Colloidal particles at fluid interfaces: behaviour of isolated particles

Demulsification of Pickering Emulsions by Chemical Dissolution of Stabilizers

Demulsification of particle-stabilized oil-in-water emulsions is crucial in diverse fields such as treatment of produce water, recovery of valuable products of Pickering emulsion catalysis, and so on. In this work, we investigated a facile method for destabilizing emulsions by dissolving stabilizer particles by the introduction of acid or base. Nanoellipsoidal hematite-stabilized decane-in-water emulsions are destabilized by dissolving hematite with oxalic or hydrochloric acid in situ. Time required for complete demulsification decreased as the acid concentration is increased. The demulsification time is typically on the order of a few hours for the chosen protocol. Similarly, the silica-stabilized decane-water emulsion is demulsified by the addition of aqueous sodium hydroxide. Demulsification kinetics is presented as the temporal change of the emulsion volume with time. Emulsion volume decreases in two stages: an initial slow decrease followed by an exponential decrease. Scanning electron microscopy analysis shows that the stabilizing particles are completely dissolved and recrystallized as salts of respective kinds. An estimate of the desorption free energy suggests that particle size should be reduced to a few nanometers for inducing destabilization. This work describes a facile method to destabilize oil-in-water emulsion, and it can be generalized to any other particle-stabilized emulsions by choosing appropriate chemical reagent for dissolution.

Deformation of confined liquid interfaces by inhomogeneous electric fields and localized particle forces

HYPOTHESIS

Oil-water interfaces that are created by confining a certain amount of oil in a square shaped pixel (∼200 x 200 μm2 with a height of ∼10 μm) topped by a layer of water, have a curvature that depends on the amount of oil that happens to be present in the confining area. Under the application of an electric field normal to the interface, the interface will deform due to inhomogeneities in the electric field. These inhomogeneities are expected to arise from the initial curvature of the meniscus, from fringe fields that emerge at the confining pixel walls and, if applicable, from interfacially adsorbed particles.

MODELING AND EXPERIMENTS

We model the shape of the confined oil-water interface invoking capillarity and electrostatics. Furthermore, we measure the initial curvature by tracking the position of interfacially adsorbed particles depending on sample tilt.

FINDINGS

We found that the pixels exhibited meniscus curvature radii ranging from 0.6-7 mm. The corresponding model based minimum oil film thicknesses range between 0.7 and 9 μm. Furthermore, the model shows that the initial meniscus curvature can increase up to 76 percent relative to the initial curvature by the electric field before the oil film becomes unstable. The pixel wall and particles are shown to have minimal impact on the interface deformation.

Dry Ball-Milled Quinoa Starch as a Pickering Emulsifier: Preparation, Microstructures, Hydrophobic Properties and Emulsifying Properties

This research supplied a "cleaner-production" way to produce "clean-label" quinoa starch-based Pickering emulsifier with excellent emulsifying properties. The effects of dry ball-milling time and speed on the multi-scale structures and emulsifying properties of quinoa starch were studied. With increasing ball-milling time and speed, particle size first decreased and then increased, the crystallinity, lamellar structure and short-range ordered structure gradually decreased, and contact angle gradually increased. The increased contact angle might be related to the increased oil absorption properties and the decreased water content. The emulsification properties of ball-milled quinoa starch (BMQS)-based Pickering emulsions increased with the increase in ball-milling time and speed, and the emulsions of BMQS-4 h, 6 h, 8 h, and 600 r reached the full emulsification state. After 120 days' storage, the oil droplets of BMQS-2 h (BMQS-400 r) deformed, the oil droplets increased, and the emulsification index decreased. The emulsification index and the oil droplets of BMQS-4 h, 6 h, 8 h and 600 r-based emulsions did not show obvious changes after storage, indicating the good emulsifying stability of these BMQS-based emulsions, which might be because that the relatively larger amount of starch particles that dispersed in the voids among the oil droplets could act as stronger network skeletons for the emulsion gel. This Pickering emulsifier was easily and highly efficiently produced and low-cost, having great potential to be used in the food, cosmetic and pharmaceutical industries.

Yield stress-enabled microencapsulation of field responsive microparticle suspensions

Try and encapsulate microparticles inside the cores of microcapsules and you will often find that particles adhere to the liquid-liquid interface in a phenomenon known as Pickering stabilization. Particles will remain irreversibly trapped and embedded within the subsequently formed microcapsule membrane. In cases where the encapsulant particles must remain suspended inside the microcapsule core to retain their desired properties or behaviours, Pickering stabilization is detrimental. Here we demonstrate a general procedure using yield stress materials as the core material, where the yield stress of the gel is strong enough to suspend particles against sedimentation, but weak enough to allow spatial manipulation of encapsulant particles using an external field. This external field imparts enough force on particles to disrupt the supporting network and allow particle mobility after encapsulation.

