Electric forces induced by a charged colloid particle attached to the water-nonpolar fluid interface

Here, we solve the problem about the electric field of a charged dielectric particle, which is adsorbed at the water-nonpolar fluid (oil, air) boundary. The solution of this problem is a necessary step for the theoretical prediction of the electrodipping force acting on such particle, as well as of the electrostatic repulsion and capillary attraction between two adsorbed particles. In accordance with the experimental observations, we consider the important case when the surface charges are located at the particle-nonpolar fluid boundary. To solve the electrostatic problem, the Mehler-Fock integral transform is applied. In the special case when the dielectric constants of the particle and the nonpolar fluid are equal, the solution is obtained in a closed analytical form. In the general case of different dielectric constants, the problem is reduced to the numerical solution of an integral equation, which is carried out by iterations. The long-range asymptotics of the solution indicates that two similar particles repel each other as dipoles, whose dipole moments are related to the particle radius, contact angle, dielectric constant and surface charge density. The investigated short-range asymptotics ensures accurate calculation of the electrodipping force. For a fast and convenient application of the obtained results, the derived physical dependencies are tabulated as functions of the contact angle and the dielectric constants.     

Spontaneous formation of mesostructures in colloidal monolayers trapped at the air-water interface: a simple explanation

The spontaneous formation of loosely bound ordered aggregates, foam, voids, chains, striations, and loops (see Figure 1a), called mesostructures hereafter, has been observed in colloidal monolayers trapped at the air-water interface. The distance between particles in these mesostructures is of the order of the particle radius (micrometers), implying that the colloidal interaction potential has a minimum at such distances, which could induce the phase separation of colloidal monolayers in dense and dilute regions. This is at odds with the accepted theory (Derjaguin-Landau-Verwey-Overbeek (DLVO)) of colloidal interactions, which predicts a secondary minimum at distances of nanometers between pairs of interacting particles. Moreover, the introduction of capillary, hydrophobic, and dipolar interactions between particles in an extended DLVO theory is not able to explain the spontaneous formation of mesostructures either. Recently, a great deal of effort has focused on understanding the mechanism behind the phenomenon of long-range attraction between colloidal particles confined in interfaces. In particular, this attraction has been employed to explain the spontaneous formation of mesostructures. Here, we show that the appearance of our mesostructures is due to the contamination of colloidal monolayers by silicone oil (poly(dimethylsiloxane)), which arises from the coating of the needles and syringes used to deposit and spread the particle solution at the air-water interface. The difference in the interfacial tension of water and silicone oil accounts for the formation of the experimentally observed mesostructures.     

Effect of electric-field-induced capillary attraction on the motion of particles at an oil-water interface

Here, we investigate experimentally and theoretically the motion of spherical glass particles of radii 240-310 microm attached to a tetradecane-water interface. Pairs of particles, which are moving toward each other under the action of lateral capillary force, are observed by optical microscopy. The purpose is to check whether the particle electric charges influence the particle motion, and whether an electric-field-induced capillary attraction could be detected. The particles have been hydrophobized by using two different procedures, which allow one to prepare charged and uncharged particles. To quantify the hydrodynamic viscous effects, we developed a semiempirical quantitative approach, whose validity was verified by control experiments with uncharged particles. An appropriate trajectory function was defined, which should increase linearly with time if the particle motion is driven solely by the gravity-induced capillary force. The analysis of the experimental results evidences for the existence of an additional attraction between two like-charged particles at the oil-water interface. This attraction exceeds the direct electrostatic repulsion between the two particles and leads to a noticeable acceleration of their motion.     

Shape of the capillary meniscus around an electrically charged particle at a fluid interface: comparison of theory and experiment

Here, we consider in detail the problem of the shape of the capillary meniscus around a charged colloidal particle, which is attached to a fluid interface: oil/water or air/water. The meniscus profile is influenced by the electric field created by charges at the particle/nonpolar fluid boundary. We digitized the coordinates of points from the meniscus around silanized glass spheres (200-300 mum in radius) attached to the tetradecane/water interface. The theoretical meniscus shape is computed in three different ways that give numerically coincident results. It is proven that for sufficiently small particles the meniscus profile can be expressed as a superposition of pure electric and gravitational deformations. Special attention is paid to the comparison of theory and experiment. A procedure for data processing is developed that allows one to obtain accurate values of the contact angle and surface charge density from the fit of the experimental meniscus profile. For all investigated particles, excellent agreement between theory and experiment is achieved. The results indicate that the electric field gives rise to an interfacial deformation of medium range and considerable amplitude.     

