency. By changing the species of healing agent, curing agent and wall substances (photo-curable monomer), the as-prepared dual-chamber microcapsules can meet different requirements of versatile self-healing system.

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Dye-filled microcapsules are an attractive way to identify microscopic damage of materials by the naked eye. However, there are many disadvantages in traditional microcapsule-based self-reporting materials, such as a poor self-reporting effect. A new concept for the design of self-reporting microcapsules is presented here. Our work develops a novel kind of dual-compartmental microcapsule via Pickering emulsion photopolymerization, which can encapsulate two interacting species ("pro-dye" and "developer") separately in a single microcapsule. In our strategy, SiO2 microspheres encapsulating polyetheramine (PEA, developer) were first prepared and employed as a Pickering emulsifier to stabilize oil-in-water emulsions, in which the oil phase consisted of 2',7'-dichlorofluorescein (DCF, pro-dye) and a monomer. After the monomer polymerization, a dual-compartment microcapsule, which encapsulated the pro-dye in the core and the developer in the shell, was obtained. Upon the rupture of the microcapsule, the pro-dye and the developer were released simultaneously and reacted to yield a pronounced chromogenic response. Compared with traditional double-microcapsule systems, this dual-compartment microcapsule system demonstrated a more efficient and pronounced self-reporting effect. This is the first time that a double-encapsulation scheme involving the compartmentalized release of two interacting species within a single microcapsule has been demonstrated for self-reporting, which overcomes the tough problems of the uneven distribution of the traditional double-microcapsule systems.

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Thermo-responsive microgels are unique stabilizers for stimuli-sensitive Pickering emulsions that can be switched between the state of emulsification and demulsification by changing the temperature. However, directly temperature-triggering the phase inversion of microgel-stabilized emulsions remains a great challenge. Here, a hybrid poly(N-isopropylacrylamide)-based microgel has now been successfully fabricated with tunable wettability from hydrophilicity to hydrophobicity in a controlled manner. Engineered microgels are synthesized from an inverse emulsion stabilized with hydrophobic silica nanoparticles, and the swelling-induced feature can make the resultant microgel behave like either hydrophilic or hydrophobic colloids. Remarkably, the phase inversion of such microgel-stabilized Pickering emulsions can be in situ regulated by temperature change. Moreover, the engineered microgels were capable of stabilizing water-in-oil Pickering emulsions and encapsulation of enzymes for interfacial bio-catalysis, as well as rapid cargo release triggered by phase inversion.

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The use of soft microgel particles for stabilizing emulsions has captured increasing attention across a wide range of disciplines in the past decades. Being soft, the nanoparticles, which are spherical in solution, undergo a structure change when adsorbed at the oil-water interface. This morphology change leads to the special dynamic properties of interface layers and packing structures, which then alter the interfacial tension and rheological properties of the interface. In addition, emulsions stabilized by these particles, known as Pickering emulsions, can be triggered by changing a variety of environmental conditions, which is especially desirable in industrial applications such as oil transportation processes and biphasic catalysis, where the emulsions can be stabilized and destabilized on demand. Although many studies of the behavior of soft microgel nanoparticles at interfaces have been reported, there are still many challenges in gaining a full understanding of the structure, dynamics, and effective interactions between microgels at the interface. In this Feature Article, we address some of the most important findings and problems in the field. They include the adsorption kinetics of soft microgel particles, particle conformation at the interface, pH and thermal responsiveness, and the interfacial rheological properties of soft-particle-occupied interfaces. We also discuss some potential benefits of using emulsions stabilized by soft particles for food applications as an alternative to conventional surfactant-based systems. We hope to encourage further investigation of these problems, which would be very beneficial to extending this knowledge to all other related soft matter systems.

