Foam drainage placed on a porous substrate

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.

Natural surfactant system: precursor flow in one-shot two-dimensional foam drainage

Foam drainage, the downward flow of the intervening liquid due to gravity, surface tension and viscosity, is an important factor for foam stability. Forced drainage (adding liquid to a foam column from the top) is used to study liquid flow characteristics in foam. The different competing phenomena involved in the vertical and horizontal movement is studied by visually monitoring the draining liquid using a dye. Hence, this study is called two-dimensional (2D) drainage, horizontal movement being considered as just one dimension. We report, for the first time, 2D forced drainage in a natural surfactant extracted from Sapindus mukorossi. Draining wave front flows vertically and spreads horizontally. Vertical front position proceeds with time in a power law whose exponent indicates a Poiseuille flow, like synthetic surfactants. The wave front begins as a conic form downstream producing an expanding ellipsoid with time. The liquid fraction, taken as light intensity at any point, follows a Gaussian distribution along the horizontal. There is an asymmetric Gaussian distribution along the vertical which distorts after 10 s, indicating two types of flow occurring simultaneously. Two such liquid movements through foam have not been reported in the literature. The reason behind these two types of flow needs further investigations.

Foam Quality of Foams Formed on Capillaries and Porous Media Systems

Foams are of great importance as a result of their expansive presence in everyday life—they are used in the food, cosmetic, and process industries, and in detergency, oil recovery, and firefighting. There is a little understanding of foam formation using soft porous media in terms of the quality of foam and foam formation. Interaction of foams with porous media has recently been investigated in a study by Arjmandi-Tash et al., where three different regimes of foam drainage in contact with porous media were observed. In this study, the amount of foam generated using porous media with surfactant solutions is investigated. The aim is to understand the quality of foam produced using porous media. The effect of capillary sizes and arrangement of porous in porous media has on the quality of foam is investigated. This is then followed by the use of soft porous media for foam formation to understand how the foam is generated on the surface of the porous media and the effect that different conditions (such as concentration) have on the quality of the foam. The quality of foam is a blanket term for bubble size, liquid volume fraction, and stability of the foam. The liquid volume fraction is calculated using a homemade dynamic foam analyser, which is used to obtain the distribution of liquid volume fraction along with the foam height. Soft porous media does not influence substantially the rate of decay of foam produced, however, it decreases the average diameter of the bubbles, whilst increasing the range of bubble sizes due to the wide range of pore sizes present in the soft porous media. The foam analyser showed the expected behaviour that, as the foam decays and becomes drier, the liquid volume fraction of the foam falls, and therefore the conductivity of foam also decreases, indicating the usefulness of the home-made device for future investigations.

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 Formation and Interaction with Porous Media

Foams are a common occurrence in many industries and many of these applications require the foam to interact with porous materials. For the first time interaction of foams with porous media has been investigated both experimentally and theoretically by O. Arjmandi-Tash et al. It was found that there are three different regimes of the drainage process for foams in contact with porous media: rapid, intermediate and slow imbibition. Foam formation using soft porous media has only been investigated recently, the foam was made using a compression device with soft porous media containing surfactant solution. During the investigation, it was found that the maximum amount of foam is produced when the concentration of the foaming agent (dishwashing surfactant) is in the range of 60–80% m/m. The amount of foam produced was independent of the pore size of the media in the investigated range of pore sizes. This study is expanded using sodium dodecyl sulphate (SDS), which has the same critical micelle concentration as the commercial dishwashing surfactant, where the foam is formed using the same porous media and compression device. During the investigation, it was found that 10 times the critical micelle concentration (CMC) is the optimum concentration for a pure SDS surfactant solution to create foam. Any further increase in concentration after that point resulted in no further mass of foam being generated.

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.

