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[Display omitted] Analytical expressions for calculating Hamaker constant (HC) and van der Waals (VDW) energy/force for interaction of a particle with a solid water interface has been reported for over eighty years. This work further developed novel analytical expressions and numerical approaches for determining HC and VDW interaction energy/force for the particle approaching and penetrating air–water interface (AWI), respectively. The expressions of HC and VDW interaction energy/force before penetrating were developed through analysis of the variation in free energy of the interaction system with bringing the particle from infinity to the vicinity of the AWI. The surface element integration (SEI) technique was modified to calculate VDW energy/force after penetrating. We explain why repulsive VDW energy exists inhibiting the particle from approaching the AWI. We found very significant VDW repulsion for a particle at a concave AWI after penetration, which can even exceed the capillary force and cause strong retention in water films on a solid surface and at air–water-solid interface line. The methods and findings of this work are critical to quantification and understanding of a variety of engineered processes such as particle manipulation (e.g., bubble flotation, Pickering emulsion, and particle laden interfaces). [ABSTRACT FROM AUTHOR]

Micron and nano-scale particles are increasingly used to stabilize water-in-oil (W/O) emulsions. Though remarkably stable, the resulting emulsions can be broken by adding low molecular weight surfactants that modify the wettability of the interfacially-adsorbed particles. W/O emulsions were prepared using lipophilic crystals of the monoglyceride glycerol monostearate (GMS), followed by addition of sorbitan monooleate (SMO) at concentrations below and above its critical micelle concentration (CMC). Systematic measurements of interfacial tension and three-phase contact angles, as well as characterization of emulsion sedimentation and microstructure, were used to assess GMS crystal wettability and emulsion destabilization. GMS crystals formed shells around the dispersed droplets, resulting in emulsions stable against breakdown under quiescent conditions. With SMO concentrations added below CMC, emulsion stability was not significantly affected. At SMO concentrations above CMC, the integrity of the crystalline shell was markedly affected. Notably, the GMS crystals transitioned from preferential oil-wet to water-wet behavior, eventually leading to their diffusion into the droplets. Therefore, in-situ modification of particle wettability at the oil–water interface was responsible for emulsion breakdown. Findings from this study may provide a pathway for the design of particle-stabilized W/O emulsions with controllable breakdown properties for applications such as tailored release of aqueous bioactive compounds. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Hypothesis Hydrophobic spheres may exhibit three impact modes after impacting the liquid surface. For the impact of common non-spherical particles in practical processes, particle shape affects the flow of surrounding fluid and forces acting on them, thus the impact behaviors of non-spherical particles may differ from those of spherical particles and remain to be revealed. Simulation The impact of hydrophobic micron ellipsoidal particles is numerically studied by solving the coupled equations of particle motion and surrounding fluid flow combining VOF method and dynamic meshing technique. The motion characteristics of the ellipsoids and fluids, and the main forces acting on the ellipsoids during impact are analyzed. Findings The increase in the axis ratio (AR) of ellipsoid decreases the surface tension and fluid force, resulting in a smaller total force acting on the ellipsoid. Correspondingly, the ellipsoid’s impact behavior changes. An impact-mode phase diagram is presented for the studied ellipsoids. Three impact modes, namely, submergence, rebound, and oscillation, exist when AR > 0.8, while only submergence and oscillation exist when AR ≤ 0.8. The critical velocities decrease with the increase in AR , which is well illustrated by the analysis of energy conversion under critical conditions. [ABSTRACT FROM AUTHOR]

In this work, we present a simple droplet microfluidic approach for generating graphene oxide (GO) hydrogel composite particles for oil decontamination. By stepwise solvent exchanges, the resulted hydrophilic GO hydrogel composite particles were transferred into organic media without any chemical modifications. As the GOs were locked tightly in hydrogel network, they were hardly changed during the processes of solvent exchanges. Thus, their polar surfaces remained in direct contact with the exchanged organic media, which indicated that the transferred GO particles were capable of effectively adsorbing polar impurities. Attractively, by encapsulating hollow cores and additional magnetic nanoparticles into the emulsion templates during the fabrication, the GO hydrogel composite particles were imparted with hierarchical porous structures and controllable movement capability, both of which could improve their efficiency of impurities adsorbing. These features make the GO hydrogel composite particles described here ideal for oil decontamination. [ABSTRACT FROM AUTHOR]

