Advances in Colloid and Interface Science (v.161, #1-2)

Foreword by Victor M. Starov; Reinhard Miller; Stefano Guido (1).

Plasma nanostructuring of porous polymer membranes by Marek Bryjak; Irena Gancarz; Katarzyna Smolinska (2-9).
Several methods for membrane modification have been presented. Chemical modification of a neat polymer followed by membrane formation and modification of just formed membranes have been compared to plasma action. The following plasma modes are discussed in detail: treatment with non-polymerizable gases, treatment with vapors and plasma initiated grafting. Some examples of modified membrane properties are given. Finally, it was concluded that plasma treatment offers the fastest, environment friendly and versatile method that allows tailoring brand new membranes.
Keywords: Membrane modification; Molecular architecture; Plasma treatment;

On a tweezer for droplets by John W.M. Bush; François Peaudecerf; Manu Prakash; David Quéré (10-14).
We describe the physics behind a peculiar feeding mechanism of a certain class of shorebirds, in which they transport their prey in droplets from their beak tips mouthwards. The subtle interplay between the drop and the beak's tweezering motion allows the birds to defy gravity through driving the drop upwards. This mechanism provides a novel example of dynamic boundary-driven drop motion, and suggests how to design tweezers for drops, able to trap and to move small amounts of liquid.
Keywords: Contact angle hysteresis; Self-propulsion; Capillary ratchet;

Fluctuations in Rayleigh breakup induced by particulates by A. Clarke; S. Rieubland (15-21).
A jet of liquid is intrinsically unstable to radial perturbations and will spontaneously break to form a series of droplets. This well known instability, the Rayleigh–Plateau instability, is controlled and used commercially in continuous inkjet printing. In this application it is important that fluctuations in drop velocity are minimised. However, the addition of particulates to the liquid is observed to strongly increase these fluctuations. The particulates are usually in the form of pigment particles of size O(100 nm) and at a concentration where they may hydrodynamically interact, particularly in the strong shear field within the nozzle (O(107  s− 1)). The boundary layer thickness within the nozzle is O(1 μm) and therefore the particulate size is a significant fraction. We therefore expect that the particles are capable of perturbing the boundary layer and hence the jet. Measurement of jet breakup fluctuation leads to a description of particulates interacting within and with the shear field associated with the boundary layer at the nozzle wall.
Keywords: Jet; Rayleigh instability; Particles in shear; Fluctuations;

Wetting studies regarding amphiphilic molecules and adsorption properties on highly water repellent solid surfaces play key roles in research and technology, with increasing interest both in fundamental and application fields. Nevertheless the wetting properties of aqueous surfactant solutions, non aqueous liquids or immiscible phases on superhydrophobic (SH) solid surfaces have been so far rarely investigated.In this work the authors give an overview on this topic reviewing the literature available together with preliminary results concerning the influence of the distribution properties of surfactants between two immiscible phases. Transition between wetting states can be also considered a possible development of these studies based on switching mechanisms.
Keywords: Wetting; Surfactants; Superhydrophobic; Coating; Partitioning;

Adsorption equilibrium is the state in which the chemical potential of each species in the interface and bulk is the same. Dynamic phenomena at fluid–fluid interfaces in the presence of surface active species are often probed by perturbing an interface or adjoining bulk phase from the equilibrium state. Many methods designed for studying kinetics at fluid–fluid interfaces focus on removing the system from equilibrium through dilation or compression of the interface. This modifies the surface excess concentration Γ i and allows the species distribution in the bulk C i to respond. There are only a few methods available for studying fluid–fluid interfaces which seek to control C i and allow the interface to respond with changes to Γ i . Subphase exchange in pendant drops can be achieved by the injection and withdrawal of liquid into a drop at constant volumetric flow rate R E during which the interfacial area and drop volume V D are controlled to be approximately constant. This can be accomplished by forming a pendant drop at the tip of two coaxial capillary tubes. Although evolution of the subphase concentration C i (t) is dictated by extrinsic factors such as R E and V D , complete subphase exchange can always be attained when a sufficient amount of liquid is used. This provides a means to tailor driving forces for adsorption and desorption in fluid–fluid systems and in some cases, fabricate interfacial materials of well-defined composition templated at these interfaces. The coaxial capillary pendant drop (CCPD) method opens a wide variety of experimental possibilities. Experiments and theoretical frameworks are reviewed for the study of surfactant exchange kinetics, macromolecular adsorption equilibrium and dynamics, as well as the fabrication of a wide range of soft surface materials and the characterization of their mechanics. Future directions for new experiments are also discussed.
Keywords: Adsorption; Desorption; Kinetics; Convection; Surfactant; Protein; Polymer; Macromolecule; Assembly; Interface; Structure; Nanomechanics; Elasticity; Nanomembrane; Viscoelastic; Rheology; Constitutive; Soft matter;