Structure and stability of silica particle monolayers at horizontal and vertical octane-water interfaces

Monolayers of silica particles at horizontal and vertical octane-water interfaces have been studied by microscopy. It is found that their structure and stability depend strongly on the particle hydrophobicity. Very hydrophobic silica particles, with a contact angle of 152 degrees measured through the water, give well-ordered monolayers at interparticle distances larger than 5 particle diameters which are stable toward aggregation and sedimentation. In contrast, monolayers of less-hydrophobic particles are disordered and unstable. Two-dimensional particle sedimentation has been observed in the case of vertical monolayers. The results have been analyzed with a simple two-particle model considering the sedimentation equilibrium as a balance between the long-range electrostatic repulsion through the oil, the gravity force, and the capillary attraction due to deformation of the fluid interface around particles. The value of the charge density at the particle-octane interface, 14.1 muC/m(2), found for the most hydrophobic particles is reasonable. It drastically decreases for particles with lower hydrophobicity, which is consistent with the order-disorder transition in monolayer structure reported by us before. The pair interactions between particles at a horizontal octane-water interface have been analyzed including the capillary attraction due to undulated three-phase contact line caused by nonuniform wetting (the contact angle hysteresis). The results are in agreement with the great stability of very hydrophobic silica particle monolayers detected experimentally, even at low pH at the point of zero charge of the particle-water interface, and with the aggregated structure of hydrophilic particle monolayers.     

Interfacial characterization of beta-lactoglobulin networks: displacement by bile salts

The competitive displacement of a model protein (beta-lactoglobulin) by bile salts from air-water and oil-water interfaces is investigated in vitro under model duodenal digestion conditions. The aim is to understand this process so that interfaces can be designed to control lipid digestion thus improving the nutritional impact of foods. Duodenal digestion has been simulated using a simplified biological system and the protein displacement process monitored by interfacial measurements and atomic force microscopy (AFM). First, the properties of beta-lactoglobulin adsorbed layers at the air-water and the olive oil-water interfaces were analyzed by interfacial tension techniques under physiological conditions (pH 7, 0.15 M NaCl, 10 mM CaCl2, 37 degrees C). The protein film had a lower dilatational modulus (hence formed a weaker network) at the olive oil-water interface compared to the air-water interface. Addition of bile salt (BS) severely decreased the dilatational modulus of the adsorbed beta-lactoglobulin film at both the air-water and olive oil-water interfaces. The data suggest that the bile salts penetrate into, weaken, and break up the interfacial beta-lactoglobulin networks. AFM images of the displacement of spread beta-lactoglobulin at the air-water and the olive oil-water interfaces suggest that displacement occurs via an orogenic mechanism and that the bile salts can almost completely displace the intact protein network under duodenal conditions. Although the bile salts are ionic, the ionic strength is sufficiently high to screen the charge allowing surfactant domain nucleation and growth to occur resulting in displacement. The morphology of the protein networks during displacement is different from those found when conventional surfactants were used, suggesting that the molecular structure of the surfactant is important for the displacement process. The studies also suggest that the nature of the oil phase is important in controlling protein unfolding and interaction at the interface. This in turn affects the strength of the protein network and the ability to resist displacement by surfactants.     

Emulsions stabilised by food colloid particles: role of particle adsorption and wettability at the liquid interface

We study the effect of the particle wettability on the preferred type of emulsion stabilised solely by food colloid particles. We present results obtained with the recently developed gel trapping technique (GTT) for characterisation of wettability and surface structuring of individual food colloid particles adsorbed at air-water and oil-water interfaces. This method allows us to replicate a particle monolayer onto the surface of polydimethylsiloxane (PDMS) without altering the position of the particles. By observing the polymer surface with scanning electron microscopy (SEM), we are able to determine the contact angle of the individual particles at the initial liquid interface. We demonstrate that the GTT can be applied to fat crystal particles, calcium carbonate particles coated with stearic acid and spray-dried soy protein/calcium phosphate particles at air-water and oil-water interfaces. Subsequently, we prepare emulsions of decane and water stabilised by the same food colloid particles and correlate the wettability data obtained for these particles to the preferred type of emulsions they stabilise.     