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HYPOTHESIS: Hollow particles have been used in a variety of applications and many methods have been developed. Hollow particles templated from Pickering emulsions due to nanoparticle adsorption at the oil-water interface usually suffer from the collapsed morphologies and low thermal and solvent stability and enhancement of the shell can significantly improve the hollow particle performance. EXPERIMENTS: This paper reports hollow particles templated from Pickering emulsion droplets in combination with UV photopolymerization. The Pickering emulsions were stabilized by functional silica nanoparticles at the O/W interface and the oil phase contains photosensitive reactants, initiator, catalyst and volatile solvents. The effects of nanoparticles concentration, O/W volume ratio, pH, dispersion speed and time on the stabilization of Pickering emulsion were firstly carried out and the properties of hollow particles formed by traditional interfacial crosslinking and UV photopolymerization were systematically investigated. FINDINGS: Compared with previous interfacial crosslinking method, the UV photopolymerization method gives much more robust shells and we show in the paper that the hollow particles have much higher solvent resistance and thermal stability. The enhancement of thermal stability and solvent resistance of the hollow particle could extend its applications to more harsh fields such as self-healing coatings used in deep sea conditions.

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Here we report on the successful preparation of open-cellular macroporous 3D scaffolds templated from gelatin nanoparticle-stabilized HIPEs with acrylamide (AM) as the monomer in the continuous phase. Tuning the gelatin nanoparticle concentration or AM content led to different porous structures with void diameters varying between 30 and 78 µm. More importantly, keeping HIPEs at room temperature to undergo a limited kinetic coarsening before polymerization could greatly improve the interconnectivity and pore size of the scaffolds, with the average diameters (approx. 118 µm) being enlarged 1.5-fold. Additionally, the scaffolds had a character of soft tissue with compressive modulus more than 150 kPa. The cell culture assay confirmed that HepG2 cells not only could adsorb on but also were grown inside the scaffolds, representing a characteristic of the good biocompatibility of the scaffolds. Our work suggests that the 3D scaffolds fabricated from gelatin nanoparticle-stabilized HIPE templates are promising culture substrates for a wide range of applications in the biomedical field.

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Coating a liquid with a particle shell not only renders a droplet superhydrophobic but also isolates a well-confined microenvironment for miniaturized chemical processes. Previously, we have demonstrated that particles at the liquid marble interface provide an ideal platform for the site-selective modification of superhydrophobic particles. However, the need for a special chemical reaction limits their potential use for the fabrication of Janus particles with various properties. Herein, we combine the employment of liquid marbles as microreactors with the remarkable adhesive ability of polydopamine to develop a general route for the synthesis of Janus particles from micrometer-sized superhydrophobic particles. We demonstrate that dopamine polymerization and deposition inside liquid marbles could be used for the selective surface modification of microsized silica particles, resulting in the formation of Janus particles. Moreover, it is possible to manipulate the Janus balance of the particles via the addition of surfactants and/or organic solvents to tune the interfacial energy. More importantly, owing to the many functional groups in polydopamine, we show that versatile strategies could be introduced to use these partially polydopamine-coated silica particles as platforms for further modification, including nanoparticle immobilization, metal ion chelation and reduction, as well as for chemical reactions. Given the flexibility in the choice of cores and the modification strategies, this developed method is distinctive in its high universality, good controllability, and great practicability.

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Nanoparticles at the air/liquid interface can serve as solid separating barriers to form stable foams or liquid marbles depending on the wettability of the nanoparticles. This paper presents an effect that enables the insertion and confinement of air bubbles inside a liquid marble, based on encapsulating an air bubble surrounded by surfactant molecules or hydrophilic particles. We have demonstrated that more than one bubble can be inserted and trapped inside one liquid marble so that liquid marbles can be divided into several separate compartments. The findings presented here may stimulate fundamental studies of this novel bubble-marble phenomenon, as well as developments of various practical applications.