Mixed biopolymer solutions are found in many food systems and household products, and are also employed in industrial processes such as bio-separation and purification. They display a rich phase behaviour, ranging from association and precipitation to the more common segregative phase separation into two liquid phases. Understanding the underlying physics of their phase behaviour and of the rheology-morphology relationships of the resulting phases is a topic of interest and importance in terms of being able to reliably design and produce products containing mixed biopolymer solutions and predicting their behaviour. The science of mixed biopolymer solutions is complicated by the fact that they are ternary systems, typically comprising mostly water, and that the biopolymers themselves are liable to structural transitions such as gelation. Both of these factors can play an important role in the phase behaviour of the mixtures, and the morphology of the resulting phases. In the following, an introduction is given to the physics of mixed biopolymer solutions and the behaviour of their phases, with a view to highlighting the unique aspects of such materials in comparison to other liquid-liquid mixtures, such as emulsions and polymer blends, and also the more interesting topics for future research in these fascinating materials.

Spreading and retraction as a function of drop size by Moniraj Ghosh; Kathleen J. Stebe (61-76).
We simulate the spreading and retraction of a two-dimensional drop over a thin film in the small slope limit for drop heights ranging from a few nanometers to hundreds of nanometers. Drop motion is initiated by an impulsive change in surface wettability expressed in terms of disjoining pressure. Owing to the presence of the film, these simulations require no closure condition at the ‘apparent’ contact line. Rather, we study the relationships that emerge between the apparent contact line velocity and dynamic contact angles. The disjoining pressure that we study includes stabilizing van der Waals interactions and destabilizing acid–base interactions. Changes in wetting conditions that promote spreading place the thin film surrounding the drop out of equilibrium; the drop spreads as the film thickens to its new equilibrium value. Changes in wetting conditions that promote retraction can either place the thin film out of equilibrium in a stable regime, or they can place the thin film in a spinodally unstable regime. We study drop rearrangement as a function of drop scale for these three cases. Small drops, with heights on the same order as the film thickness, are strongly influenced by disjoining pressure gradients everywhere beneath them. Larger drops, with heights at least an order of magnitude greater than the film thickness, have disjoining pressure gradients isolated near the apparent contact line at all times. For these larger drops, after initial dynamics, macroscopic behavior is recovered; drops move in agreement with Tanner's law. However, dynamics associated with the thin film can play a leading role in the ensuing drop response even after Tanner's law emerges. In particular, when drops retract over spinodally unstable films, retraction occurs in three regimes. Rims form near the apparent contact line over time scales comparable to the time scale for the instability. The rim geometry can be characterized in terms of spinodal film thicknesses. The rims then propagate toward the bulk drop. Finally, the rim disappears and the drop assumes a cap-like shape. Tanner's law is obeyed during the latter two regimes. Attempts to simulate drop rearrangements disregarding the thin film dynamics before Tanner's law manifests can lead to erroneous outcomes, as shown in simulations of drop retraction on a solid surface with an imposed Navier slip length.
Keywords: Dewetting; Contact line; Disjoining pressure; Tanner's law; Slip; Rim formation;

