Interaction of Nanoparticles in Electrolyte Solutions.

The interaction between nanoparticles includes several components; however, the most frequently used are electrostatic, caused by overlapping double electrical layers, and London-van der Waals interactions, caused by quantum and thermodynamic fluctuations of electromagnetic fields. Only these two kinds of interaction are considered below. The electrostatic interaction is calculated based on the linearized Poisson-Boltzmann equation for particles with constant electrical potential of the surfaces (constant ζ potentials).

Foam-Based Electrophoretic Separation of Charged Dyes.

Electrophoretic separation of a fluorescent dye mixture, containing rhodamine B (RB) and fluorescein, in liquid foams stabilized by anionic, cationic, or non-ionic surfactants in water-glycerol mixtures was studied in a custom-designed foam separation device. The effects of the external electric field applied across the foam and the initial pH of the solution on the effectiveness of separation were also studied. The fluid motion due to electroosmosis and the resulting back pressure within the foam and local pH changes were found to be complex and affected the separation.

Stability of Two-Dimensional Liquid Foams under Externally Applied Electric Fields.

Liquid foams are highly complex systems consisting of gas bubbles trapped within a solution of surfactant. Electroosmotic effects may be employed to induce fluid flows within the foam structure and impact its stability. The impact of external electric fields on the stability of a horizontally oriented monolayer of foam (2D foam) composed of anionic, cationic, non-ionic, and zwitterionic surfactants was investigated, probing the effects of changing the gas-liquid and solid-liquid interfaces.

Crude Oil Drop Penetration into Permeates Using a Slotted Pore Membrane.

The primary focus of the presented research is to come up with a model that could be utilized to evaluate the permeate content (concentration) of oil drops using a straight (nonconverging) slotted microstructured membrane. The content (concentration) of crude drops in the permeate with a nonconverging slit structure membrane has not been studied before, and the study presented would be a good contribution to the literature. A comparison between the use of a converging (narrowing toward the inside) and a nonconverging slotted pore microstructured membrane is made for the purpose of removing oil content from the produced water.

Effect of synthetic surfactants on the environment and the potential for substitution by biosurfactants.

The environmental impacts of the use of synthetic surfactants are discussed in this work such as their high levels of toxicity and low biodegradability. These materials destroy aquatic microbial populations, damage fish and other aquatic life, and reduce photochemical energy conversion efficiency of plants as well as adversely affecting waste-water treatment processes. With global usage of surfactants being over 15 million tonnes annually, and an estimated 60% of surfactant ending up in the aquatic environment, there is an urgent need for alternatives with lower adverse environmental effects; this review explores biosurfactants as potential alternatives.

Electrokinetic Transport of a Charged Dye in a Freely Suspended Liquid Film: Experiments and Numerical Simulations.

Electrokinetic transport of a charged dye within a free liquid film stabilized by a cationic surfactant, trimethyl(tetradecyl)ammonium bromide, subjected to an external electric field was investigated. Confocal laser scanning microscopy was used to visualize fluorescein isothiocyanate (FITC) separation within the stabilized liquid film. Numerical simulations were performed using the finite element method to model the dynamics of charged dye separation fronts observed in the experiments.

Foam drainage placed on a thin porous layer.

Drainage of foams placed on porous substrates has only recently been theoretically investigated (O. Arjmandi-Tash, N. Kovalchuk, A.

Equilibrium Droplets on Deformable Substrates: Equilibrium Conditions.

Equilibrium conditions of droplets on deformable substrates are investigated, and it is proven using Jacobi's sufficient condition that the obtained solutions really provide equilibrium profiles of both the droplet and the deformed support. At the equilibrium, the excess free energy of the system should have a minimum value, which means that both necessary and sufficient conditions of the minimum should be fulfilled. Only in this case, the obtained profiles provide the minimum of the excess free energy.

Electroosmotic flow measurements in a freely suspended liquid film: Experimhents and numerical simulations.

Fluid flow profiles in free liquid films stabilised by anionic and cationic surfactants under an external electric field were investigated. Depthwise velocity fields were measured at the mid region of the free liquid film by confocal micron-resolution particle image velocimetry and corresponding numerical simulations were performed using Finite Element Method to model the system. Depthwise change in velocity profiles was observed with electroosmotic flow dominating in the vicinity of the gas-liquid and solid-liquid interfaces while backpressure drives fluid in the opposite direction at the core of the film.

Kinetics of Wetting and Spreading of Droplets over Various Substrates.

There has been a substantial increase in the number of publications in the field of wetting and spreading since 2010. This increase in the rate of publications can be attributed to the broader application of wetting phenomena in new areas. It is impossible to review such a huge number of publications; that is, some topics in the field of wetting and spreading are selected to be discussed below.

Hysteresis of the Contact Angle of a Meniscus Inside a Capillary with Smooth, Homogeneous Solid Walls.

A theory of contact angle hysteresis of a meniscus inside thin capillaries with smooth, homogeneous solid walls is developed in terms of surface forces (disjoining/conjoining pressure isotherm) using a quasi-equilibrium approach. The disjoining/conjoining pressure isotherm includes electrostatic, intermolecular, and structural components. The values of the static receding θr, advancing θa, and equilibrium θe contact angles in thin capillaries were calculated on the basis of the shape of the disjoining/conjoining pressure isotherm.