Electrokinetic movement of spherical polystyrene particles at different liquid-fluid interfaces was experimentally investigated in this paper. A novel method was developed to place the particles rightly at the interfaces formed in a large plastic container. The velocity was measured by an optical microscope. The experimental results show that the particles (3, 5 and 10 μm in diameter) at the water-air interface, water-dodecane interface and NaCl solution-air interface move in the opposite direction of the applied electric field. The magnitude of the particles’ velocity increases linearly with the increase in the applied electric field. Moreover, for particles of the same size, the electrokinetic velocity at the liquid-fluid interfaces is larger than particles’ electrophoretic velocity in the bulk liquid phase. Under the same electric field, however, the electrokinetic velocity of smaller particles at the liquid-fluid interfaces is larger than that of larger particles. Such results are attributed to the surface charges at the liquid-fluid interface and the particle-liquid interface. [ABSTRACT FROM AUTHOR]

Development of an electrostatic stabilization mechanism for colloidal suspensions in nonpolar fluids requires an improved understanding of the interactions between the inverse micelles and particles as well as the roles that steric and electrostatic effects play. A droplet-based millifluidic device is designed and used to investigate the stabilization effects of surfactants on colloidal suspensions. A system containing carbon black and the surfactant OLOA 11000 suspended in dodecane is chosen as a well-characterized system to study sedimentation quantitatively. This device takes advantage of sub-millimeter optical path lengths to characterize sedimentation at concentrations at which sedimentation is not observable in the bulk and to achieve higher resolution in composition. A simple image analysis algorithm has been developed and applied to quantify sedimentation. Conductivity measurements using electrochemical impedance spectroscopy (EIS) are coupled with the sedimentation experiments to identify the concentration ranges in which steric and electrostatic effects are dominant. A more gradual transition is observed than previously reported. [ABSTRACT FROM AUTHOR]

Foam-based materials are promising micro-structured materials with interesting thermal and acoustical properties. The control of the material morphology requires counteracting all the destabilizing mechanisms during their production, starting with the drainage process, which remains to be understood in the case of the complex fluids that are commonly used to be foamed. Here we perform measurements for the drainage velocity of aqueous foams made with granular suspensions of hydrophilic monodisperse particles and we show that the effect of the particles can be accounted by two parameters: the volume fraction of particles in the suspension ( φ p ) and the confinement parameter ( λ ), that compares the particle size to the size of passage through constrictions in the foam network. We report data over wide ranges for those two parameters and we identify all the regimes and transitions occurring in the φ p - λ diagram. In particular, we highlight a transition which refers to the included/excluded configuration of the particles with respect to the foam network, and makes the drainage velocity evolve from its minimal value (fully included particles) to its maximal one (fully excluded particles). We also determine the conditions ( φ p , λ ) leading to the arrest of the drainage process. [ABSTRACT FROM AUTHOR]

Abstract: Capillary rise is the basis of some methods that are widely applied for the determination of contact angles as well as wettabilities of small particles. The equivalent hydraulic radius r d in the Classical Washburn equation is assumed to be particle-specific. But it seems that r d is not always constant when the type of liquids is different. The new equation with the pressure increment and the hydrostatic effects are theoretically derived based on the Washburn equation, so contact angles of small particles can be measured experimentally independently of r d. The result shows the validity of the proposed method, and therefore, it becomes possible to accurately measure the wettability of small particles. [Copyright &y& Elsevier]

