Efficient formation of oil-in-oil Pickering emulsions with narrow size distributions by using electric fields

Biocompatible Hydrogel-Based Liquid Marbles with Magnetosomes

Liquid marbles are widely known for their potential biomedical applications, especially due to their versatility and ease of preparation. In the present work, we prepared liquid marbles with various cores composed of water, agar-based hydrogels, magnetic fluids, or non-aqueous substances. As a coating material, we used biocompatible particles of plant origin, such as turmeric grains and Lycopodium pollen. Additionally, we provided marbles with magnetic properties by incorporating either magnetosomes or iron oxide nanoparticles as a powder or by injecting another magnetic fluid. Structures obtained in this way were stable and susceptible to manipulation by an external magnetic field. The properties of the magnetic components of our marbles were verified using electron paramagnetic resonance (EPR) spectroscopy and vibrating sample magnetometry (VSM). Our approach to encapsulation of active substances such as antibiotics within a protective hydrogel core opens up new perspectives for the delivery of hydrophobic payloads to the inherently hydrophilic biological environment. Additionally, hydrogel marbles enriched with magnetic materials showed promise as biocompatible heating agents under alternating magnetic fields. A significant innovation of our research was also the fabrication of composite structures in which the gel-like core was surrounded without mixing by a magnetic fluid covered on the outside by the particle shell. Our liquid marbles, especially those with a hydrogel core and magnetic content, due to the ease of preparation and favorable properties, have great potential for biomedical use. The fact that we were able to simultaneously produce, functionalize (by filling with predefined cargo), and manipulate (by means of an external magnetic field) several marbles also seems to be important from an application point of view.

Impact of Processing Parameters on Contactless Emulsification via Corona Discharge

A contactless emulsification method is presented using corona discharge. The corona discharge forms using a pin-to-plate configuration, creating a non-uniform electric field. This results in a simultaneous electrohydrodynamic (EHD) pumping of silicone oil and an electroconvection of water droplets that accelerate and submerge inside the oil, leading to a continuous water-in-oil (W/O) emulsion formation process. The impact of the oil viscosity and corona generating AC and DC electric fields (i.e., voltage and frequency) on the characteristics of the emulsions is studied. The emulsification power consumption using the AC and DC electric fields is calculated and compared to traditional emulsion formation methods. While using the DC electric field results in the formation of uniform emulsions, the AC electric field is readily available and uses less power for the emulsification. This is facile, contactless, and energy-efficient for the continuous formation of W/O emulsions.

Stimuli-Responsive Pickering Emulsions Regulated via Polymerization-Induced Self-Assembly Nanoparticles

With the development of reversible deactivated radical polymerization techniques, polymerization-induced self-assembly (PISA) is emerging as a facile method to prepare block copolymer nanoparticles in situ with high concentrations, providing wide potential applications in different fields, including nanomedicine, coatings, nanomanufacture, and Pickering emulsions. Polymeric emulsifiers synthesized by PISA have many advantages comparing with conventional nanoparticle emulsifiers. The morphologies, size, and amphiphilicity can be readily regulated via the synthetic process, post-modification, and external stimuli. By introducing stimulus responsiveness into PISA nanoparticles, Pickering emulsions stabilized with these nanoparticles can be endowed with "smart" behaviors. The emulsions can be regulated in reversible emulsification and demulsification. In this review, the authors focus on recent progress on Pickering emulsions stabilized by PISA nanoparticles with stimuli-responsiveness. The factors affecting the stability of emulsions during emulsification and demulsification are discussed in details. Furthermore, some viewpoints for preparing stimuli-responsive emulsions and their applications in antibacterial agents, diphase reaction platforms, and multi-emulsions are discussed as well. Finally, the future developments and applications of stimuli-responsive Pickering emulsions stabilized by PISA nanoparticles are highlighted.