Vesicles are hollow aggregates, composed of bilayers of amphiphilic molecules, dispersed into and filled with a liquid solvent. These aggregates can be formed either as equilibrium or as out of equilibrium meta-stable structures and they exhibit a rich variety of different morphologies. The surprising richness of structures, the vast range of industrial applications and the presence of vesicles in a number of biological systems have attracted the interest of numerous researchers and scientists. In this article, we review both the thermodynamics and the kinetics aspects of the phenomena of formation of vesicles.We start presenting the thermodynamics of bilayer membranes formation and deformation, with the aim of deriving the conditions for the existence of equilibrium vesicles. Specifically, we use the results from continuum thermodynamics to discuss the possibility of formation of stable equilibrium vesicles, from both mixed amphiphiles and single component systems. We also link the bilayer membrane properties to the molecular structure of the starting amphiphiles.In the second part of this article, we focus on the dynamics and kinetics of vesiculation. We review the process of vesicles formation both from planar lamellar phase under shear and from isotropic micelles. In order to clarify the physical mechanisms of vesicles formation, we continuously draw a parallel between emulsification and vesiculation processes. Specifically, we compare the experimental results, the driving forces and the relative scaling laws identified for the two processes. Describing the dynamics of vesicles formation, we also discuss why non equilibrium vesicles can be formed by kinetics control and why they are meta-stable.Understanding how to control the properties, the stability and the formation process of vesicles is of fundamental importance for a vast number of industrial applications.
Keywords: Vesicles; Amphiphiles systems; Vesicles formation; Thermodynamics of aggregates; Non equilibrium vesicles;

Droplet deformation under confined Poiseuille flow by Stefano Guido; Valentina Preziosi (89-101).
The flow behavior of droplet-based liquid–liquid systems, such as emulsions, polymer blends, and foodstuff, which are ubiquitous in everyday life, has attracted scientific interest in different disciplines. In this review, we focus on the pressure-driven confined flow behavior of isolated droplets in circular and rectangular cross-section channels, which are valuable model geometries to gain insight into more complex flow conditions found in industrial applications. The effect of the relevant nondimensional parameters governing droplet deformation and breakup, such as viscosity ratio, capillary number, and ratio of droplet to tube radius, is presented both for axisymmetric and off-axis droplets, including cross-stream migration. The role of surfactants is also discussed. Ongoing research directions include the field of microfluidics techniques, where confined flow geometries can be exploited to manipulate droplets with a variety of possible applications.
Keywords: Droplet; Poiseuille; Confined; Surfactant; Tube;

Capillary pressure studies under low gravity conditions by V.I. Kovalchuk; F. Ravera; L. Liggieri; G. Loglio; P. Pandolfini; A.V. Makievski; S. Vincent-Bonnieu; J. Krägel; A. Javadi; R. Miller (102-114).
For the understanding of short-time adsorption phenomena and high-frequency relaxations at liquid interfaces particular experimental techniques are needed. The most suitable method for respective studies is the capillary pressure tensiometry. However, under gravity conditions there are rather strong limitations, in particular due to convections and interfacial deformations. This manuscript provides an overview of the state of the art of experimental tools developed for short-time and high-frequency investigations of liquid drops and bubbles under microgravity. Besides the brief description of instruments, the underlying theoretical basis will be presented and limits of the applied methods under ground and microgravity conditions will be discussed. The results on the role of surfactants under highly dynamic conditions will be demonstrated by some selected examples studied in two space shuttle missions on Discovery in 1998 and Columbia in 2003.
Keywords: Microgravity conditions; Interfacial dynamics; Capillary pressure tensiometry; Oscillating drops and bubbles; Surfactant adsorption layers;