Mechanism of the Decrease in Surface Tension by Bulk Nanobubbles (Ultrafine Bubbles)

The mechanism of the decrease in the surface tension of water containing bulk nanobubbles (ultrafine bubbles) is studied theoretically by numerical simulations of the adsorption of bulk nanobubbles at the liquid's surface based on the dynamic equilibrium model for the stability of a bulk nanobubble under the conditions of the Tuziuti experiment (Tuziuti, T., et al., Langmuir, 2023, 39, 5771-5778). It is predicted that the concentration of bulk nanobubbles in the bulk liquid decreases considerably with time, as many bulk nanobubbles are gradually adsorbed at the liquid's surface. A part of the decrease in surface tension is due to the Janus-like structure of a bulk nanobubble that could partly break the hydrogen bond network of water molecules at the liquid's surface because more than 50% of the bubble's surface is covered with hydrophobic impurities, according to the dynamic equilibrium model. The theoretically estimated decrease in surface tension due to the Janus-like structure of a bulk nanobubble agrees with the experimental data of the decrease in surface tension solely by bulk nanobubbles obtained by the comparison of before and after the elimination of bulk nanobubbles by the freeze-thaw process. This effect cannot be explained by the electric charge stabilization model widely discussed for the stability of a bulk nanobubble, although the present model is only applicable to the solution containing hydrophobic impurities. Another part of the decrease in surface tension should be due to impurities produced from a nanobubble generator, such as a mechanical seal, which was partly confirmed by the TOC measurements.

Capillary detachment of a microparticle from a liquid-liquid interface

The attachment and detachment of microparticles at a liquid-liquid interface are common in many material systems, from Pickering emulsions and colloidal assemblies to capillary suspensions. Properties of these systems rely on how the particles interact with the liquid-liquid interface, including the detachment process. In this study, we simultaneously measure the capillary detachment force of a microparticle from a liquid-liquid interface and visualize the shape of the meniscus by combining colloidal probe microscopy and confocal microscopy. The capillary behavior is studied on both untreated (hydrophilic) and fluorinated (hydrophobic) glass microparticles. The measured force data show good agreement with theoretical calculations based on the extracted geometric parameters from confocal images of the capillary bridge. It is also evident that contact line pinning is an important aspect of detachment for both untreated and fluorinated particles.

Nanoscale Porosity in Microellipsoids Cloaks Interparticle Capillary Attraction at Fluid Interfaces

Anisotropic particles pinned at fluid interfaces tend toward disordered multiparticle configurations due to large, orientationally dependent, capillary forces, which is a significant barrier to exploiting these particles to create functional self-assembled materials. Therefore, current interfacial assembly methods typically focus on isotropic spheres, which have minimal capillary attraction and no dependence on orientation in the plane of the interface. In order to create long-range ordered structures with complex configurations via interfacially trapped anisotropic particles, control over the interparticle interaction energy via external fields and/or particle engineering is necessary. Here, we synthesize colloidal ellipsoids with nanoscale porosity and show that their interparticle capillary attraction at a water-air interface is reduced by an order of magnitude compared to their smooth counterparts. This is accomplished by comparing the behavior of smooth, rough, and porous ellipsoids at a water-air interface. By monitoring the dynamics of two particles approaching one another, we show that the porous particles exhibit a much shorter-range capillary interaction potential, with scaling intriguingly different than theory describing the behavior of smooth ellipsoids. Further, interferometry measurements of the fluid deformation surrounding a single particle shows that the interface around porous ellipsoids does not possess the characteristic quadrupolar symmetry of smooth ellipsoids, and quantitatively confirms the decrease in capillary interaction energy. By engineering nanostructured surface features in this fashion, the interfacial capillary interactions between particles may be controlled, informing an approach for the self-assembly of complex two-dimensional microstructures composed of anisotropic particles.

Janus Ligand-Tethered Nanoparticles at Liquid-Liquid Interfaces

We investigate the structural properties of Janus ligand-tethered nanoparticles at liquid-liquid interfaces using coarse-grained molecular dynamics simulations. The effect of interactions between different chains and liquids is discussed. We consider the Janus particles with symmetrical interactions with the liquids which correspond to supplementary wettability and particles with uncorrelated interactions. Simulation results indicate that the Janus hairy particles trapped in the interface region have different configurations characterized by the vertical displacement distance, the orientation of the Janus line relative to the interface, and the particle shape. The Janus hairy particles present abundant morphologies, including dumbbell-like and typical core-shell, at the interface. The shape of adsorbed particles is analyzed in detail. The simulation data are compared with those predicted by a simple phenomenological approach. This work can promote the applications of Janus hairy particles in nanotechnology.

A review of oil and water retention in emulsified meat products: The mechanisms of gelation and emulsification, the application of multi-layer hydrogels

Emulsified meat products are key deep-processing products due to unique flavor and high nutritional value. Myosin dissolves, and protein aggregation and heat-induced gelation occur after myosin unfolds and hydrophobic groups are exposed. Myosin could form interfacial protein membranes and wrap fat globules. Emulsified fat globules may be filled in heat-induced gel networks. Therefore, this review intends to discuss the influences of heat-induced gelation and interfacial adsorption behavior on oil and water retention. Firstly, the mechanism of heat-induced gelation was clarified from the perspective of protein conformation and micro-structure. Secondly, the mechanism of emulsification stability and its factors affecting interfacial adsorption were demonstrated as well as limitations and challenges. Finally, the structure characteristics and application of multi-layer hydrogels in the gelation and emulsification were clarified. It could conclude that the characteristic morphology, spatial conformation and structure adjustment affected heat-induced gelation and interfacial adsorption behavior. Spatial conformation and microstructure were adjusted to improve the oil and water retention by pH, ionic strength, amino acid, oil phase characteristic and protein interaction. Multi-layer hydrogels facilitated oil and water retention. The comprehensive review of gelation and emulsification mechanisms could promote the development of meat products and improvement of meat processing technology.