Naturally occurring spore particles at planar fluid interfaces and in emulsions

We have investigated the potential of utilizing naturally occurring spore particles of Lycopodium clavatum as sole emulsifiers of oil and water mixtures. The preferred emulsions, prepared from either oil-borne or aqueous-borne dispersions of the monodispersed particles of diameter 30 microm, are oil-in-water. The particles act as efficient stabilizers for oils of different polarity. Droplets as large as several millimeters are stable to coalescence indefinitely, despite the low coverage of interfaces by particles observed microscopically. Consistent with the emulsion findings, we discover that particles spontaneously adsorb to bare oil-water interfaces of single drops from oil dispersions, whereas adsorption is less spontaneous and extensive from aqueous dispersions. Monolayers of the spore particles at both air-water and oil-water planar interfaces contain particles in an aggregated state forming clusters and chains. The influence of particle concentration, oil/water ratio, and additives in the aqueous phase is studied.     

Particle bridging between oil and water interfaces

Particle bridging between a water drop and a flat oil-water interface has been observed when the drop is brought into contact with the interface, leading to the formation of a dense particle monolayer of disc shape (namely, particle disc) that prevents the drop from coalescing into the bulk water phase. Unlike previous observations where particles from opposite interfaces appear to register with each other before bridging, the present experiment demonstrates that the particle registry is not a necessity for bridging. In many cases, the particles from one of the interfaces were repelled away from the contact region, leaving behind the particles from the other interface to bridge the two interfaces. This is confirmed by particle bridging experiments between two interfaces covered with different sized particles, and between a particle-covered interface and a clean interface. The dynamics associated with the growth of the particle disc due to particle bridging follows a power law relationship between the radius of the disc and time: r proportional, variant t0.32+/-0.03. A scaling analysis assuming capillary attraction as the driving force and a hydrodynamic resistance leads to the power law r proportional, variant t1/3, in good agreement with the experiment. In addition, we found that binary mixtures of two different sized particles can undergo phase segregation driven by the particle bridging process.     

Contact angles of colloid silica and gold particles at air-water and oil-water interfaces determined with the gel trapping technique

We have used the recently developed gel trapping technique (GTT) to determine the three-phase contact angles of submicrometer silica particles partially coated with octadecyl groups. The particles were spread at air-water and decane-water surfaces, and the aqueous phase was subsequently gelled with a nonadsorbing polysaccharide. The particles trapped at the surface of the aqueous gel were lifted by molding with curable poly(dimethylsiloxane) and imaged with scanning electron microscopy (SEM) to determine the particle contact line diameter which allows their contact angle at the original air-water or oil-water interface to be estimated. We report for the first time the use of the GTT for characterizing the contact angle of individual submicrometer particles adsorbed at liquid interfaces. The SEM images also reveal the structure of the particle monolayer at the interface and the structure of adsorbed particle aggregates. We have also determined the contact angles of agglomerated gold powder microparticles at the air-water and the decane-water interfaces. It was found that agglomerated gold particles demonstrate considerably higher contact angles than those on flat gold-coated surfaces.     

Simulation of the flow of polymer blends in convergent channels

The method for the simulation of various cases of collisions between spherical solid particles in suspensions or emulsions (for example, in mixtures of polymers) moving in convergent flows is proposed. These flows arise in many technological schemes for the processing of polymers. The derived formulas make it possible to relate interparticle distances with the dimensions of particles and their volume fraction. The effect of the distribution of particles with respect to their dimensions and distances between them is considered. The flow lines are proposed to be described by two functions: (i) a parabolic dependence for a reservoir and (ii) a logarithmic dependence at a capillary inlet. The height of the conjugation of the above dependences appears to be similar for both flow lines. Under similar conditions, the number of collisions between particles and their rate dramatically increase after passage from a reservoir to a capillary as well as with a decrease in the radius of the particles (at a constant volume fraction). The experiment on microimaging of the frozen emulsion of PMMA in PS at different regions of convergent flow shows a fair qualitative correlation between experimental and calculated data.     