Abstract: Understanding of the effects of the boundary – the channel walls – on the electrophoretic motion of particles in microchannels is very important. This paper developed an analytical solution of the electrophoretic mobility for eccentric motion of a rectangular particle in a rectangular microchannel. The simple geometry of the system does not limit the generality of the qualitative prediction of the model and the analytical solution. Several special cases are studied, and the effects of the degree of the eccentricity, the particle’s size relative to the channel’s size, and the relative zeta potentials on the particle’s mobility are discussed. For the case where the particle’s cross-section area is close to the cross-section area of the microchannel, the model’s predictions are compared with the published experimental results and good agreement was found. [Copyright &y& Elsevier]

Abstract: Mechanism of inter-bubble coalescence by an aqueous fatty alcohol particle suspension antifoam containing a nonionic surfactant has been investigated. By observing visually two colliding air bubbles in a liquid pool in the presence of the antifoam, a four-step mechanism is identified. The role of the surfactant in the antifoam is, for the first time, proposed. A surface tension gradient due to the local surfactant concentration difference enables a surfactant laden hydrophobic particle located on bubble surface to move from the periphery of a liquid film between two colliding air bubbles to their region of contact. Drop volume tensiometry and macroscopic foam column experiments are used to further prove this observation. Subsequently, the particle bridges and dewets the bubbles resulting in film rupture. The rate of drainage of the liquid film depends on the particle hydrophobicity, which necessitates complete surfactant desorption from particle surface. This is corroborated experimentally by Wilhelmy plate tensiometry. In addition, cryo-scanning electron and atomic force microscopy are used to determine the particle shape and the force for its entry into the bubble. [Copyright &y& Elsevier]

Abstract: The distribution and movement of fluids in porous media are important in a variety of situations arising naturally and industrially (e.g., water migration in soils, oil recovery, chromatography, filtration and separation processes). Our specific interest is in deriving advancing and receding contact angles from capillary pressure measurements in packed beds of particles partially saturated with liquids. The simplest model of a porous medium treats the porous body as an equivalent uniform capillary giving rise to the same capillary pressure. Pressure measurements were performed successfully with advancing as well as receding liquids. For an advancing liquid front a measurement with a second liquid is needed to calibrate the equivalent capillary radius and obtain the advancing contact angle. For a receding liquid front – an additional determination of the amount of liquid trapped behind in smaller pores is required. The equivalent capillary radius is mainly determined by the porosity of the packed bed and is easily corrected to account for capillary retention. Only then can the receding contact angle be obtained reliably. This new methodology for contact angle measurement was validated with model systems and applied successfully to various real particulate systems. [Copyright &y& Elsevier]

Abstract: An experimental circulating water loop has been constructed to study the deposition of hematite particles of average diameter 320 nm on polypropylene pipe walls in the ranges pH 4–11, Re 3300–17,700 at 25 °C. Real-time turbidimetric measurements were used to measure the deposited concentrations. Results showed that the deposition rate increased when pH decreased and when the flow rate increased. Adhesion was observed even under repulsive electrostatic conditions (pH>7), where the surfaces of hematite and polypropylene were both negative, indicating that the kinetic energy of at least a part of the particles surpassed the electrostatic repulsive potentials. The experimental curves were fitted by a model assuming simultaneous adhesion and removal of particles, leading to adhesion and removal rate constants, whose values depend on pH and flow rate. Removal is negligible below pH 9. [Copyright &y& Elsevier]

Abstract: We investigate theoretically forces acting on a porous particle in an oscillating viscous incompressible flow. We use the unsteady equations describing the creeping flow, namely the Stokes equations exterior to the particle and the Darcy or Brinkman equations inside it. The effect of particle permeability and oscillation frequency on the flow and forces is expressed via the Brinkman parameter and the frequency parameter , respectively. Here a is particle radius, k is its permeability, ω is the angular frequency, δ is the thickness of Stokes layer (penetration depth) and ν is the fluid kinematic viscosity. It is shown that the oscillations interact with permeable structure of the particle and affect both the Stokes-like viscous drag and the added mass force components. [Copyright &y& Elsevier]