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Little attention has been paid to the participation of the shell of silica-particle-based liquid marbles and their influence on chemical reactions. The fabrication of liquid marbles with the encapsulating particle shells not only act as protecting layers to provide a confined environment, but also provide the reactive substrate surfaces to regulate the classical silver mirror reaction. Fabrication of silver mirrors with different morphologies was achieved by modifying particle surface properties, which could further lead to Janus liquid marbles. The different evaporation behavior of microreactors was demonstrated. Micrometer-sized silica particles were used for the preparation of monolayer-stabilized liquid marbles, which show great potential in fabricating Janus particles from superhydrophobic particles that are not attainable from Pickering emulsions.

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The coating of solid particles on the surface of liquid in air makes liquid marbles a promising approach in the transportation of a small amount of liquid. The stabilization of liquid marbles by polymeric latex particles imparts extra triggers such as pH and temperature, leading to the remote manipulation of droplets for many potential applications. Because the functionalized polymeric latexes can exist either as colloidally stable latex or as flocculated latex in a dispersion, the drying of latex dispersions under different conditions may play a significant role in the stabilization of subsequent liquid marbles. This article presents the investigation of liquid marbles stabilized by poly(styrene-co-methacrylic acid) (PS-co-MAA) particles drying under varied conditions. Protonation of the particles before freeze drying makes the particles excellent liquid marble stabilizers, but it is hard to stabilize liquid marbles for particles dried in their deprotonated states. The static properties of liquid marbles with increasing concentrations of protonating reagent revealed that the liquid marbles are gradually undermined by protonating the stabilizers. Furthermore, the liquid marbles stabilized by different particles showed distinct behaviors in separation and merging manipulated by tweezers. This study shows that the initial state of the particles should be carefully taken into account in formulating liquid marbles.

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In this paper, we report for the first time the use of a well-dispersed gelatin particle as a representative of natural and biocompatible materials to be an effective particle stabilizer for high internal phase emulsion (HIPE) formulation. Fairly monodispersed gelatin particles (∼200 nm) were synthesized through a two-step desolvation method and characterized by dynamic light scattering, ζ-potential measurements, scanning electron microscopy, and atomic force microscopy. Those protein latexes were then used as sole emulsifiers to fabricate stable oil-in-water Pickering HIPEs at different concentrations, pH conditions, and homogenization times. Most of the gelatin particles were irreversibly adsorbed at the oil-water interface to hinder droplet coalescence, such that Pickering HIPEs can be formed by a small amount of gelatin particles (as low as 0.5 wt % in the water phase) at pH far away from the isoelectric point of the gelatin particles. In addition, increasing homogenization time led to narrow size distribution of droplets, and high particle concentration resulted in more solidlike Pickering HIPEs. In vitro controlled-release experiments revealed that the release of the encapsulated ß-carotene can be tuned by manipulating the concentration of gelatin particles in the formulation, suggesting that the stable and narrow-size-distributed gelatin-stabilized HIPEs had potential in functional food and pharmaceutical applications.

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Various aspects of particle-stabilized emulsions (or so-called Pickering emulsions) have been extensively investigated during the last two decades, but the preparation of uniform Pickering emulsion droplets via a simple and scalable method has been sparingly realized. We report the preparation of uniform Pickering emulsions by Shirasu porous glass (SPG) membrane emulsification. The size of the emulsion droplets ranging from 10-50 µm can be precisely controlled by the size of the membrane pore. The emulsion droplets have a high monodispersity with coefficients of variation (CV) lower than 15% in all of the investigated systems. We further demonstrate the feasibility of locking the assembled particles at the interface, and emulsion droplets have been shown to be excellent templates for the preparation of monodisperse colloidosomes that are necessary in drug-delivery systems.

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We report a facile method for preparing porous structured TiO2 materials by templating from Pickering high-internal phase emulsions (HIPEs). A Pickering HIPE with an internal phase of up to 80 vol %, stabilized by poly(N-isopropylacrylamide)-based microgels and TiO2 solid nanoparticles, was first formulated and employed as a template to prepare the porous TiO2 materials with an interconnected structure. The resultant materials were characterized by scanning electron microscopy, X-ray diffraction, and mercury intrusion. Our results showed that the parent emulsion droplets promoted the formation of macropores and interconnecting throats with sizes of ~50 and ~10 µm, respectively, while the interfacially adsorbed microgel stabilizers drove the formation of smaller pores (~100 nm) throughout the macroporous walls after drying and sintering. The interconnected structured network with the bimodal pores could be well preserved after calcinations at 800 °C. In addition, the photocatalytic activity of the fabricated TiO2 was evaluated by measuring the photodegradation of Rhodamine B in water. Our results revealed that the fabricated TiO2 materials are good photocatalysts, showing enhanced activity and stability in photodegrading organic molecules.