Recent advances and applications of microspheres and nanoparticles in transarterial chemoembolization for hepatocellular carcinoma

Transarterial chemoembolization (TACE) is a recommended treatment for patients suffering from intermediate and advanced hepatocellular carcinoma (HCC). As compared to the conventional TACE, drug-eluting bead TACE demonstrates several advantages in terms of survival, treatment response, and adverse effects. The selection of embolic agents is critical to the success of TACE. Many studies have been performed on the modification of the structure, size, homogeneity, biocompatibility, and biodegradability of embolic agents. Continuing efforts are focused on efficient loading of versatile chemotherapeutics, controlled sizes for sufficient occlusion, real-time detection intra- and post-procedure, and multimodality imaging-guided precise treatment. Here, we summarize recent advances and applications of microspheres and nanoparticles in TACE for HCC. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Contactless Method of Emulsion Formation Using Corona Discharge

Electroemulsification methods use electrohydrodynamic (EHD) forces to manipulate fluids and droplets for emulsion formation. Here, a top-down method is presented using a contactless corona discharge for simultaneous emulsion formation and its pumping/collection. The corona discharge forms using a sharp conductive electrode connected to a high-voltage source that ionizes water vapor droplets (formed by a humidifier) and creates an ionic wind (electroconvection), dragging them into an oil medium. The nonuniform electric field induced by the corona discharge also drives the motion of the oil medium via an EHD pumping effect utilizing a modulated bottom electrode geometry. By these two effects, this contactless method enables the immersion of the water droplets into the moving oil medium, continuously forming a water-in-oil (W/O) emulsion. The impact of corona discharge voltage, vertical and horizontal distances between the two electrodes, and depth of the silicone oil on sizes of the formed emulsions is studied. This is a low-cost and contactless process enabling the continuous formation of the W/O emulsions.

Semi-batch and continuous production of Pickering emulsion via direct contact steam condensation

We propose a versatile strategy for the production of highly stable water in oil Pickering emulsion by direct contact condensation of steam. In contrast to conventional methods that use mechanical energy for creating drops, the condensation of steam brought in contact with a non-aqueous colloidal dispersion is exploited to produce Pickering emulsions in two modes of operation, namely, semi-batch and continuous. As steam that comes in contact with oil condenses into water drops, the particles adsorb to the interface and thus arrest drop-drop coalescence. The adsorption of particles on the drop's surface imparts kinetic stability to the emulsions. The dependence of size of the emulsions as a function of parameters such as steam temperature, flow rate, particle type and particle concentration is investigated. We show that the tailoring of these parameters allows a precise control over droplet size distribution. The flexibility of continuous mode of operation makes it a potential technique for large scale production of emulsions suited for many applications.

Interfacial structures of particle-stabilized emulsions examined by ultrasonic scattering analysis with a core-shell model

Pickering emulsions comprising liquid droplets stabilized by solid microparticles have gained much attention in the field of cosmetics, inks, and drug delivery systems. To ensure that microparticles in Pickering emulsions are localized at the surface of liquid droplets, ultrasonic spectroscopy analysis combined with scattering function theory was conducted in this study. Two specific cases were investigated: (1) silica particles and liquid droplets independently dispersed in liquid and (2) silica particles effectively localized at the surface of the droplets. It was found that the core-shell model was effective for analyzing nanoparticles anchored at the surface of oil droplets. Conversely, it was found that an effective shell comprised of solid particles was no longer observed as the particle size or the distance between solid particles increased. When a large solid particle was applied, the ultrasonic spectra resembled those of conventional surfactant-stabilized emulsions without solid stabilizers.

Armored Droplets as Soft Nanocarriers for Encapsulation and Release under Flow Conditions

Technical challenges in precision medicine and environmental remediation create an increasing demand for smart materials that can select and deliver a probe load to targets with high precision. In this context, soft nanomaterials have attracted considerable attention due to their ability to simultaneously adapt their morphology and functionality to complex ambients. Two major challenges are to precisely control this adaptability under dynamic conditions and provide predesigned functionalities that can be manipulated by external stimuli. Here, we report on the computational design of a distinctive class of soft nanocarriers, built from armored nanodroplets, able to selectively encapsulate or release a probe load under specific flow conditions. First, we describe in detail the mechanisms at play in the formation of pocket-like structures in armored nanodroplets and their stability under external flow. Then we use that knowledge to test the capacity of these pockets to yield flow-assisted encapsulation or expulsion of a probe load. Finally, the rheological properties of these nanocarriers are put into perspective with those of delivery systems employed in pharmaceutical and cosmetic technology.

