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. Fluorescein dye molecules, which have a positive or negative charge depending on the solution pH, aggregated in the vicinity of an electrode, leaving a pure band of neutral dye RB. The effectiveness of the separation was quantified by the percentage width of the pure RB band, which was found to be between 29 and 42%. This study demonstrates the potential of liquid foam as a platform for electrophoretic 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. Time-lapse recordings were analyzed to investigate the evolution of foam over time subject to varying electric field strengths. Numerical simulations of electroosmotic flow of the same system were performed using the finite element method. Foam stability was affected by the presence of an external electric field in all cases and depended on the surfactant type, strength of the electric field, and the solid material used to construct the foam cell. For the myristyltrimethylammonium bromide (MTAB) foam in a glass cell, the time to collapse 50% of the foam was increased from ∼25 min under no electric field to ∼85 min under an electric field strength of 2000 V/m. In comparison, all other surfactants trialed exhibited faster foam collapse under external electric fields. Numerical simulations provided insight as to how different zeta potentials at the gas-liquid and solid-liquid interfaces affect fluid flow in different elements of the foam structure under external electric fields, leading to a more stable or unstable foam.

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. The sources and natural function of biosurfactants are presented, together with their advantages compared with their synthetic counterparts, including their low toxicity and biodegradability. Their comparable effectiveness as surfactants has been demonstrated by surface tension reduction, achieved at much lower critical micelle concentrations that those of synthetic surfactants. The limitations and challenges for the use of biosurfactants are discussed, particularly low production yields; such limitations must be addressed before wide range industrial use of biosurfactants can be achieved. Although there has been focus on achieving greater production yields, a remaining issue is the lack of research into the use of biosurfactants in a greater range of industrial and consumer applications to demonstrate their efficacy and identify candidate biosurfactants for production. This review highlights such research as deserving of further investigation, alongside the ongoing work to optimize the production process.

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. Because of the electrochemical reactions at the electrodes, significant spatial and temporal pH changes were observed within the liquid film. These local pH changes could affect the local zeta potential at the gas-liquid and solid-liquid film boundaries; hence, the flow field was found to be highly dynamic and complex. The charged dye (FITC) used in the experiments is pH-sensitive, and therefore, electrophoresis of the dye also depended on the local pH. The pH and the electroosmotic flow field predicted from the numerical simulations were useful for understanding charged dye separation near both the anode and the cathode.

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. Trybala, V. Starov, Foam Drainage Placed on a Porous Substrate, Soft Matter, 2015, 11(18), 3643-3652), where an equation describing foam drainage (with non-slip boundary conditions on the liquid-air interfaces) was combined with that of imbibition of liquid into the thick porous substrate. Foam-based applications have been used as a method of drug delivery, which is a recent and promising area of research related to application of medicinal products onto the skin or hair, which are both thin porous layers. A theory of foam drainage (taking into account surface viscosity) placed on a completely wettable thin porous layer is developed: the rate of foam drainage and imbibition inside the porous layer and other characteristics of the process are predicted. The "effective slip" caused by the surface viscosity increased a movement of the top boundary of the foam. The theoretical predictions are compared with experimental observations of foam drainage placed on thin porous layers. The comparison showed a reasonable agreement between the theoretical predictions and experimental observations. One of the phenomena during foam application is the possibility of a build-up of a free liquid layer on the foam/porous layer interface, which can be very useful for applications. Three different regimes of spreading/imbibition process have been predicted. Conditions and durations of free liquid layer formation have been theoretically predicted and compared with experimental observations.

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. It was also found that the direction of the flow at various sections of the films depends on the type of surfactant used, but flow features remained the same. Numerical simulations predicted the flow profiles with reasonable accuracy; however, asymmetry of the actual film geometry caused deviations at the top half of the computational domain. Overall, electroosmotic flow profiles within a free liquid film are similar to that of the closed-end solid microchannel. However, the flow direction and features of the velocity profiles can be changed by selecting various types of surfactants. The free liquid films thickness was selected to match dimensions of foam Plateau border. Hence, these findings will be useful in developing a separation system based on foam electrokinetics.

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. These topics are as follows: (i) Contact angle hysteresis on smooth homogeneous solid surfaces via disjoining/conjoining pressure. It is shown that the hysteresis contact angles can be calculated via disjoining/conjoining pressure. The theory indicates that the equilibrium contact angle is closer to a static receding contact angle than to a static advancing contact angle. (ii) The wetting of deformable substrates, which is caused by surface forces action in the vicinity of the apparent three-phase contact line, leading to a deformation on the substrate. (iii) The kinetics of wetting and spreading of non-Newtonian liquid (blood) over porous substrates. We showed that in spite of the enormous complexity of blood, the spreading over porous substrate can be described using a relatively simple model: a power low-shear-thinning non-Newtonian liquid. (iv) The kinetics of spreading of surfactant solutions. In this part, new results related to various surfactant solution mixtures (synergy and crystallization) are discussed, which shows some possible direction for the future revealing of superspreading phenomena. (v) The kinetics of spreading of surfactant solutions over hair. Fundamental problems to be solved are identified.