Some mechanisms of immiscible fluid displacement in small networks

We report experimental observations on immiscible displacement in two small networks using three different pairs of fluids, air-oil, air-water, and oil-water, to vary the wettability. The experiments were run for a wide range of capillary number, from 10⁻⁷ to 10⁻³. Various mechanisms are observed. These are film spreading and drainage, Haines' jump, free slip and stick-slip meniscus motion, contact angle hysteresis, snap-off, coalescence, and blocking of film and bubble. For the air-oil case, oil is perfectly wetting in the network. In imbibition, the displacement occurs first via thin film spreading, followed by snap-off of menisci, and then by piston-like displacement at low flow rates. As the flow rate increases, piston-like displacement dominates because film spreading is comparatively slow. Snap-off of menisci in the throats is a necessary condition for air trapping. In drainage, meniscus snap-off and coalescence are observed in one network. For both imbibition and drainage, during each snap-off or piston-like displacement event, all menisci move freely along the channels to adjust their curvatures, due to the lubrication of the wetting film. For the other two fluid pairs at low flow rates, this curvature readjustment through free slipping of meniscus is not observed, presumably due to the absence of wetting film during the displacement. At high flow rate, oscillation of menisci due to volumetric competition is observed. Neither wetting film spreading nor throat snap-off is observed. Stick and slip motion of meniscus is observed, probably due to the roughness and/or heterogeneous wettability of the solid surface. For the oil-water system the wettability seems to be time dependent. Coalescence between two menisci can occur in the throat, in the pore, or at the pore-throat boundary during displacement. Trapping of the displaced phase is due to its being bypassed or snapped off in the throat.     

Atomic-scale roughness effect on capillary force in atomic force microscopy

We study the capillary force in atomic force microscopy by using Monte Carlo simulations. Adopting a lattice gas model for water, we simulated water menisci that form between a rough silicon-nitride tip and a mica surface. Unlike its macroscopic counterpart, the water meniscus at the nanoscale gives rise to a capillary force that responds sensitively to the tip roughness. With only a slight change in tip shape, the pull-off force significantly changes its qualitative variation with humidity.     

Resonance frequencies of meniscus waves as a physical mechanism for a DNA biosensor

This paper presents a liquid surface biosensor whose potential applications are analogue to the well-known quartz crystal microbalance. The technique involved is based on the resonance of meniscus capillary waves here excited at a functionalized air-water interface. The strategy proposed in this paper can be seen as a promising way to avoid as much as possible any transfer of Blodgett type. Meniscus capillary waves supplied by the electrodynamical vibration of a brimful cylinder filled with water are used as a way to characterize the surface aging of an air-water interface covered by a lipidic monolayer. An optical technique based on one-dimensional interferometry is developed to measure continuously the resonant behavior of the surface elevation at the center of the cell around the natural frequencies of the meniscus waves. The frequency dependence of the wave amplitude is investigated during the transient regime associated to the immobilization of DNA strands at the lipidic matrix. Resonant frequencies are found to be very sensitive to the chemical loading supported by the air-water interface. The technique is seen as a mean to discriminate between single- and double-stranded DNA.     

Effect of pH on the Aggregation Stability of Microcrystalline Cellulose Dispersions in Aqueous 0.1 M NaCl Solutions

Effect of pH on the coagulation kinetics of microcrystalline cellulose dispersions in an aqueous 0.1 M NaCl solution is studied by the flow ultramicroscopy. The lowest coagulation rate is observed at pH 4.9. A decrease or an increase in pH gives rise to the coagulation rate approaching the rate of fast coagulation (according to Smoluchowski) at pH 1.0. Results of calculating the particle pair interaction energy in terms of the DLVO theory with allowance for only the molecular and ion electrostatic components suggest the dominance of attraction forces at any interparticle distances and cannot explain the data of experimental methods. The allowance for the structural component, which arises upon the overlap of water boundary layers surrounding hydrophilic particles of microcrystalline cellulose, makes it possible to treat the experimental results and estimate possible values of the K andl parameters of the equation for the structural component.     