Particle-covered droplet and a particle shell under compressive electric stress

Understanding of the behavior of an individual droplet suspended in a liquid and subjected to a stress is important for studying and designing more complex systems, such as emulsions. Here, we present an experimental study of the behavior of a particle-covered droplet and its particle shell under compressive stress. The stress was induced by an application of a DC electric field. We studied how the particle coverage (φ), particle size (d), and the strength of an electric field (E) influence the magnitude of the droplet deformation (D). The experimental results indicate that adding electrically insulating particles to a droplet interface drastically changes the droplet deformation by increasing its magnitude. We also found that the magnitude of the deformation is not retraceable during the electric field sweeping, i.e., the strain-stress curves form a hysteresis loop due to the energy dissipation. The field-induced droplet deformation was accompanied by structural and morphological changes in the particle shell. We found that shells made of smaller particles were more prone to jamming and formation of arrested shells after removal of an electric stress.

Electric-field-induced deformation, yielding, and crumpling of jammed particle shells formed on non-spherical Pickering droplets

Droplets covered with densely packed solid particles, often called Pickering droplets, are used in a variety of fundamental studies and practical applications. For many applications, it is essential to understand the mechanics of such particle-laden droplets subjected to external stresses. Several research groups have studied theoretically and experimentally the deformation, relaxation, rotation, and stability of Pickering droplets. Most of the research concerns spherical Pickering droplets. However, little is known about non-spherical Pickering droplets with arrested particle shells subjected to compressive stress. The experimental results presented here contribute to filling this gap in research. We deform arrested non-spherical Pickering droplets by subjecting them to electric fields, and study the effect of droplet geometry and size, as well as particle size and electric field strength, on the deformation and yielding of arrested non-spherical Pickering droplets. We explain why a more aspherical droplet and/or a droplet covered with a shell made of larger particles required higher electric stress to deform and yield. We also show that an armored droplet can absorb the electric stress differently (i.e., through either in-plane or out-of-plane particle rearrangements) depending on the strength of the applied electric field. Furthermore, we demonstrate that particle shells may fail through various crumpling instabilities, including ridge formation, folding, and wrinkling, as well as inward indentation.

Electrorotation of particle-coated droplets: from fundamentals to applications

Electrically insulating objects immersed in a weakly conducting liquid may Quincke rotate when subjected to an electric field. Experimental and theoretical investigations of this type of electrorotation typically concern rigid particles and particle-free droplets. This work provides the basic features of electric field-induced rotation of particle-covered droplets that expand the current knowledge in this area. Compared to pure droplets, we show that adding particles to the droplet interface considerably changes the parameters of electrorotation. We study in detail deformation magnitude (D), orientation (β) and rotation rate (ω) of a droplet subjected to a DC E-field. Our experimental results reveal that both the critical electric field (for electrorotation) and the rotational rate depend on droplet size, particle shell morphology (smooth vs. brush-like), and composition (loose vs. locked particles). We also demonstrate the importance of the electrical parameters of the surface particles by comparing the behavior of droplets covered by (insulating) polymeric particles and droplets covered by (non-ohmic) clay mineral particles. The knowledge acquired from the electrorotation experiments is directly translated into practical applications: (i) to form arrested droplets with shells of different permeability; (ii) to study solid-to-liquid transition of particle shells; (iii) to mix particles on shells; and (iv) to increase the formation efficiency of Pickering emulsions.