Hybrid Nanoparticles at Fluid-Fluid Interfaces: Insight from Theory and Simulation

Hybrid nanoparticles that combine special properties of their different parts have numerous applications in electronics, optics, catalysis, medicine, and many others. Of the currently produced particles, Janus particles and ligand-tethered (hairy) particles are of particular interest both from a practical and purely cognitive point of view. Understanding their behavior at fluid interfaces is important to many fields because particle-laden interfaces are ubiquitous in nature and industry. We provide a review of the literature, focusing on theoretical studies of hybrid particles at fluid-fluid interfaces. Our goal is to give a link between simple phenomenological models and advanced molecular simulations. We analyze the adsorption of individual Janus particles and hairy particles at the interfaces. Then, their interfacial assembly is also discussed. The simple equations for the attachment energy of various Janus particles are presented. We discuss how such parameters as the particle size, the particle shape, the relative sizes of different patches, and the amphiphilicity affect particle adsorption. This is essential for taking advantage of the particle capacity to stabilize interfaces. Representative examples of molecular simulations were presented. We show that the simple models surprisingly well reproduce experimental and simulation data. In the case of hairy particles, we concentrate on the effects of reconfiguration of the polymer brushes at the interface. This review is expected to provide a general perspective on the subject and may be helpful to many researchers and technologists working with particle-laden layers.

Effect of surface roughness on the solar evaporation of liquid marbles

HYPOTHESIS

Nanostructured materials are widely used for solar energy harvesting and conversion due to their excellent photothermal properties. It is generally accepted that the better the light absorption ability, the better the photothermal conversion efficiency.

EXPERIMENT

A series of experiments in solar evaporation of liquid marbles (LMs) by coating the droplets with Fe3O4, Ni nanoparticles (NPs) and carbon nanotubes (CNTs) are conducted.

FINDINGS

Conversely, we found that the surface roughness of solar absorber plays a significant role in solar evaporation rather than the light absorption. The results disclose that the Fe3O4 NPs with the lowest absorptivity has the largest roughness on drop surface, while that of CNTs show the opposite properties. The evaporation dynamics of LMs are featured with dome or constant spherical collapse with different roughness. Such dynamic difference arises from the mechanical competition between the capillary force and interparticle interaction. Besides, the strong light-harvesting and near-field radiation enabled by the rough surfaces enhance the solar evaporation. The Fe3O4-LM shows the highest evaporation rate of 6.55 μg/s, which is 1.09 and 1.30 times larger than that of Ni-LM and CNT-LM, respectively. Numerical analysis reveals that the rough surface with stacking arrangement of NPs greatly enhances the light-induced electromagnetic field and heat concentration over the interface, leading to a plasmon-coupling boundary with high temperature for the fast evaporation. Uncovering these properties could be of much help for developments of heatable miniature evaporators or reactors and their counterparts, permitting a broad range of processes with precise temperature and kinetic control.

Evaporation-driven liquid flow in sessile droplets

The evaporation of a sessile droplet spontaneously induces an internal capillary liquid flow. The surface-tension driven minimisation of surface area and/or surface-tension differences at the liquid-gas interface caused by evaporation-induced temperature or chemical gradients set the liquid into motion. This flow drags along suspended material and is one of the keys to control the material deposition in the stain that is left behind by a drying droplet. Applications of this principle range from the control of stain formation in the printing and coating industry, to the analysis of DNA, to forensic and medical research on blood stains, and to the use of evaporation-driven self-assembly for nanotechnology. Therefore, the evaporation of sessile droplets attracts an enormous interest from not only the fluid dynamics, but also the soft matter, chemistry, biology, engineering, nanotechnology and mathematics communities. As a consequence of this broad interest, knowledge on evaporation-driven flows in drying droplets has remained scattered among the different fields, leading to various misconceptions and misinterpretations. In this review we aim to unify these views, and reflect on the current understanding of evaporation-driven liquid flows in sessile droplets in the light of the most recent experimental and theoretical advances. In addition, we outline open questions and indicate promising directions for future research.

Forces Controlling the Assembly of Particles at Fluid Interfaces

The interaction of particles with fluid interfaces is ubiquitous in synthetic and natural work, involving two types of interactions: particle-interface interactions (trapping energy) and interparticle interactions. Therefore, it is urgent to gain a deep understanding of the main forces controlling the trapping of particles at fluid interfaces, and their assembly to generate a broad range of structures characterized by different degrees of order. This Perspective tries to provide an overview of the main contributions to the energetic landscape controlling the assembly of particles at fluid interfaces, which is essential for exploiting this type of interfacial systems as platforms for the fabrication of interface-based soft materials with technological interest.