Electrowetting: A versatile tool for drop manipulation, generation, and characterization by Frieder Mugele; Michel Duits; Dirk van den Ende (115-123).
Electrowetting is arguably the most flexible tool to control and vary the wettability of solid surfaces by an external control parameter. In this article we briefly discuss the physical origin of the electrowetting effect and subsequently present a number of approaches for selected novel applications. Specifically, we will discuss the use of EW as a tool to extract materials properties such as interfacial tensions and elastic properties of drops. We will describe some modifications of the EW equation that apply at finite AC voltage for low conductivity fluids when the electric field can partially penetrate into the drops. We will discuss two examples where finite conductivity effects have important consequences, namely electrowetting of topographically structured surfaces as well as the generation of drops in AC electric fields. Finally, we review recent attempts to incorporate electrowetting into conventional channel-based microfluidic devices in order to enhance the flexibility of controlling the generation of drops.
Keywords: Microfluidics; Electrowetting; Two-phase flow; Drop generation; Wetting; Interfacial tension;

An introduction to superhydrophobicity by Neil J. Shirtcliffe; Glen McHale; Shaun Atherton; Michael I. Newton (124-138).
This paper is derived from a training session prepared for COST P21. It is intended as an introduction to superhydrophobicity to scientists who may not work in this area of physics or to students. Superhydrophobicity is an effect where roughness and hydrophobicity combine to generate unusually hydrophobic surfaces, causing water to bounce and roll off as if it were mercury and is used by plants and animals to repel water, stay clean and sometimes even to breathe underwater. The effect is also known as The Lotus Effect® and Ultrahydrophobicity. In this paper we introduce many of the theories used, some of the methods used to generate surfaces and then describe some of the implications of the effect.
Keywords: Superhydrophobic; Rough; Lotus effect; Ultrahydrophobic; Textured;

The most important problem in kinetics of wetting and spreading from the author's point of view is a consideration of combined surface forces and capillary action in a vicinity of the apparent three phase contact line. The latter is equally important at the consideration of static or dynamics. Other current trends in kinetics of wetting and spreading are also briefly reviewed. It is impossible to cover the whole literature on the subject: it was around 5000 publications on that subject in 2009 only and the total number of publication in the area is 65,917 (according to Science Direct). The problems to be solved in the area are marked in italic bold and underlined.

Why do aqueous surfactant solutions spread over hydrophobic substrates? by Victor Starov; Natalia Ivanova; Ramon G. Rubio (153-162).
Spreading of aqueous surfactant solution droplets over hydrophobic substrates proceeds in one slow stage at concentration of surfactants below some critical value and in two stages if the surfactant concentration is above the critical value: the fast and relatively short first stage is followed by a slower second stage. It is shown that the kinetics of a slow spreading at concentrations below the critical value and the second stage at concentrations above the critical value are determined by a transfer of surfactant molecules on a bare hydrophobic substrate in front of the moving three-phase contact line (autophilic phenomenon). The latter process results in an increase of the solid–vapour interfacial tension of the hydrophobic solid surface in front of the moving three-phase contact line and spreading as a result. It is proven that the adsorption of surfactant molecules in front of the moving three-phase contact line results in a decrease of the total free energy of the droplet. Hence, the adsorption of surfactants molecules on a bare hydrophobic substrate in front of the moving three-phase contact line is a spontaneous process in spite of an increase of the local solid–vapour interfacial tension. The duration of the first stage of spreading in the case of the surfactant concentration above the critical value correlates well with the duration of adsorption of surfactant molecules onto a liquid–vapour interface. The latter allows assuming that the adsorption on the liquid–vapour interface is the driving mechanism of spreading during the first fast stage of spreading at surfactant concentrations above the critical value. It is discussed why the first stage of spreading does not take place in the case of surfactant concentrations below the critical concentration in spite of the longer duration of adsorption on liquid–vapour interface in this case.
Keywords: Spreading; Surfactant solution; Hydrophobic substrate;