A many-body dissipative particle dynamics study of forced water-oil displacement in capillary

The forced water-oil displacement in capillary is a model that has important applications such as the groundwater remediation and the oil recovery. Whereas it is difficult for experimental studies to observe the displacement process in a capillary at nanoscale, the computational simulation is a unique approach in this regard. In the present work, the many-body dissipative particle dynamics (MDPD) method is employed to simulate the process of water-oil displacement in capillary with external force applied by a piston. As the property of all interfaces involved in this system can be manipulated independently, the dynamic displacement process is studied systematically under various conditions of distinct wettability of water in capillary and miscibility between water and oil as well as of different external forces. By analyzing the dependence of the starting force on the properties of water/capillary and water/oil interfaces, we find that there exist two different modes of the water-oil displacement. In the case of stronger water-oil interaction, the water particles cannot displace those oil particles sticking to the capillary wall, leaving a low oil recovery efficiency. To minimize the residual oil content in capillary, enhancing the wettability of water and reducing the external force will be beneficial. This simulation study provides microscopic insights into the water-oil displacement process in capillary and guiding information for relevant applications.     

Bubble-solid interactions in water and electrolyte solutions

Surface forces between an air bubble and a flat mica surface immersed in aqueous electrolyte solutions have been investigated using a modified surface force apparatus. An analysis of the deformation of the air bubble with respect to the mutual position of the bubble and the mica surface, the capillary pressure, and the disjoining pressure allows the air-liquid surface electrical potential to be determined. The experiments show that a long-range, double-layer repulsion acts between the mica (which is negatively charged) and an air bubble in water and in various electrolyte solutions at low concentration, thereby indicating that the air bubble surface is negatively charged. However, there is clear evidence that charge regulation occurs at the air-water interface to maintain a constant surface potential, and as a result of this, the charge at this interface changes from negative to positive as the bubble approaches the mica surface. Because of the attraction that arises as a result of the charge reversal, a finite force is required to separate the bubble from the mica, though the mica remains wetted by the aqueous phase. At the low concentrations investigated, the potential on the gas-liquid interface is independent of the electrolyte type within experimental uncertainty.     

Partitioning of clay colloids at air-water interfaces

Colloid sorption onto air-water interfaces in a variety of natural environments has been previously recognized, but better quantification and understanding is still needed. Affinities of clay colloids for the air-water interface were measured using a bubble-column method and reported as partition coefficients (K). Four types of dilute clay suspensions were measured in NaCl solutions under varying pH and ionic strength conditions: kaolinite KGa-1, illite IMt-2, montmorillonite SWy-2, and bentonite. The K values of three types of polystyrene latex particles with different surface-charge properties were also measured for comparison. Kaolinite exhibited extremely high affinity to the air-water interface at pH values below 7. Illite has lower affinity to air-water interfaces than kaolinite, but has similar pH dependence. Na-montmorillonite and bentonite clay were found excluded from the air-water interface at any given pH and ionic strength. Positively and negatively charged latex particles exhibited sorption and exclusion, respectively, at the air-water interface. These results show the importance of electrostatic interactions between the air-water interface and colloids, especially the influence of pH-dependent edge charges, and influence of particle shape.     

Electric field-induced self-assembly of micro- and nanoparticles of various shapes at two-fluid interfaces

Particle lithography which explores the capability of particles to self-assemble offers an attractive means to manufacture nanostructured materials. Although traditional techniques typically lead to the formation of dense crystals, adjustable non-close-packed crystals are crucial in a number of applications. We have recently proposed a novel method to assemble spherical micro- and nanoparticles into monolayers. The technique consists of trapping particles at a liquid-fluid interface and applying an electric field normal to the interface. Particles rearrange themselves under the influence of interfacial and electrostatic forces to form 2-D hexagonal arrays of long-range order and whose lattice constant depends on the electric field strength and frequency. Furthermore, the existence of an electric field-induced capillary force makes the technique applicable to submicron and nanosized particles. Although spherical particles are often used, non-spherical particles can be beneficial in practice. Here, we review the method, discuss its applicability to particles of various shapes, and present results for particles self-assembly on air-liquid and liquid-liquid interfaces. In the case of non-spherical particles, the self-assembly process, while still taking place, is more complex as particles experience a torque which causes them to rotate relative to one another. This leads to a final arrangement displaying either a dominant orientation or no well-defined orientation. We also discuss the possibility of dislodging the particles from the interface by applying a strong electric field such that the Weber number is of order 1 or larger, a phenomenon which can be utilized to clean particles from liquid-fluid surfaces.