Darsh T. Wasan

Interfacial transport processes and rheology

Interfacial rheology and its applications basic properties of interfacial transport processes interfacial transport of momentum interfacial transport of species measurement of dynamic interfacial tension and dilatational elasticity measurement of interfacial shear viscosity measurement of the interfacial pilatational viscosity measurement of non-Newtonian interfacial theological behaviour interfacial stability foam rheology a surface-excess theory of interfacial transport processes surface-excess transport of momentum surface-excess transport of species a line-excess theory of equilibrium line tension.

Spreading of nanofluids on solids

Suspensions of nanometre-sized particles (nanofluids) are used in a variety of technological contexts. For example, their spreading and adhesion behaviour on solid surfaces can yield materials with desirable structural and optical properties. Similarly, the spreading behaviour of nanofluids containing surfactant micelles has implications for soil remediation, oily soil removal, lubrication and enhanced oil recovery. But the well-established concepts of spreading and adhesion of simple liquids do not apply to nanofluids. Theoretical investigations have suggested that a solid-like ordering of suspended spheres will occur in the confined three-phase contact region at the edge of the spreading fluid, becoming more disordered and fluid-like towards the bulk phase. Calculations have also suggested that the pressure arising from such colloidal ordering in the confined region will enhance the spreading behaviour of nanofluids. Here we use video microscopy to demonstrate both the two-dimensional crystal-like ordering of charged nanometre-sized polystyrene spheres in water, and the enhanced spreading dynamics of a micellar fluid, at the three-phase contact region. Our findings suggest a new mechanism for oily soil removal—detergency.

Ordered Micelle Structuring in Thin Films Formed from Anionic Surfactant Solutions I. Experimental

Reflected light micro-interferometry was used to observe stratification (i.e., the kinetics of layered structuring) in thinning foam films formed from micellar solutions of sodium n-alkyl sulfates at concentrations much lower than those at which liquid crystalline structures form in bulk solutions. The effects of surfactant concentration and chain length, added electrolyte, and capillary pressure on the step-wise jump transition thicknesses have been investigated. We have further conducted a model study with films formed from aqueous dispersions of latex particles. In similar fashion to those formed from micellar solutions, the thinning films changed thickness with regular step-wise jump transitions, and the films exhibited a number of metastable equilibria. These observations verify that the stratification of thin liquid films can be explained as a layer-by-layer thinning of ordered structures of micelles or colloidal particles formed inside the film. The stratification-nonstratification phase diagram is presented for anionic micelles and is similar to the order-disorder phase diagram for latexes. In the accompanying paper (Part II) we show that the phenomenon of stratification and ordered micellar structures is governed by the long range electrostatic repulsion beteen the ionic micelles and the restricted volume of the film. This present study provides evidence, for the first time, for the presence of structural forces in thin films with thicknesses on the order of 100 nm and strongly suggests that the free liquid film dynamics may be used as a probe to study the ordering in, and stability of, microheterogeneous systems.

A possible mechanism of stabilization of emulsions by solid particles

Abstract Possible explanations of some experimental findings with emulsions stabilized by small adsorbed solid particles are proposed. Films consisting of a particle monolayer are considered and the stability of the liquid menisci between the particles is investigated theoretically. The effect of contact angle hysteresis on the effective disjoining pressure isotherms is also taken into account.

Superspreading driven by Marangoni flow

The spontaneous spreading (called superspreading) of aqueous trisiloxane ethoxylate surfactant solutions on hydrophobic solid surfaces is a fascinating phenomenon with several practical applications. For example, the ability of trisiloxane ethoxylate surfactants to enhance the spreading of spray solutions on waxy weed leaf surfaces, such as velvetleaf (Abutilion theophrasti), makes them excellent wetting agents for herbicide applications. The superspreading ability of silicone surfactants has been known for decades, but its mechanism is still not well understood. In this paper, we suggest that the spreading of trisiloxane ethoxylates is controlled by a surface tension gradient, which forms when a drop of surfactant solution is placed on a solid surface. The proposed model suggests that, as the spreading front stretches, the surface tension increases (the surfactant concentration becomes lower) at the front relative to the top of the droplet, thereby establishing a dynamic surface tension gradient. The driving force for spreading is due to the Marangoni effect, and our experiments showed that the higher the gradient, the faster the spreading. A simple model describing the phenomenon of superspreading is presented. We also suggest that the superspreading behavior of trisiloxane ethoxylates is a consequence of the molecular configuration at the air/water surface (i.e. small and compact hydrophobic part), as shown by molecular dynamics modeling. We also found that the aggregates and vesicles formed in trisiloxane solutions do not initiate the spreading process and therefore these structures are not a requirement for the superspreading process.