Interfacial self-assembly of SiO2-PNIPAM core-shell particles with varied crosslinking density

Spherical particles confined to liquid interfaces generally self-assemble into hexagonal patterns. It was theoretically predicted by Jagla two decades ago that such particles interacting via a soft repulsive potential are able to form complex, anisotropic assembly phases. Depending on the shape and range of the potential, the predicted minimum energy configurations include chains, rhomboid and square phases. We recently demonstrated that deformable core-shell particles consisting of a hard silica core and a soft poly(N-isopropylacrylamide) shell adsorbed at an air/water interface can form chain phases if the crosslinker is primarily incorporated around the silica core. Here, we systematically investigate the interfacial self-assembly behavior of such SiO₂-PNIPAM core-shell particles as a function of crosslinker content and core size. We observe chain networks predominantly at low crosslinking densities and smaller core sizes, whereas higher crosslinking densities lead to the formation of rhomboid packing. We correlate these results with the interfacial morphologies of the different particle systems, where the ability to expand at the interface and form a thin corona at the periphery depends on the degree of crosslinking close to the core. We perform minimum energy calculations based on Jagla-type pair potentials with different shapes of the soft repulsive shoulder. We compare the theoretical phase diagram with experimental findings to infer to which extent the interfacial interactions of the experimental system may be captured by Jagla pair-wise interaction potentials.

Effect of particle size on the stripping dynamics during impact of liquid marbles onto a liquid film

The robust attachment of particles at fluid interfaces is favorable for engineering new materials due to the large capillary energy, but it meets significant challenges when particle removal is a requirement. A previous study has shown that soap films can be utilized to achieve particle separation from liquid marbles. Here, we investigate the effects of particle size on the particle separation from liquid marbles using fast dynamics of drop impact on a soap film. Experimental observations disclose that the fast dynamics of the liquid marble involves coalescence, bouncing, stripping, or tunneling through the film by controlling the falling height and drop volume. More importantly, the active regime of the stripping mode can be selective-controlled by tuning the particle size, and the smaller stabilizing particles make a wider stripping regime. This is attributed to the smaller change of the surface energy resulting from the larger surface tension of LMs wrapped by smaller particles. Theoretical analysis reveals that the stripping thresholds are determined by the energy competition between kinetic energy, the increased surface energy and viscous dissipation, which offers important insights into particle separation by tuning the particle size. The present study provides guidelines for applications that involve phase separation.

Rotating Micro-Spheres for adsorption monitoring at a fluid interface

HYPOTHESIS

A broad range of phenomena, such as emulsification and emulsion stability, foam formation or liquid evaporation, are closely related to the dynamics of adsorbing colloidal particles. Elucidation of the mechanisms implied is key to a correct design of many different types of materials.

EXPERIMENTS

Microspheres forced to rotate near a fluid interface exhibit a roto-translational hydrodynamic mechanism that is hindered by capillary torques as soon as the particles protrude the interface. Under these conditions, the time evolution in the ratio of moving spheres provides a direct description of the adsorption kinetics, while microscopy monitoring of particle acceleration\deceleration informs about the adsorption\desorption dynamics. In this work, the proposed strategy is applied at an air/water interface loaded with spherical magnetic particles negatively charged, forced to rotate by the action of a rotating magnetic field.

FINDINGS

The proposed method enables the adsorption/desorption dynamics to be followed during the earliest phase of the process, when desorption of a small fraction of particles is detected, as well as to estimate approximated values of the adsorption/desorption constants. The results obtained show that the addition of a monovalent salt or a cationic (anionic) surfactant promotes (inhibits) both adsorption and formation of permanent bonds between particles.

A broad perspective to particle-laden fluid interfaces systems: from chemically homogeneous particles to active colloids

Particles adsorbed to fluid interfaces are ubiquitous in industry, nature or life. The wide range of properties arising from the assembly of particles at fluid interface has stimulated an intense research activity on shed light to the most fundamental physico-chemical aspects of these systems. These include the mechanisms driving the equilibration of the interfacial layers, trapping energy, specific inter-particle interactions and the response of the particle-laden interface to mechanical perturbations and flows. The understanding of the physico-chemistry of particle-laden interfaces becomes essential for taking advantage of the particle capacity to stabilize interfaces for the preparation of different dispersed systems (emulsions, foams or colloidosomes) and the fabrication of new reconfigurable interface-dominated devices. This review presents a detailed overview of the physico-chemical aspects that determine the behavior of particles trapped at fluid interfaces. This has been combined with some examples of real and potential applications of these systems in technological and industrial fields. It is expected that this information can provide a general perspective of the topic that can be exploited for researchers and technologist non-specialized in the study of particle-laden interfaces, or for experienced researcher seeking new questions to solve.

Non-specific adhesive forces between filaments and membraneless organelles

Many membraneless organelles are liquid-like domains that form inside the active, viscoelastic environment of living cells through phase separation. To investigate the potential coupling of phase separation with the cytoskeleton, we quantify the structural correlations of membraneless organelles (stress granules) and cytoskeletal filaments (microtubules) in a human-derived epithelial cell line. We find that microtubule networks are substantially denser in the vicinity of stress granules. When microtubules are depolymerized, the sub-units localize near the surface of the stress granules. We interpret these data using a thermodynamic model of partitioning of particles to the surface and bulk of the droplets. In this framework, our data are consistent with a weak (≲k B T) affinity of the microtubule sub-units for stress granule interfaces. As microtubules polymerize, their interfacial affinity increases, providing sufficient adhesion to deform droplets and/or the network. Our work suggests that proteins and other objects in the cell have a non-specific affinity for droplet interfaces that increases with the contact area and becomes most apparent when they have no preference for the interior of a droplet over the rest of the cytoplasm. We validate this basic physical phenomenon in vitro through the interaction of a simple protein-RNA condensate with microtubules.