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Pickering emulsions stabilized by solid particles have been widely studied in the past decades due to improved stability and reduced use of small molecular surfactants. Recently, the application of Pickering emulsions in pharmaceutics has been attracting increasing attention but very limited practical use has been demonstrated, because most of the investigated particles possess poor biodegradability, which is inappropriate in pharmaceutics. Some reported biodegradable particles were too hydrophilic to stabilize emulsions, which needs further particle modification or additional surfactants. Fortunately, biodegradable poly(d,l-lactic-co-glycolic acid) (PLGA) with tunable hydrophilicity makes itself a promising material to prepare Pickering emulsions. However, the mechanism of emulsion stabilization still remains unknown. Moreover, fabrication of large amounts of uniform-sized and size-controlled PLGA particles by traditional methods is very difficult, which further increases the difficulty to perform the research. In the present study, we applied Shirasu Porous Glass (SPG) premix membrane emulsification to solve this problem. The stabilization mechanism of Pickering emulsions stabilized by PLGA particles was systematically studied for the first time. The factors including oil type, particle properties, concentration, molecular weight (Mw) and oil-water volume ratio were analyzed through particle wettability and interfacial influence. We found that octanol was an appropriate oil type, and its small particle size, high particle concentration and high Mw were favorable for emulsion stability. By changing the oil-water volume ratio, stable emulsions were also readily achieved. These studies proved that Pickering emulsions stabilized by PLGA particles had wide potential applications in pharmaceutics and tissue engineering.

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Bimodal colloidal mixtures of nanoparticles and microparticles may show different phase behaviors depending upon the interparticle interaction of both species. In the present work, we examined the stabilization of spherical microparticles using highly charged, spherical nanoparticles. Total internal reflection microscopy (TIRM) was used to measure the interaction forces between a charged microparticle and flat glass substrate in aqueous solutions at varying volume fractions of nanoparticles of the same sign. We found that, in the system containing of highly charged nanoparticles, microparticle, and glass substrate, non-adsorbing charged nanoparticles in solution did not lead to depletion attraction. Instead, the addition of nanoparticles was to consistently create a repulsive force between the microparticle and glass substrate even at a very low nanoparticle volume fraction. This result might attributed to the formation of thin shells (halos) with a high local nanoparticle volume fraction to the region near the glass surface, resulting in electrostatic repulsion between the decorated surfaces. This study demonstrates that nanoparticle halos can also arise in binary systems of mutually but highly repulsive microparticle/nanoparticle dispersions.

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Multiple emulsions are complex polydispersed systems in which both oil-in-water (O/W) and water-in-oil (W/O) emulsion exists simultaneously. They are often prepared accroding to a two-step process and commonly stabilized using a combination of hydrophilic and hydrophobic surfactants. Recently, some reports have shown that multiple emulsions can also be produced through one-step method with simultaneous occurrence of catastrophic and transitional phase inversions. However, these reported multiple emulsions need surfactant blends and are usually described as transitory or temporary systems. Herein, we report a one-step phase inversion process to produce water-in-oil-in-water (W/O/W) multiple emulsions stabilized solely by a synthetic diblock copolymer. Unlike the use of small molecule surfactant combinations, block copolymer stabilized multiple emulsions are remarkably stable and show the ability to separately encapsulate both polar and nonpolar cargos. The importance of the conformation of the copolymer surfactant at the interfaces with regards to the stability of the multiple emulsions using the one-step method is discussed.

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