The Effect of Particle Shell on Cooling Rates in Oil-in-Oil Magnetic Pickering Emulsions

Pickering emulsions (particle-stabilized emulsions) are usually considered because of their unique properties compared to surfactant-stabilized emulsions including better stability against emulsion aging. However, the interesting feature of particle-stabilized emulsions could be revealed during their magnetic heating. When magnetic particles constitute a shell around droplets and the sample is placed in an alternating magnetic field, a temperature increase appears due to energy dissipation from magnetic relaxation and hysteresis within magnetic particles. We hypothesize that the solidity of the magnetic particle shell around droplets can influence the process of heat transfer from inside the droplet to the surrounding medium. In this way, particle-stabilized emulsions can be considered as materials with changeable heat transfer. We investigated macroscopically heating and cooling of oil-in-oil magnetic Pickering emulsions with merely packed particle layers and these with a stable particle shell. The change in stability of the shell was obtained here by using the coalescence of droplets under the electric field. The results from calorimetric measurements show that the presence of a stable particle shell caused a slower temperature decrease in samples, especially for lower intensities of the magnetic field. The retarded heat transfer from magnetic Pickering droplets can be utilized in further potential applications where delayed heat transfer is desirable.

Advances and Opportunities of Oil-in-Oil Emulsions

Emulsions are mixtures of two immiscible liquids in which droplets of one are dispersed in a continuous phase of the other. The most common emulsions are oil-water systems, which have found widespread use across a number of industries, for example, in the cosmetic and food industries, and are also of advanced scientific interest. In addition, the past decade has seen a significant increase in both the design and application of nonaqueous emulsions. This has been primarily driven by developments in understanding the mechanism of effective stabilization of oil-in-oil (o/o) systems, either using block copolymers (BCPs) or solid (Pickering) particles with appropriate surface functionality. These systems, as highlighted in this review, have enabled emergent applications in areas such as pharmaceutical delivery, energy storage, and materials design (e.g., polymerization, monolith, and porous polymer synthesis). These o/o emulsions complement traditional emulsions that utilize an aqueous phase and allow the use of materials incompatible with water. We assess recent advances in the preparation and stabilization of o/o emulsions, focusing on the identity of the stabilizer (BCP or particle), the interplay between stabilizer and oils, and highlighting applications and opportunities associated with o/o emulsions.

Size Determination of Polystyrene Sub-Microspheres Using Transmission Spectroscopy

Nano/micro polystyrene (PS) beads have found many applications in different fields spanning from drug delivery, bio-diagnostics, and hybrid plasmonics to advanced photonics. The sizes of the PS beads are an important parameter, especially in plasmonic and photonic experiments. In this work, we demonstrate a quick and straightforward method to estimate the diameters of sub-microspheres (0.2 μm to 0.8 μm) using the transmission spectra of a close-packed monolayer of polystyrene beads on glass or quartz substrates. Experimental transmission spectra of the PS monolayers were verified against finite-difference time-domain (FDTD) simulation and showed good agreement. The effects of the substrates on the transmission spectra and, hence, the accuracy of the method were also studied by simulation, which showed that common transparent substrates only cause minor deviation of the PS bead sizes calculated by the proposed method.

Particle Stabilization of Oil-Fluorocarbon Interfaces and Effects on Multiphase Oil-in-Water Complex Emulsion Morphology and Reconfigurability

Stabilization of oil-oil interfaces is important for nonaqueous emulsions as well as for multiphase oil-in-water emulsions, with relevance to a variety of fields ranging from emulsion polymerization to sensors and optics. Here, we focus on examining the ability of functionalized silica particles to stabilize interfaces between fluorinated oils and other immiscible oils (such as hydrocarbons and silicones) in nonaqueous emulsions and also on the particles' ability to affect the morphology and reconfigurability of complex, biphasic oil-in-water emulsions. We compare the effectiveness of fluorophilic, lipophilic, and bifunctional fluorophilic-lipophilic coated nanoparticles to stabilize these oil-oil interfaces. Sequential bulk emulsification steps by vortex mixing, or emulsification by microfluidics, can be used to create complex droplets in which particles stabilize the oil-oil interfaces and surfactants stabilize the oil-water interfaces. We examine the influence of particles adsorbed at the internal oil-oil interface in complex droplets to hinder the reconfiguration of these complex emulsions upon addition of aqueous surfactants, creating "metastable" droplets that resist changes in morphology. Such metastable droplets can be triggered to reconfigure when heated above their upper critical solution temperature. Thus, not only do these bifunctional silica particles enable the stabilization of a broad array of oil-fluorocarbon nonaqueous emulsions, but the ability to address the oil-oil interface within complex O/O/W droplets expands the diversity of oil chemical choices available and the accessibility of droplet morphologies and sensitivity.