A series of surfactant-encapsulated polyoxometalates which have different compositions, shapes, and sizes, are able to self-assemble to the highly ordered honeycomb-structured macroporous films at the air/water interface without any extra moist airflow across the solution surface. The honeycomb film pores in the average diameter of 2–3 μm are obtained, which are independent of the polyoxometalates. It is speculated that the cooled micrometer water droplets act as the necessary templates for the formation of macropores, and the stability of these water droplets is crucial during the self-assembly. With increasing the concentration of surfactants, various morphologies from lowly ordered honeycomb films to highly ordered honeycomb films and then to disordered fragments can be modulated. The interfacial tension between chloroform solution and water droplets induces the changes of films. High-resolution TEM observations indicate a close-packed lamellar structure in the ordered honeycomb film walls. The self-assembly successfully performs the transfer of functional polyoxometalates from bulk solutions to interfacial films. Consequently, the produced honeycomb films present electronic activities, such as ferromagnetism and electrochemical properties. These detailed researches will enrich the studies based on materials obtained by encapsulations in cationic surfactants to construct newly nanostructures of polyoxometalates at interfaces, and promote the potential applications of the honeycomb films of surfactant-encapsulated polyoxometalates in advanced materials.
Keywords: Surfactant-encapsulated polyoxometalates; Honeycomb films; Self-assembly; Micrometer water droplets; Electrochemical properties;

We review the thermodynamic approach to determining the surface tension of solid–fluid interfaces. If the pressure is in the narrow range where the contact angle, θ, can exist, then for isothermal systems, adsorption at the solid–liquid interface affects γ SL or θ, but γ SV is very nearly equal γ LV, the surface tension of the adsorbing fluid. For a liquid partially filling a cylinder, the pressure in the liquid phase at the three-phase line, x 3 L, depends on the curvature of the three-phase line, C cl, but the line tension can play no role, since it acts perpendicular to the cylinder wall. C cl is decreased as the cylinder diameter is increased; x 3 L is increased; and θ increases. For a given value of C cl, x 3 L can be changed by rotating the cylinder or by changing the height of the three-phase line in a gravitational field. In all cases, for water in borosilicate glass cylinders, the value of θ is shown to increase as x 3 L is increased. This behaviour requires the Gibbsian adsorption at the solid–liquid interface to be negative, indicating the liquid concentration in the interphase is less than that in the bulk liquid. For sessile droplets, the value of θ depends on both x 3 L and C cl. If the value of θ for spherical sessile droplets is measured as a function of C cl, the adsorption at the solid–liquid interface that would give that dependence can be determined. It is unnecessary to introduce the line tension hypothesis to explain the dependence of θ on C cl. Adsorption at the solid–liquid interface gives a full explanation.
Keywords: Solid–liquid surface tension; Solid–vapour surface tension; Surface tension of a solid in the absence of adsorption; Line tension;

A SDC electrolyte film with gradient structure rooted on porous alumina substrate has been prepared by using a gas-phase controlling convection–diffusion approach. Investigation on the fabrication principles and the co-precipitation kinetics turned out the gradient distribution of hydroxide product of Ce(OH)3 and Sm(OH)3 in a porous substrate could be formed as induced by the down-toward diffusion of NH3·H2O in polar solvent along vertical direction and the up-toward convection of Sm3+ and Ce3+ ions over the cross-section of porous substrate, and the aim ratio of Ce to Sm of 4:1 in the sediment phase would be achieved by controlling component concentration in bulk solution. As a result, Sm0.2Ce0.8O2.0(SDC) electrolyte film with gradient microstructure could be fabricated after a subsequent sintering treatment at a high temperature. Investigation of crystal phase, structural, compositional characteristics of the sintered SDC/substrate specimens proved that a uniform and dense SDC film with an average grain size of ∼ 500 nm spread over on the surface of substrate, and a correct cubic fluorite phase has been formed. Gradient variation presented in both the microstructure of SDC/substrate and the component contents over the cross-section of the SDC/substrate. Numerical analysis on the EDX data presented three component parts were sectioned, including a dense SDC layer of ∼ 25 μm, a uniform filling layer of ∼ 140 μm and a successive diffuse layer stretching as far as ∼ 250 μm. Effect of bulk pH on thickness and surface microstructure of SDC film has been discussed. This microstructure-optimization approach will be applicable to fabricate electrode-supported gradient electrolyte films for IT-SOFC.
Keywords: Samarium doped ceria (SDC); Electrolyte film; Gradient variation in microstructure; Gradient variation in component content; IT-SOFCs;