Wiebke Drenckhan

Particle-stabilised foams: an interfacial study

In an attempt to elucidate the remarkable stability of foams generated from dispersions of partially hydrophobic nanoparticles (fumed silica), we present investigations into the static and dilational properties of the gas–liquid interfaces of such dispersions. By relating the dynamic surface tension γ(t) and the dilational elasticity E measured using an oscillating bubble device, we confirm that the Gibbs stability criterion E > γ/2 against foam coarsening is fulfilled. We complement these studies using ellipsometry and Brewster angle microscopy, which provide evidence for a pronounced adsorption barrier for the particles and a network-like structure in the interface at sufficiently high concentrations. We observe this structure also in freely suspended films drawn from the same particle dispersions.

Unusually stable liquid foams.

Obtaining stable liquid foams is an important issue in view of their numerous applications. In some of these, the liquid foam in itself is of interest, in others, the liquid foam acts as a precursor for the generation of solid foam. In this short review, we will make a survey of the existing results in the area. This will include foams stabilised by surfactants, proteins and particles. The origin of the stability is related to the slowing down of coarsening, drainage or coalescence, and eventually to their arrest. The three effects are frequently coupled and in many cases, they act simultaneously and enhance one another. Drainage can be arrested if the liquid of the foam either gels or solidifies. Coalescence is slowed down by gelified foam films, and it can be arrested if the films become very thick and/or rigid. These mechanisms are thus qualitatively easy to identify, but they are less easy to model in order to obtain quantitative predictions. The slowing down of coarsening requests either very thick or small films, and its arrest was observed in cases where the surface compression modulus was large. The detail of the mechanisms at play remains unclear.

The fluid dynamics of foams

Liquid foam is an example of soft matter (or a complex fluid) with a very well-defined structure, first clearly described by Joseph Plateau in the 19th century. Current research addresses many aspects of the fluid dynamics of this system. How is liquid transported through it in response to a pressure gradient or gravity? How does it respond to stress, particularly above the yield stress? What is the nature of the local fluid flow in the Plateau borders and their junctions? Simple first-order answers to many such questions exist but ongoing experiments continue to challenge our understanding.

On the origin of the stability of foams made from catanionic surfactant mixtures

Using mixtures of the anionic myristic acid (C13COOH) and the cationic cetyl trimethylammonium chloride (C16TA+Cl−) in aqueous solutions at a 2:1 ratio, we show that the outstanding stability of foams generated from sufficiently concentrated “catanionic” surfactant mixtures can be explained by a synergy effect between two fundamentally different mechanisms. Applying a multi-scale approach, in which we link static and dynamic properties of the bulk solutions, isolated gas/liquid interfaces, thin liquid films and foams, we identify these two mechanisms to be as follows: firstly, cationic mixtures create tightly packed surfactant layers at gas/liquid interfaces, which are strongly viscoelastic and also confer high disjoining pressures when two interfaces are approaching each other to form a thin liquid film. Foams created with such kind of interfaces tend to be extremely stable against coalescence (film rupture) and coarsening (gas exchange). However, typical time scales to cover the interfaces are much longer than typical foaming times. This is why a second mechanism plays a key role, which is due to the presence of micron-sized catanionic vesicles in the foaming solution. The bilayers of these vesicles are in a gel-like state, therefore leading to nearly indestructible objects which act like elastic micro-spheres. At sufficiently high concentrations, these vesicles jam in the presence of the confinement between bubbles, slowing down the drainage of liquid during the initial foaming process and therefore providing time for the interfaces to be covered. Furthermore, the tightly packed vesicles strongly reduce bubble coalescence and gas transfer between bubbles.

On the long-term stability of foams stabilised by mixtures of nano-particles and oppositely charged short chain surfactants

We have studied the foaming properties of aqueous dispersions containing mixtures of silica nano-particles (Ludox TMA) and a short-chain amphiphile (n-amylamine). By combining standard hand shaking methods and microfluidic techniques we show that stable foams can be obtained at amine concentrations above approximately 0.5 wt%, which appears to be a critical concentration for cooperative association between particles and amine. In contrast to foams stabilised solely by nano-particles, these foams suffer from slow coarsening due to gas exchange between bubbles. “Superstable” foams for which coarsening is inhibited can only be produced at sufficiently high particle and amine concentrations (typically 10 and 3 wt%, respectively) for which the dispersions also gel in the continuous phase of the foam. We combine investigations of the static and dynamic properties of the particle-laden air–water interfaces in an attempt to elucidate some of the key mechanisms which control the observed behaviour.

Wall slip of bubbles in foams

We present a computational analysis of the flow of liquid foam along a smooth wall, as encountered in the transport of foams in vessels and pipes. We concentrate on the slip of the bubbles at the wall and present some novel finite element calculations of this motion for the case of fully mobile gas/liquid interfaces. Our two-dimensional simulations provide for the first time the bubble shapes and entire flow field, giving detailed insight into the distribution of stresses and dissipation in the system. In particular, we investigate the relationship between the drag force and the slip velocity of the bubble, which for small slip velocities obeys power laws, as predicted by previous semianalytical treatments.

The science of foaming

The generation of liquid foams is at the heart of numerous natural, technical or scientific processes. Even though the subject of foam generation has a long-standing history, many recent progresses have been made in an attempt to elucidate the fundamental processes at play. We review the subject by providing an overview of the relevant key mechanisms of bubble generation within a coherent hydrodynamic context; and we discuss different foaming techniques which exploit these mechanisms.

Liquid dispersions under gravity: volume fraction profile and osmotic pressure

Many physical properties of concentrated dispersions of immiscible fluids are captured by the concept of an osmotic pressure, which measures how much energy is required to deform the bubbles or drops upon compaction. This pressure has a strong impact on the flow and drainage behavior of dispersions. Nevertheless, theoretical models describing its variation with the volume fraction ϕ of the continuous phase are so far available only in the limits of low or high ϕ and experimental data are scarce. We report an experimental study of osmotic pressure in foams and emulsions, showing how the effects of ϕ, disorder, grain size, polydispersity and interfacial tension can all be captured by a single law which satisfies previously established theoretical constraints. Building on this result, we propose the first equation which accurately describes the variation of the volume fraction with the height of a fluid dispersion under gravity.

Synthesis of Macroporous Polystyrene by the Polymerization of Foamed Emulsions

Dependingontheapplication,afoammustmeetspecific requirements. Thus great effort has been invested inthe determination and manipulation of foam properties.Material composition and cellular structure constitute crucialparameters when it comes to the tailoring of foams. Asconventional manufacturing, where foams are produced frompolymer melts and blowing agents, is a very complex process,it is hard to control the product!s morphology and properties.In recent years alternative methods for the synthesis ofpolymer foams have been proposed which make use oftemplates: a template is generated first and the actualpolymer is subsequently synthesized. For example, emulsionshave been found to be suitable templates for the synthesis ofporous materials. In particular water-in-oil emulsions witha high concentration of the dispersed phase (high-internal-phase emulsions, HIPEs) have attracted a great deal ofattention.

Bubble size control and measurement in the generation of ferrofluid foams

Ordered ferrofluid foams in tubes or channels offer promising possibilities for transporting and processing small gas (or liquid) samples. For this purpose, monodisperse bubbles can be produced, with fine control of their volume over at least three orders of magnitude, by the application of a variable magnetic field gradient during bubble generation. Electrical resistance measurements can be used to count bubbles, determine their volume, and identify the foam structure they form in the tube or channel.