Particle Separation from Liquid Marbles by the Viscous Folding of Liquid Films

Particle separation from fluid interfaces is one of the major challenges due to the large capillary energy associated with particle adsorption. Previous approaches rely on physicochemical modification or tuning the electrostatic action. Here, we show experimentally that particle separation can be achieved by fast dynamics of drop impact on soap films. When a droplet wrapped with particles (liquid marble) collides with a soap film, it undergoes bouncing and coalescence, stripping and viscous separation, or tunneling through the film. Despite the violence of splashing events, the process robustly yields the stripping in a tunable range. This viscous separation is supported by the transfer front of dynamic contact among the film, particle crust, and drop and can be well controlled in a deterministic manner by selectable impact parameters. By extensive experiments, together with thermodynamic analysis, we disclose that the separation thresholds depend on the energy competition between the kinetic energy, the increased surface energy, and the viscous dissipation. The mechanical cracking of the particle crust arises from the complex coupling between interfacial stress and viscous forces. This study is of potential benefit in soft matter research and also permits the study of a drop with colloid and surface chemistry.

Microfluidics-Enabled Soft Manufacture of Materials with Tailorable Wettability

Microfluidics and wettability are interrelated and mutually reinforcing fields, experiencing synergistic growth. Surface wettability is paramount in regulating microfluidic flows for processing and manipulating fluids at the microscale. Microfluidics, in turn, has emerged as a versatile platform for tailoring the wettability of materials. We present a critical review on the microfluidics-enabled soft manufacture (MESM) of materials with well-controlled wettability and their multidisciplinary applications. Microfluidics provides a variety of liquid templates for engineering materials with exquisite composition and morphology, laying the foundation for precisely controlling the wettability. Depending on the degree of ordering, liquid templates are divided into individual droplets, one-dimensional (1D) arrays, and two-dimensional (2D) or three-dimensional (3D) assemblies for the modular fabrication of microparticles, microfibers, and monolithic porous materials, respectively. Future exploration of MESM will enrich the diversity of chemical composition and physical structure for wettability control and thus markedly broaden the application horizons across engineering, physics, chemistry, biology, and medicine. This review aims to systematize this emerging yet robust technology, with the hope of aiding the realization of its full potential.

Effect of Janus particles and non-ionic surfactants on the collapse of the oil-water interface under compression

HYPOTHESIS

Janus particles (JPs) and surfactants express different behaviors at the oil-water interface under compression. When both are present at the interface, their synergies result in a different collapse mechanism than when present individually depending on the concentration of the JPs and surfactants.

EXPERIMENTS

Coarse-grained modeling methods were used to probe the synergies between Janus nanoparticles and nonionic surfactants on the stability of an oil-water interface under compression. When both JPs and surfactants were present, the interface was covered at 0-55% area by JPs and contained surfactants at 0-40% of the interfacial surfactant concentration corresponding to the critical micelle concentration (CMC).

FINDINGS

Compression of the interface with only surfactants resulted in the expulsion of surfactant molecules to the water phase once the interfacial concentration of surfactant molecules reached the CMC value. Compression of a Janus particle-laden interface past the closed-packing point led to a buckled interface, so that the total interfacial area remained constant upon further compression. When both surfactants and JPs were present on the interface, JPs still caused buckling, which helped retain the surfactant molecules on the interface. The interface exhibited a higher level of deformation in presence of surfactants. When the surfactant concentration was high, under compression, the surfactants partitioned into the water phase, but the buckling of the interface persisted.

Nanotherapeutics and nanotheragnostics for cancers: properties, pharmacokinetics, biopharmaceutics, and biosafety

With the worldwide increasing rate of chronic diseases, such as cancer, the development of novel techniques to improve the efficacy of therapeutic agents is highly demanded. Nanoparticles are especially well suited to encapsulate drugs and other therapeutic agents, bringing additional advantages, such as less frequent dosage requirements, reduced side effects due to specific targeting, and therefore increased patient compliance. However, with the increasing use of nanoparticles and their recent launch on the pharmaceutical market it is important to achieve high quality control of these advanced systems. In this review, we discuss the properties of different nanoparticles, the pharmacokinetics, the biosafety issues of concern, and conclude with novel nanotherapeutics and nanotheragnostics for cancer drug delivery.