The potential of magnetic heating for fabricating Pickering-emulsion-based capsules

Pickering emulsions (particle-stabilized emulsions) have been widely explored due to their potential applications, one of which is using them as precursors for the formation of colloidal capsules that could be utilized in, among others, the pharmacy and food industries. Here, we present a novel approach to fabricating such colloidal capsules by using heating in the alternating magnetic field. When exposed to the alternating magnetic field, magnetic particles, owing to the hysteresis and/or relaxation losses, become sources of nano- and micro-heating that can significantly increase the temperature of the colloidal system. This temperature rise was evaluated in oil-in-oil Pickering emulsions stabilized by both magnetite and polystyrene particles. When a sample reached high enough temperature, particle fusion caused by glass transition of polystyrene was observed on surfaces of colloidal droplets. Oil droplets covered with shells of fused polystyrene particles were proved to be less susceptible to external stress, which can be evidence of the successful formation of capsules from Pickering emulsion droplets as templates.

How Do Charged End-Groups on the Steric Stabilizer Block Influence the Formation and Long-Term Stability of Pickering Nanoemulsions Prepared Using Sterically Stabilized Diblock Copolymer Nanoparticles?

Reversible addition-fragmentation chain transfer (RAFT) solution polymerization is used to prepare well-defined poly(glycerol monomethacrylate) (PGMA) chains bearing carboxylic acid, tertiary amine, or neutral end-groups. Each of these PGMA precursors was then chain-extended in turn via RAFT aqueous emulsion polymerization of 2,2,2-trifluoroethyl methacrylate to form spherical nanoparticles as confirmed by transmission electron microscopy (TEM) analysis. Dynamic light scattering studies indicated an intensity-average diameter of approximately 25 nm. Aqueous electrophoresis measurements confirmed that the amine-functional nanoparticles became cationic at low pH owing to end-group protonation. In contrast, carboxylic acid-functional nanoparticles became appreciably anionic at pH 10 owing to end-group ionization. Finally, nanoparticles bearing neutral end-groups exhibited zeta potentials close to zero over a range of solution pH. High-shear homogenization of n-dodecane in the presence of such sterically stabilized nanoparticles led to the formation of oil-in-water Pickering macroemulsions with volume-average diameters of 20-30 μm. High-pressure microfluidization was then used to prepare the three corresponding Pickering nanoemulsions. Each Pickering nanoemulsion was characterized by analytical centrifugation and TEM studies of the dried nanoemulsion droplets confirmed their original nanoparticle superstructure. The nanoparticle adsorption efficiency at the oil-water interface was assessed by gel permeation chromatography (using a UV detector) for each nanoparticle type at both pH 3 and 7. Nanoparticles with charged end-groups exhibited relatively low adsorption efficiency, whereas up to 90% of the neutral nanoparticles were adsorbed onto the oil droplets. This observation was supported by small-angle X-ray scattering experiments, which indicated that the packing efficiency of neutral nanoparticles around oil droplets was higher than that of nanoparticles bearing charged end-groups. Analytical centrifugation was used to evaluate the colloidal stability of the aged Pickering nanoemulsions. Pickering nanoemulsions stabilized with nanoparticles bearing charged end-groups proved to be significantly less stable than those prepared using neutral end-groups.

Modeling of Sedimentation and Creaming in Suspensions and Pickering Emulsions

Suspensions and emulsions are prone to kinetic instabilities of sedimentation and creaming, wherein the suspended particles and droplets fall or rise through a matrix fluid. It is important to understand and quantify sedimentation and creaming in such dispersed systems as they affect the shelf-life of products manufactured in the form of suspensions and emulsions. In this article, the unhindered and hindered settling/creaming behaviors of conventional emulsions and suspensions are first reviewed briefly. The available experimental data on settling/creaming of concentrated emulsions and suspensions are interpreted in terms of the drift flux theory. Modeling and simulation of nanoparticle-stabilized Pickering emulsions are carried out next. The presence of nanoparticles at the oil/water interface has a strong influence on the creaming/sedimentation behaviors of single droplets and swarm of droplets. Simulation results clearly demonstrate the strong influence of three-phase contact angle of nanoparticles present at the oil/water interface. This is the first definitive study dealing with modeling and simulation of unhindered and hindered creaming and sedimentation behaviors of nanoparticle-stabilized Pickering emulsions.