Abstract: Liquid droplets are proposed as transport vehicles for manipulating microobjects. An atomic force microscope (AFM) cantilever is used as a gripper to pick-up a droplet and then a particle from a hydrophobic substrate. The droplet and the particle are then released to a hydrophilic substrate from the cantilever. During the pick-up and release, the liquid bridge that formed between the gripper and the substrate is studied. The efficiency of micromanipulation is quantified by the term “volumetric distribution ratio,” which is the liquid volume retained by the substrate divided by the whole volume of the droplet during the rupture of the liquid bridge. Based on the theoretical analysis, an optimized micromanipulation is suggested which could achieve 100% efficiency by carefully controlling the wetting properties of the gripper and the substrate. [Copyright &y& Elsevier]

Abstract: Lipid-based particles (Cubosome® particles) were surface-modified by chitosan and the ratio between particles and chitosan was optimized to minimize the free chitosan concentration in the dispersion. The modified particles were characterized by electrophoretic measurements and the pH dependence of the zeta potential could be directly related to the protonation of chitosan. Interaction between the modified particles and mucin-coated silica surfaces were subsequently investigated in situ by ellipsometry to assess the mucoadhesive properties at physiologically relevant conditions. The result showed that a substantial amount of modified particles was adsorbed to mucin-coated silica surfaces at both pH 4 and pH 6, probably due to electrostatic interactions between amino groups in chitosan and negatively charged groups in mucin. Furthermore, the amount of bound particles decreased by less than 15% upon rinsing indicating relatively strong interactions. This investigation demonstrates that ellipsometry is a useful tool to study mucoadhesive properties of particles in the submicrometer range. Moreover, the novel chitosan-modified particles may be of interest for mucosal drug delivery applications. [Copyright &y& Elsevier]

Abstract: Monodispersed BaSO4 micrometer and submicrometer sized particles were produced by a simple precipitation reaction in the presence of poly(methacrylic acid) (PMAA) at the ambient temperature. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron diffraction (ED) were used to characterize the obtained BaSO4 particles, and polycrystalline nature of the BaSO4 products was revealed. A “polymer–M in situ template” model was proposed to elucidate the formation process of such polycrystalline BaSO4 particles. [Copyright &y& Elsevier]

Abstract: The main purpose of the present paper is to investigate the effect of the normal Gaussian size distribution on the deposition of Brownian particles onto a spherical collector, by applying the Brownian dynamic simulation method and the Kuwabara flow field model with different types of DLVO interaction energy curves and the shadow effect. The simulation results show that the collection efficiency of Brownian particles always increases with a wider particle size distribution region. The same increased tendencies are also observed for the case of increasing Reynolds number and for the case of increasing the particle size to the collector size ratio. When compared to the available experimental data, the present simulation method fits well with the experimental data when the specific deposit per collector is not large. [Copyright &y& Elsevier]

Abstract: We have observed the condensation of a mixture of γ-glycidoxypropylmethyldiethoxysilane and 3-aminopropyltriethoxysilane in dilute aqueous solutions. NMR and IR spectroscopy have allowed to follow the condensation process in the mixture, which is noticeably enhanced and proceeds faster than for each silane on its own. Cross-condensation between the two silanes was evidenced. When the hydrophilic aminosilane is in excess, the condensation, as evidenced by dynamic light scattering, proceeds toward gel formation because the oligomers formed are essentially hydrophilic. When the more hydrophobic epoxysilane is in excess, oligomer growth proceeds slowly and results in a destabilization of the solution: due to their hydrophobic character, the oligomers formed coalesce suddenly into 200-nm-diameter aggregates. Coatings deposited from such solutions with high epoxysilane content can be used to strengthen glass. We show that the progress of condensation in solution results in a wetting transition during deposition of the silane film on glass by dip coating. The production of increasingly hydrophobic oligomers as the reaction time increases results in adsorption of more hydrophobic aggregates at the surface, which eventually leads to dewetting of the film: in the absence of film, glass strengthening disappears. [Copyright &y& Elsevier]