Particle-laden fluid/fluid interfaces: physico-chemical foundations

Particle-laden fluid/fluid interfaces are ubiquitous in academia and industry, which has fostered extensive research efforts trying to disentangle the physico-chemical bases underlying the trapping of particles to fluid/fluid interfaces as well as the properties of the obtained layers. The understanding of such aspects is essential for exploiting the ability of particles on the stabilization of fluid/fluid interface for the fabrication of novel interface-dominated devices, ranging from traditional Pickering emulsions to more advanced reconfigurable devices. This review tries to provide a general perspective of the physico-chemical aspects associated with the stabilization of interfaces by colloidal particles, mainly chemical isotropic spherical colloids. Furthermore, some aspects related to the exploitation of particle-laden fluid/fluid interfaces on the stabilization of emulsions and foams will be also highlighted. It is expected that this review can be used for researchers and technologist as an initial approach to the study of particle-laden fluid layers.

Liquid-liquid interfaces: a unique and advantageous environment to prepare and process thin films of complex materials

Thin film technology is pervasive for many fields with high impact in our daily lives, which makes processing materials such as thin films a very important subject in materials science and technology. However, several paramount materials cannot be prepared as thin films through the well-known and consolidated deposition routes, which strongly limits their applicability. This is particularly noticeable for multi-component and complex nanocomposites, which present unique properties due to the synergic effect between the components, but have several limitations to be obtained as thin films, mainly if homogeneity and transparence are required. This review highlights the main advances of a novel approach to both process and synthesize different classes of materials as thin films, based on liquid/liquid interfaces. The so-called liquid/liquid interfacial route (LLIR) allows the deposition of thin films of single- or multi-component materials, easily transferable over any kind of substrate (plastics and flexible substrates included) with precise control of the thickness, homogeneity and transparence. More interesting, it allows the in situ synthesis of multi-component materials directly as thin films stabilized at the liquid/liquid interface, in which problems related to both the synthesis and processing are solved together in a single step. This review presents the basis of the LLIR and several examples of thin films obtained from different classes of materials, such as carbon nanostructures, metal and oxide nanoparticles, two-dimensional materials, organic and organometallic frameworks, and polymer-based nanocomposites, among others. Moreover, specific applications of those films in different technological fields are shown, taking advantage of the specific properties emerging from the unique preparation route.

Textured Microcapsules through Crystallization

This work demonstrates the fabrication of surface-textured microcapsules formed from emulsion droplets, which are stabilized by an interlocking mesh of needle-like crystals. Crystals of the small-organic-compound decane-1,10-bis(cyclohexyl carbamate) are formed within the geometric confinement of the droplets, through precipitation from a binary-solvent-dispersed phase. This binary mixture consists of a volatile solvent and nonvolatile carrier oil. Crystallization is facilitated upon supersaturation due to evaporation of the volatile solvent. Microcapsule diameter can be easily tuned using microfluidics. This approach also proves to be scalable when using conventional mixers, yielding spikey microcapsules with diameters in the range of 10-50 μm. It is highlighted that the capsule shape can be molded and arrested by jamming using recrystallization in geometric confinement. Moreover, it is shown that these textured microcapsules show a promising enhanced deposition onto a range of fabric fibers.

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.

A miniaturized radial Langmuir trough for simultaneous dilatational deformation and interfacial microscopy

INNOVATION

Interfacial rheological properties of complex fluid-fluid interfaces are strongly influenced by the film microstructure. Experimental investigations for correlating interfacial morphology and rheology are notoriously challenging. A miniaturized radial Langmuir trough was developed to study complex fluid-fluid interfaces under purely dilatational deformations that operates in tandem with a conventional inverted microscope for simultaneous interfacial visualization.

EXPERIMENTS

Two materials were investigated at an air-water interface: poly(tert-butyl methacrylate) (PtBMA) and dipalmitoylphosphatidylcholine (DPPC). Surface pressure measurements made in the radial Langmuir trough were compared with a commercial rectangular Langmuir trough. Interfacial in situ visualization for each material was performed during the compression cycle in the radial trough. Challenges associated with the small size of the radial Langmuir trough, such as the influence of capillary deformation on the measured surface pressure, are also quantified.

FINDINGS

Measured surface pressures between the newly developed radial trough and the rectangular Langmuir trough compare well. Micrographs obtained in the radial Langmuir trough were used to obtain film properties such as Young's modulus. The new advance in colloid and interface science is the ability to capture structure-property relationships of planar interfaces using microscopy and purely dilatational deformation. This will advance the development of constitutive modeling of complex fluid-fluid interfaces.

The fabrication of phospholipid vesicle-based artificial cells and their functions

Phospholipid vesicles as artificial cells are used to simulate the cellular structure and function.

Interfacial viscoelasticity and jamming of colloidal particles at fluid-fluid interfaces: a review

Colloidal particles can be adsorbed at fluid-fluid interfaces, a phenomenon frequently observed in particle-stabilized foams, Pickering emulsions, and bijels. Particles adsorbed at interfaces exhibit unique physical and chemical behaviors, which affect the mechanical properties of the interface. Therefore, interfacial colloidal particles are of interest in terms of both fundamental and applied research. In this paper, we review studies on the adsorption of colloidal particles at fluid-fluid interfaces, from both thermodynamic and mechanical points of view, and discuss the differences as compared with surfactants and polymers. The unique particle interactions induced by the interfaces as well as the particle dynamics including lateral diffusion and contact line relaxation will be presented. We focus on the rearrangement of the particles and the resultant interfacial viscoelasticity. Particular emphasis will be given to the effects of particle shape, size, and surface hydrophobicity on the interfacial particle assembly and the mechanical properties of the obtained particle layer. We will also summarize recent advances in interfacial jamming behavior caused by adsorption of particles at interfaces. The buckling and cracking behavior of particle layers will be discussed from a mechanical perspective. Finally, we suggest several potential directions for future research in this area.