Equilibrium and dynamics of adsorption of surfactants at fluid-fluid interfaces

Abstract Surfactant adsorption at fluid-fluid interfaces is modeled with respect to both equilibrium and dynamic behavior. A thermodynamic treatment is developed wherein the surface concentration of ionic surfactants is distinguished from their surface excess concentration by the contribution from the electrical double layer. Only the surface concentrations are assigned to the dividing surface and the electrical double layer is considered to be a part of the aqueous phase. A general adsorption isotherm is derived by including non-idealities in the bulk as well as at the dividing surface. The corresponding equation of state is derived using the Gibbs adsorption isotherm. Several well-known adsorption isotherms and equations of state arise as limiting cases of the general equations of this model. The model is shown to correlate with several published surface and interfacial tension measurements on ionic surfactant systems with or without indifferent electrolytes. Ideal behavior is observed at the interfaces of all liquid-liquid systems investigated. Surface non-idealities are observed only in gas-liquid systems. It is concluded that the non-idealities do not arise from the electrical interactions but rather from the attractive interactions between the hydrophobic tails of the adsorbed surfactant. This approach is superior to the approaches based on a Gibbsian dividing surface since it is readily applicable to the treatment of electrostatic and electrokinetic phenomena as demonstrated by a remarkable correlation achieved between the calculated values of interfacial potentials and observed electrophoretic mobilities. The model is extended using phenomenological considerations to the case of systems not at equilibrium. A continuum treatment of adsorption dynamics is formulated wherein surfactant transport in the bulk phases is described using transport equations with constant coefficients. Activation energy barriers to solute exchange between the bulk and the dividing surface are represented by kinetics of a reversible reaction. The kinetic expression corresponding to the general adsorption isotherm is derived from phenomenological considerations.

The kinetics of adsorption of surface active agents at gas-liquid surfaces

Abstract A general model of adsorption of surfactants at a gas-liquid surface has been developed, which accounts for both the diffusion in the bulk and the barrier to adsorption. In this model, the activation energy barrier to adsorption is accounted for by means of a kinetic expression. A numerical scheme for the solution of the resulting equation of the diffusion-kinetic model is illustrated for the case of the Frumkin isotherm. The model is used to estimate the extent of the adsorption barrier from the dynamic surface tension data of two aqueous surfactant systems reported in the literature. A simplification of the diffusion-kinetic model is proposed which accounts for diffusion in the bulk using diffusion-penetration theory. The simplified model compares very well with the exact model, especially for high barrier resistances.

Effect of oil on foam stability: Aqueous foams stabilized by emulsions

Abstract The interactions between oil drops and the foam lamella were investigated by following the aging phenomena (i.e., drainage, bubble disproportionation, and rupture) of foams containing emulsified oils. It was found that the stability of the film (“pseudoemulsion film”) between an oil drop and an air/water surface was an important factor for the stability of foams containing emulsified oils. The mechanism of foam stability in the presence of stable emulsions was investigated for the case where the pseudoemulsion film was stable. It was found that these emulsions accumulated within the Plateau borders of the draining foam, thus inhibiting foam drainage. As a result, the foams were found to be stabilized by emulsified oil. Important parameters which affect foam stability, such as oil volume fraction, drop size, and oil phase density, were identified and the effect of these parameters on foam stability was experimentally verified. It was found that the stability increased with the oil fraction in the foam. The foam stability was found to go through a minimum as a function of drop size, due to competing effects of emulsion viscosity at smaller drop sizes and the higher accumulation of oil at larger drop sizes. The density of the oil phase directly influenced the oil accumulation and thus the foam stability.

Ordered micelle structuring in thin films formed from anionic surfactant solutions: II. Model development

Abstract We observed the phenomenon of the stratification of thinning liquid films with both micellar solutions of anionic surfactants and solutions containing latex particles. To explain this phenomenon, we suggest that the stratification is a layer-by-layer decrease of the thickness of an ordered micellar (or latex) structure inside the film. To interpret the available experimental data for stratifying films from micellar solutions of sodium dodecyl sulfate (NaDS), a simple cell model is suggested. It permits calculation of the disjoining pressure contribution which is due to the presence of micellar structure inside the film. The micelles interact via screened electrostatic repulsion forming an ordered structure due to the restricted volume of the film. The calculated excess energy per unit area of the film exhibits a number of minima corresponding to the metastable states with micellar layers inside the film. The values of the film thickness at the metastable states were predicted by the model and agreed with the experiment.

Characterization of oil—water interfaces containing finely divided solids with applications to the coalescence of water-in-oil Emulsions: A review

Abstract In this paper we have reviewed the recent developments and highlighted the status of research in the area of three-phase systems with applications to solids-stabilized emulsions. The various factors affecting the formation and stability of these emulsions such as contact angle, demulsifier concentration, temperature, interfacial rheology, and interfacial structure are discussed. The phenomenon of oil loss due to entrainment in emulsion sludge layers is also described and a semi-empirical approach is suggested for estimating oil loss.