Kinetics of Spreading over Porous Substrates

The spreading of small liquid drops over thin and thick porous layers (dry or saturated with the same liquid) is discussed in the case of both complete wetting (silicone oils of different viscosities over nitrocellulose membranes and blood over a filter paper) and partial wetting (aqueous SDS (Sodium dodecyl sulfate) solutions of different concentrations and blood over partially wetted substrates). Filter paper and nitrocellulose membranes of different porosity and different average pore size were used as a model of thin porous layers, sponges, glass and metal filters were used as a model of thick porous substrates. Spreading of both Newtonian and non-Newtonian liquid are considered below. In the case of complete wetting, two spreading regimes were found (i) the fast spreading regime, when imbibition is not important and (ii) the second slow regime when imbibition dominates. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations was derived in the case of complete wetting for both Newtonian and non-Newtonian liquids to describe the evolution with time of radii of both the drop base and the wetted region inside the porous layer. The deduced system of differential equations does not include any fitting parameter. Experiments were carried out by the spreading of silicone oil drops over various dry microfiltration membranes (permeable in both normal and tangential directions) and blood over dry filter paper. The time evolution of the radii of both the drop base and the wetted region inside the porous layer were monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactorily for the experimental data. According to the theory prediction, (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function and (ii) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in the system under investigation in the case of complete wetting. These conclusions again are in good agreement with experimental observations in the case of complete wetting for both Newtonian and non-Newtonian liquids. Addition of surfactant to aqueous solutions, as expected, improve spreading over porous substrates and, in some cases, results in switching from partial to complete wetting. It was shown that for the spreading of surfactant solutions on thick porous substrates there is a minimum contact angle after which the droplet rapidly absorbs into the substrate. Unfortunately, a theory of spreading/imbibition over thick porous substrates is still to be developed. However, it was shown that the dimensionless time dependences of both contact angle and spreading radius of the droplet on thick porous material fall on to a universal curve in the case of complete wetting.

Biological applications of kinetics of wetting and spreading

Wetting and spreading kinetics of biological fluids has gained a substantial interest recently. The importance of these fluids in our lives has driven the pace of publications. Globally scientists have ever growing interest in understanding wetting phenomena due to its vast applications in biological fluids. It is impractical to review extremely large number of publications in the field of kinetics of complex biological fluids and cosmetic solutions on diverse surfaces. Therefore, biological and cosmetic applications of wetting and spreading dynamics are considered in the following areas: (i) Spreading of Newtonian liquids in the case of non-porous and porous substrates. It is shown that the spreading kinetics of a Newtonian droplet on non-porous and porous substrate can be defined through theoretical relations for droplet base radius on time, which agree well with the experimental results; (ii) Spreading of blood over porous substrates. It is shown that blood, which has a complex non-Newtonian rheology, can be successfully modelled with the help of simple power-law model for shear-thinning non-Newtonian liquids; (iii) Simultaneous spreading and evaporation kinetics of blood. This part enlightens different underlying mechanisms present in the wetting, spreading, evaporation and dried pattern formation of the blood droplets on solid substrates; (iv) Spreading over hair. In this part the wetting of hair tresses by aqueous solutions of two widely used by industry commercially available polymers, AculynTM 22 and AculynTM 33, are discussed. The influence of non-Newtonian rheology of these polymer solutions on the drainage of foams produced from these solutions is also briefly discussed.

Structure of Polystyrenesulfonate/Surfactant Mixtures at Air-Water Interfaces and Their Role as Building Blocks for Macroscopic Foam

Air/water interfaces were modified by oppositely charged poly(sodium 4-styrenesulfonate) (NaPSS) and hexadecyltrimethylammonium bromide (CTAB) polyelectrolyte/surfactant mixtures and were studied on a molecular level with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry. In order to deduce structure property relations, our results on the interfacial molecular structure and lateral interactions of PSS-/CTA+ complexes were compared to the stability and structure of macroscopic foam as well as to bulk properties. For that, the CTAB concentration was fixed to 0.1 mM, while the NaPSS concentration was varied. At NaPSS monomer concentrations <0.1 mM, PSS-/CTA+ complexes start to replace free CTA+ surfactants at the interface and thus reduce the interfacial electric field in the process. This causes the O-H bands from interfacial H2O molecules in our SFG spectra to decrease substantially, which reach a local minimum in intensity close to equimolar concentrations. Once electrostatic repulsion is fully screened at the interface, hydrophobic PSS-/CTA+ complexes dominate and tend to aggregate at the interface and in the bulk solution. As a consequence, adsorbate layers with the highest film thickness, surface pressure, and dilatational elasticity are formed. These surface layers provide much higher stabilities and foamabilities of polyhedral macroscopic foams. Mixtures around this concentration show precipitation after a few days, while their surfaces to air are in a local equilibrium state. Concentrations >0.1 mM result in a significant decrease in surface pressure and a complete loss in foamability. However, SFG and surface dilatational rheology provide strong evidence for the existence of PSS-/CTA+ complexes at the interface. At polyelectrolyte concentrations >10 mM, air-water interfaces are dominated by an excess of free PSS- polyelectrolytes and small amounts of PSS-/CTA+ complexes which, however, provide higher foam stabilities compared to CTAB free foams. The foam structure undergoes a transition from wet to polyhedral foams during the collapse.

Enhancement of foamability and foam stability induced by interactions between a hyperbranched exopolysaccharide and a zwitterionic surfactant dodecyl sulfobetaine

Weak hydrogen bonding and electrostatic interactions between a zwitterionic surfactant dodecyl sulfobetaine (DSB) and a hyperbranched exopolysaccharide (EPS) enhanced considerably the stability and foamability of EPS/DSB foam.