Surface pressure of liquid interfaces laden with micron-sized particles

We consider the surface pressure of a colloid-laden liquid interface. As micron-sized particles of suitable wettability can be irreversibly bound to the liquid interface on experimental timescales, we use the canonical ensemble to derive an expression for the surface pressure of a colloid-laden interface. We use this expression to show that adsorption of particles with only hard-core interactions has a negligible effect on surface pressures from typical Langmuir-trough measurements. Moreover, we show that Langmuir-trough measurements cannot be used to extract typical interparticle potentials. Finally, in the case of relatively weakly interacting sterically stabilized particles at a liquid interface, we argue that the dependence of measured surface pressure on surface fraction can be explained by particle coordination number at low to intermediate particle surface fractions. At high surface fractions, where the particles are jammed and cannot easily rearrange, we argue that contact-line sliding and/or deformations of the liquid interface at the length scale of the particles might play a pivotal role.

Soft matter science and the COVID-19 pandemic

Much of the science underpinning the global response to the COVID-19 pandemic lies in the soft matter domain. Coronaviruses are composite particles with a core of nucleic acids complexed to proteins surrounded by a protein-studded lipid bilayer shell. A dominant route for transmission is via air-borne aerosols and droplets. Viral interaction with polymeric body fluids, particularly mucus, and cell membranes controls their infectivity, while their interaction with skin and artificial surfaces underpins cleaning and disinfection and the efficacy of masks and other personal protective equipment. The global response to COVID-19 has highlighted gaps in the soft matter knowledge base. We survey these gaps, especially as pertaining to the transmission of the disease, and suggest questions that can (and need to) be tackled, both in response to COVID-19 and to better prepare for future viral pandemics.

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 Electrical Double Layer Force between Spherical Particles Which Are Partially Submerged in Water

We study the EDL force between two colloidal particles that are adsorbed to the surface of an electrolyte solution. The attachment of colloidal particles to a free surface of an electrolyte solution, which may interface with another liquid or vapor phase, is a well-known phenomenon that is employed in many scientific and industrial applications, the most well-known of which is the Pickering emulsion. In addition to capillary stresses, the particles will experience an electrical double layer (EDL) force when they are close to each other. The force originates from the overlap of the diffusive layers of ions that appear in the electrolyte solution next to the charged surfaces of the particles and the charged surface of the electrolyte solution, which is free of particles. Here, we elucidate the contribution of the free surface of the electrolyte solution to the EDL force between two spherical particles, which are half-submerged in the electrolyte solution. We solve the linearized Poisson-Boltzmann equation for the excess electrical potential near the particles and integrate over the resulting excess Maxwell and osmotic stresses on the particles. We further give corresponding Páde approximations, thus enabling the use of simple formulas for the EDL force between interacting particles in cases similar to the ones in this study without the need to repeat the mathematical procedure employed here.

Recent advances of characterization techniques for the formation, physical properties and stability of Pickering emulsion

Recently, there have been increasing demand for the application of Pickering emulsions in various industries due to its combined advantage in terms of cost, quality and sustainability. This review aims to provide a complete overview of the available methodology for the physical characterization of emulsions that are stabilized by solid particles (known as Pickering emulsion). Current approaches and techniques for the analysis of the formation and properties of the Pickering emulsion were outlined along with the expected results of these methods on the emulsions. Besides, the application of modelling techniques has also been elaborated for the effective characterization of Pickering emulsions. Additionally, approaches to assess the stability of Pickering emulsions against physical deformation such as coalescence and gravitational separation were reviewed. Potential future developments of these characterization techniques were also briefly discussed. This review can act as a guide to researchers to better understand the standard procedures of Pickering emulsion assessment and the advanced methods available to date to study these emulsions, down to the minute details.

Interactions between polystyrene particles with diameters of several tens to hundreds of micrometers at the oil-water interface

HYPOTHESIS

The charged spherical colloidal particles at the fluid-fluid interface experience considerably strong and long-ranged electrostatic and capillary interactions. The contribution of capillary force becomes more significant as the particle size increases beyond a certain limit. The relative strengths of the two competing interactions between the spherical polystyrene particles at the oil-water interface are quantified depending on their size.

EXPERIMENTS

The studied particles, obtained using the microfluidic method, have diameters of tens to hundreds of micrometers. The scaling behaviors of the commercially available colloidal particles with diameters of ~3 μm are also compared. An optical laser tweezer apparatus is used to directly or indirectly measure the interparticle force. Subsequently, the capillary force that can be attributed to the gravity-induced interface deformation and contact line undulation is calculated and compared with the measured interaction force.

FINDINGS

Regardless of the particle diameter (~3-330 μm), the measured force is observed to decay as r^-4, where r denotes the center-to-center separation, demonstrating that the dipolar electrostatic interaction is important and that the gravity-induced capillary interaction is negligible. Furthermore, numerical calculations with respect to the undulated meniscus confirm that the magnitude of capillary interaction is significantly smaller than that of the measured electrostatic interaction.