Spreading of a Lidocaine Formulation on Microneedle-Treated Skin.

The spreadability of a liquid drug formulation on skin is an indication of it either remaining stationary or distributing (spreading) as a droplet. Factors determining droplet spreadability of the formulation are spreading area, diameter of the droplet base, viscosity of the liquid, contact angle, volume of droplet on skin and any others. The creation of microcavities from the application of microneedle (MN) has the potential to control droplet spreading, and hence, target specific areas of skin for drug delivery.

Spreading of blood drops over dry porous substrate: complete wetting case.

Hypothesis: The process of dried blood spot sampling involves simultaneous spreading and penetration of blood into a porous filter paper with subsequent evaporation and drying. Spreading of small drops of blood, which is a non-Newtonian liquid, over a dry porous layer is investigated from both theoretical and experimental points of view.

Experiments And Theory: A system of two differential equations is derived, which describes the time evolution of radii of both the drop base and the wetted region inside the porous medium.

Current applications of foams formed from mixed surfactant-polymer solutions.

Foams cannot be generated without the use of special foaming agents, as pure liquids do not foam. The most common foaming agents are surfactants, however often for foam stability one active agent is not enough, it is necessary to add other component to increase foam lifetime. Foams on everyday use are mostly made from mixture of different components.

Modeling the effect of surface forces on the equilibrium liquid profile of a capillary meniscus.

The equilibrium profile of a capillary meniscus formed under combined action of disjoining/conjoining and capillarity pressures is investigated. Attention is focused on the shape of a transition zone between a spherical meniscus and a thin liquid film in front of the meniscus. The Poisson-Boltzmann equation is used for calculations of electrostatic contribution to the disjoining/conjoining pressure and the liquid shape inside the transition zone.

Particle laden fluid interfaces: dynamics and interfacial rheology.

We review the dynamics of particle laden interfaces, both particle monolayers and particle+surfactant monolayers. We also discuss the use of the Brownian motion of microparticles trapped at fluid interfaces for measuring the shear rheology of surfactant and polymer monolayers. We describe the basic concepts of interfacial rheology and the different experimental methods for measuring both dilational and shear surface complex moduli over a broad range of frequencies, with emphasis in the micro-rheology methods.

Simultaneous spreading and evaporation: recent developments.

The recent progress in theoretical and experimental studies of simultaneous spreading and evaporation of liquid droplets on solid substrates is discussed for pure liquids including nanodroplets, nanosuspensions of inorganic particles (nanofluids) and surfactant solutions. Evaporation of both complete wetting and partial wetting liquids into a nonsaturated vapour atmosphere are considered. However, the main attention is paid to the case of partial wetting when the hysteresis of static contact angle takes place.

Evaporation of droplets of surfactant solutions.

The simultaneous spreading and evaporation of droplets of aqueous trisiloxane (superspreader) solutions onto a hydrophobic substrate has been studied both experimentally, using a video-microscopy technique, and theoretically. The experiments have been carried out over a wide range of surfactant concentration, temperature, and relative humidity. Similar to pure liquids, four different stages have been observed: the initial one corresponds to spreading until the contact angle, θ, reaches the value of the static advancing contact angle, θad.

Evaporation kinetics of sessile droplets of aqueous suspensions of inorganic nanoparticles.

Evaporation kinetics of sessile droplets of aqueous suspension of inorganic nanoparticles on solid substrates of various wettabilities is investigated from both experimental and theoretical points of view. Experimental results on evaporation of various kinds of inorganic nanosuspensions on solid surfaces of different hydrophobicities/hydrophilicities are compared with our theoretical predictions of diffusion limited evaporation of sessile droplets in the presence of contact angle hysteresis. The theory describes two main stages of evaporation process: (I) evaporation with a constant radius of the droplet base when the contact angle decreases from static advancing contact angle down to static receding contact angle and (II) evaporation with constant contact angle equal to the static receding contact angle when the radius of the droplet base decreases.

Computer simulations of evaporation of pinned sessile droplets: influence of kinetic effects.

The aim of the current work is to present results of computer simulations, which show the influence of kinetic effects on evaporation of pinned sessile water droplets of submicrometer size placed on a heat conductive substrate. The computer simulation model also takes into account the following phenomena: influence of curvature of the droplet's surface on saturated vapor pressure above the surface (Kelvin's equation), the effect of latent heat of vaporization, thermal Marangoni convection, and Stefan flow inside an air domain above the droplet. The suggested model combines both diffusive and kinetic models of evaporation.

Thickness, stability and contact angle of liquid films on and inside nanofibres, nanotubes and nanochannels.

While the stability of liquid films on substrates is a classical topic of colloidal science, the availability of nanostructured materials, such as nanotubes, nanofibres and nanochannels, has raised the question of how the stability of liquid films and their wetting behaviour is affected by nanoscale confinement. This paper will present the conditions for the stability of liquid films on and inside cylindrical solid substrates with nanometre scale characteristic dimensions. It is shown that the stability is determined by an effective disjoining/conjoining pressure isotherm which differs from the corresponding disjoining/conjoining pressure isotherm of flat liquid films on flat solid substrates.