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. Properly chosen combinations of two active ingredients lead to a faster foam formation and increased foam stability. During the last decade polymers (mainly polyelectrolytes and proteins) have become frequently used additives to foaming solutions. Mixtures of surfactants and polymers often demonstrate different foaming properties in comparison to surfactant only or polymer only solutions. The nature of surfactant-polymer interactions is complicated and prediction of resulting foaming properties of such formulations is not straightforward. Properties and foaming of surfactant-polymer mixtures are discussed as well as current applications of foams and foaming agents as foams are widely used in cosmetics, pharmaceutics, medicine and the food industry.

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. In the case of complete wetting the spreading/evaporation process proceeds in two stages. A theory was suggested for this case and a good agreement with available experimental data was achieved. In the case of partial wetting the spreading/evaporation of a sessile droplet of pure liquid goes through four subsequent stages: (i) the initial stage, spreading, is relatively short (1-2 min) and therefore evaporation can be neglected during this stage; during the initial stage the contact angle reaches the value of advancing contact angle and the radius of the droplet base reaches its maximum value, (ii) the first stage of evaporation is characterised by the constant value of the radius of the droplet base; the value of the contact angle during the first stage decreases from static advancing to static receding contact angle; (iii) during the second stage of evaporation the contact angle remains constant and equal to its receding value, while the radius of the droplet base decreases; and (iv) at the third stage of evaporation both the contact angle and the radius of the droplet base decrease until the drop completely disappears. It has been shown theoretically and confirmed experimentally that during the first and second stages of evaporation the volume of droplet to power 2/3 decreases linearly with time. The universal dependence of the contact angle during the first stage and of the radius of the droplet base during the second stage on the reduced time has been derived theoretically and confirmed experimentally. The theory developed for pure liquids is applicable also to nanofluids, where a good agreement with the available experimental data has been found. However, in the case of evaporation of surfactant solutions the process deviates from the theoretical predictions for pure liquids at concentration below critical wetting concentration and is in agreement with the theoretical predictions at concentrations above it.

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. Duration of this stage is rather short, and the evaporation during this stage can be neglected. The evaporation is essential during the next three stages. The next stage after the spreading, which is referred to herein as the first stage, takes place at constant perimeter and ends when θ reaches the static receding contact angle, θr. During the next, second stage, the perimeter decreases at constant contact angle θ = θr for surfactant concentration above the critical wetting concentration (CWC). The static receding contact angle decreases during the second stage for concentrations below CWC because the concentration increases due to the evaporation. During the final stage both the perimeter and the contact angle decrease. In what follows, we consider only the longest stages I and II. The developed theory predicts universal curves for the contact angle dependency on time during the first stage, and for the droplet perimeter on time during the second stage. A very good agreement between theory and experimental data has been found for the first stage of evaporation, and for the second stage for concentrations above CWC; however, some deviations were found for concentrations below CWC.

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. Theoretically predicted universal dependences for both evaporation stages are compared with experimental data, and a very good agreement is found.

Deposition of core latex particles encapsulated in polyelectrolyte shells at modified mica surfaces.

We used an oblique impinging jet (OIJ) cell to determine the initial deposition rates of model microcapsules at bare and modified by multilayer polyelectrolyte (PE) film mica surfaces. The transport conditions in the cell were quantitatively established by studying the kinetics of deposition of negatively charged latex at mica surfaces converted to positively charged by adsorption of (3-aminoprolyl)triethoxysilane. The dependence of reduced particle flux on the Reynolds number of the flow in the OIJ cell was determined by a direct counting of particles deposited on the mica surface. The results are described in terms of convective-diffusion theory taking into account hydrodynamic, dispersive, and electrostatic interactions, between the charged particles/capsules and the mica plate. In this way, transport conditions in the cell were characterized and they were used to interpret the results concerning the deposition of microcapsules with PE shells of various thickness obtained by layer-by-layer polyelectrolyte adsorption on colloidal cores. We demonstrated that the initial deposition rate of capsules is governed by the charge of the solid/liquid interface and the outermost layer of the capsule shell, and is largely independent of the thickness of the capsule shell or the number of PE layers at the mica surface. The deposition rates were in good agreement with theoretical predictions derived from the convective-diffusion theory.