Direct observation of the attachment behavior of hydrophobic colloidal particles onto a bubble surface

The attachment of solid particles to the surface of immersed gas bubbles plays a fundamental role in surface science, and hence plays key roles in various engineering fields ranging from industrial separation processes to the fabrication of functional materials. However, detailed investigation from a microscopic view on how a single particle attaches to a bubble surface and how the particle properties affect the attachment behavior has been so far scarcely addressed. Here, we observed the attachment of a single particle to a bubble surface using a high-speed camera and systematically investigated the effects of the wettability and shape of particles. We found that hydrophobic particles abruptly "jumped into" the bubble while sliding down the bubble surface to eventually satisfy their static contact angles, the behavior of which induced a much stronger attachment to the bubble surface. Interestingly, the determinant factor for the attachment efficiency of spherical particles was not the wettability of the spherical particles but the location of the initial collision with the bubble surface. In contrast, the attachment efficiency of anisotropically-shaped particles was found to increase with the hydrophobicity caused by a larger contact area to the bubble surface. Last but not least, a simple formulation is suggested to recover the contact angle based on the jump-in behavior.

Ultra-stable Pickering emulsion stabilized by a natural particle bilayer

An all-natural Pickering emulsion with a Janus interface of particle bilayer is prepared, which has unprecedented stability.

Impact of Surface Amphiphilicity on the Interfacial Behavior of Janus Particle Layers under Compression

Langmuir monolayers of silica/gold Janus particles with two different degrees of amphiphilicity have been examined to study the significance of particle surface amphiphilicity on the structure and mechanical properties of the interfacial layers. The response of the layers to the applied compression provides insight into the nature and strength of the interparticle interactions. Different collapse modes observed for the interfacial layers are linked to the amphiphilicity of Janus particles and their configuration at the interface. Molecular dynamics simulations on nanoparticles with similar contact angles provide insight on the arrangement of particles at the interface and support our conclusion that the interfacial configuration and collapse of anisotropic particles at the air/water interface are controlled by particle amphiphilicity.

Sub-micron sized cytochrome c particles adsorbing to solid surfaces: A comparison between solution phase and colloidal system

Present work reports on Cytochrome C (Cyt C) adsorption from solution and as sub-micron sized colloidal particles (pH 7.5) at fluid/solid interface. The colloidal particles from 2 methods ((method 1 freeze-thaw); (method 2-Decane/water interface)) have been characterized by UV-Visible spectroscopy, Static and Time resolved Fluorescence spectra and Circular dichroic spectroscopy. Morphology and size studies indicate that the colloids are solid and well-packed. Using Quartz crystal microbalance with dissipation, elastic compliance of the protein on quartz surface has been monitored. Properties of the protein from solution and as colloids assembled from method 2 are similar in elastic compliance (65.53 ± 1.2 and 73.5 ± 1.1 GPa^-1 respectively) due to polar/non-polar interactions at the solid surface. For particles from method 1, irregular desolvation of water on the particle surface results in higher compliance (104.3 ± 1.3 GPa^-1). Change in work of adhesion from contact angle profiles shows optimal adhesion for colloids from method 1 whereas the protein solution and as colloids from method 2, show subnormal adhesion. The work shows the role of extensive H-bonding with the hydrodynamically coupled water leading to a fluid like system in protein colloids from method 1 whereas those from method 2 behaved more like the pure protein adsorbing from solution.

Measurement of the force between uncharged colloidal particles trapped at a flat air/water interface

The radial attraction between microspheres straddling at the air/water interface (Bond number ≪1), whose origin is the irregular shape of the contact line and its concomitant distortion of the water surface, is measured using two light beams of a time-sharing optical tweezer. The colloidal particles used to make the measurements are microspheres made of hydrophobically covered silica to reduce the electrostatic interactions to a minimum. The measured radial force goes as a quadrupolar power law, r-n, with n = 5.02 ± 0.18 and n = 5.04 ± 0.18 for particles of 3 μm and 5 μm, respectively. In both cases, the electrostatic interaction is negligible.

Amphiphilic Dimers at Liquid-Liquid Interfaces: A Density Functional Approach

We apply density functional theory to study the structure of dimers at the interface between two partially miscible symmetric liquids. The dimers are built of two tangentially jointed spheres and do not solve the coexisting liquids. The interactions in the system are modeled using Lennard-Jones potentials with different interactions between segments of the dimers and the liquid components. We study how asymmetry of the interactions between dimers and molecules of the liquid, i.e., the degree of dimer amphiphilicity, influences the interfacial structure. Two unexpected phenomena have been found. First, for some systems, the liquid-liquid interface is able to accommodate only a finite amount of dimers. If the amount of added dimers is larger than a threshold value, a part or all of the dimers move to the interior one of the coexisting phase, forming an insoluble sheet inside it, or the initial interface splits into separate parts. The second is a peculiar behavior of the dependence of the interfacial width with an increase of the amount of added dimers. In this case, we observe a discontinuous jump that is connected with reorientation of dimers with respect to the interface.