Abstract: There is a concentration-polarization (CP) force acting on a particle submerged in an electrolyte solution with a concentration (conductivity) gradient under an externally applied DC electric field. This force originates from the two mechanisms: (i) gradient of electrohydrodynamic pressure around the particle developed by the Coulombic force acting on induced free charges by the concentration polarization, and (ii) dielectric force due to nonuniform electric field induced by the conductivity gradient. A perturbation analysis is performed for the electric field, the concentration field, and the hydrodynamic field, under the assumptions of creeping flow and small concentration gradient. The leading order component of this force acting on a dielectric spherical particle is obtained by integrating the Maxwell and the hydrodynamic stress tensors. The analytical results are validated by comparing the surface pressure and the skin friction to those of a numerical analysis. The CP force is proportional to square of the applied electric field, effective for electrically neutral particles, and always directs towards the region of higher ionic concentration. The magnitude of the CP force is compared to that of the electrophoretic and the conventional dielectrophoretic forces. [Copyright &y& Elsevier]

Abstract: A new multi-Monte Carlo (MMC) method is promoted to consider general dynamic equation (GDE) for particle coagulation and condensation/evaporation. MMC method introduces the concept of a “weighted fictitious particle” and is based on time-driven Monte Carlo technique, constant number of fictitious particles technique, and constant volume technique. MMC method for independent coagulation, for independent condensation/evaporation, and for simultaneous coagulation and condensation/evaporation are validated by some special cases in which analytical solutions exist, in which numerical results agree with corresponding analytical solutions well. Furthermore, the computation cost of MMC method is low enough to be applied in engineering computation and general scientific quantitative analysis. [Copyright &y& Elsevier]

In this paper mathematical expressions have been developed to describe the hydrodynamic resistance force on a colloidal particle as it slides along a slip surface of a gas bubble held stationary in a quiescent liquid. The particle size was considered to be sufficiently small relative to the bubble size so that the bubble surface could be locally approximated to a planar interface. The modeling incorporated a bispherical coordinate transformation to solve the equations governing the liquid creeping flow disturbed by the particle. Exact numerical solutions for the resistance coefficients of the particle-shearing motion parallel to the slip bubble surface were obtained as a function of the separation distance from the bubble surface. Finally, simplified analytical rational approximations for the whole range of the separation distance were presented, which were in good agreement with the exact numerical result. Importantly, the approximations for the modeling and simulation of the bubble–particle interactions are mathematically tractable. [Copyright &y& Elsevier]

A CCD high-speed video microscopy system operating at 1000 frames per second was used to obtain direct quantitative measurements of the trajectories of fine glass spheres on the surface of air bubbles. The glass spheres were rendered hydrophobic by a methylation process. Rupture of the intervening water film between a hydrophobic particle and an air bubble with the consequent formation of a three-phase contact was observed. The bubble–particle sliding attachment interaction is not satisfactorily described by the available theories. Surface forces had little effect on the particle sliding with a water film, which ruptured probably due to the submicrometer-sized gas bubbles existing at the hydrophobic particle–water interface. [Copyright &y& Elsevier]

The goal of this work is to determine the size distribution of hollow glass spheres by laser diffraction, an experiment which involves measuring angle-dependent scattering of light from particles dispersed in a liquid. The proprietary software supplied with commercial instruments is not strictly applicable to our two-layer, glass-shell, hollow-core spheres because it requires that the particles have spatially homogeneous properties. We therefore developed Fortran code to compute the scattering from core–shell spherical particles. The results show that the scattering from representative hollow glass particles diverges from homogeneous sphere scattering when the radius decreases from 10 to 3 μm. Additionally, scattering measurements on two core–shell hollow glass powders were analyzed using the exact core–shell optical model and homogeneous sphere approximations. In both cases, the size distribution determined using the exact core–shell model differs from that determined using the homogeneous-sphere approximation when the distribution covers radii smaller than about 10 μm, as expected. The size distribution based on the exact core–shell optical model was determined using a new algorithm. Although the basic equations used in the algorithm have been published previously, they are developed here in a different form, which can be implemented using Fortran and MatLab routines available commercially and in the public domain. This algorithm could be used to determine the size distribution of other kinds of particles, such as cylindrical rods, as long as their angle-dependent scattering could be computed. [Copyright &y& Elsevier]