Influence of particle concentration on multiple droplet formation in Pickering emulsions

HYPOTHESIS

Multiphase droplets form when oil and water are mixed together at the ambivalent conditions that occur close to phase inversion. In this paper we propose a mechanism for the stabilisation of multiphase droplets by a single population of particles.

EXPERIMENTS

We investigated the microstructure of emulsions formed when dodecane and water are mixed in the presence of hydrophobic fumed silica nanoparticles. We identified the range of compositions, mixing times and rates where water-in-oil-in-water emulsions are stabilised in a single mixing step. To explore how the particle availability and mixing conditions lead to multiple emulsion formation we used light scattering and microscopy techniques to probe the size and morphology of the drops, and the particle coverage of the interfaces.

FINDINGS

Our key finding is that to stabilise multiphase drops there should be sufficient particles available to coat water drops that are entrained within coalescing oil droplets. The size of an entrained drop is determined by the volume of the rupturing film that forms between the oil drops. The particle coating prevents the entrained drop from escaping into the external aqueous phase. These results suggest a simple route for controlling the formation and stability of multiple emulsions for encapsulation applications.

Opening and Closing of Particle Shells on Droplets via Electric Fields and Its Applications

Active, tunable, and reversible opening and closing of particle shells on droplets may facilitate chemical reactions in droplets and enable various small-scale laboratory operations, including online detection, measurement, and adjustment of droplet liquid. Manipulating various types of particle shells in a controlled manner requires new routes. This work provides a new strategy for controlling the spatial arrangement of particle-covered oil droplets using electric fields that expands the application of responsive droplets beyond the abovementioned examples. The behavior of stimulated particle-covered droplets is exploited in multiple ways: to form an active smart device, fabricate Janus and patchy shells, create an online diagnostic tool, and produce a tool for fundamental studies. Electric fields are used here to manipulate particle films on oil droplets through the synergetic action of droplet deformation and electrohydrodynamic liquid flows. First, the effects of electric field strength and liquid viscosity on droplet deformation, surface particle arrangements, and dynamics are examined in detail. Then three examples of applications of responsive particle-covered droplets are demonstrated. Our results show that the reversible opening and closing of the droplet's shells, composed of various types of particles, can be conveniently achieved through electric fields, opening up a new possibility for applications in optics, clinical diagnostics, microfluidics, and material engineering.

Particle-covered drops in electric fields: drop deformation and surface particle organization

Drops covered by adsorbed particles are a prominent research topic because they hold promise for a variety of practical applications. Unlocking the enormous potential of particle-laden drops in new material fabrication, for instance, requires understanding how surface particles affect the electrical and deformation properties of drops, as well as developing new routes for particle manipulation at the interface of drops. In this study, we utilized electric fields to experimentally investigate the mechanics of particle-covered silicone oil drops suspended in castor oil, as well as particle assembly at drop surfaces. We used particles with electrical conductivities ranging from insulating polystyrene to highly conductive silver. When subjected to electric fields, drops can change shape, rotate, or break apart. In the first part of this work, we demonstrate how the deformation magnitude and shape of drops, as well as their electrical properties, are affected by electric field strength, particle size, conductivity, and coverage. We also discuss the role of electrohydrodynamic flows on drop deformation. In the second part, we present the electric field-directed assembly and organization of particles at drop surfaces. In this regard, we studied various parameters in detail, including electric field strength, particle size, coverage, and electrical conductivity. Finally, we present a novel method for controlling the local particle coverage and packing of particles on drop surfaces by simply tuning the frequency of the applied electric field. This approach is expected to find uses in optical materials and applications.