Liquid and particles retention in foamed suspensions

Liquid foam templating - A route to tailor-made polymer foams

Solid foams with pore sizes between a few micrometres and a few millimetres are heavily exploited in a wide range of established and emerging applications. While the optimisation of foam applications requires a fine control over their structural properties (pore size distribution, pore opening, foam density, …), the great complexity of most foaming processes still defies a sound scientific understanding and therefore explicit control and prediction of these parameters. We therefore need to improve our understanding of existing processes and also develop new fabrication routes which we understand and which we can exploit to tailor-make new porous materials. One of these new routes is liquid templating in general and liquid foam templating in particular, to which this review article is dedicated. While all solid foams are generated from an initially liquid(-like) state, the particular notion of liquid foam templating implies the specific condition that the liquid foam has time to find its "equilibrium structure" before it is solidified. In other words, the characteristic time scales of the liquid foam's stability and its solidification are well separated, allowing to build on the vast know-how on liquid foams established over the last 20 years. The dispersed phase of the liquid foam determines the final pore size and pore size distribution, while the continuous phase contains the precursors of the desired porous scaffold. We review here the three key challenges which need to be addressed by this approach: (1) the control of the structure of the liquid template, (2) the matching of the time scales between the stability of the liquid template and solidification, and (3) the preservation of the structure of the template throughout the process. Focusing on the field of polymer foams, this review gives an overview of recent research on the properties of liquid foam templates and summarises a key set of studies in the emerging field of liquid foam templating. It finishes with an outlook on future developments. Occasional references to non-polymeric foams are given if the analogy provides specific insight into a physical phenomenon.

A review of aqueous foam in microscale

In recent years, significant progress has been achieved in the study of aqueous foams. Having said this, a better understanding of foam physics requires a deeper and profound study of foam elements. This paper reviews the studies in the microscale of aqueous foams. The elements of aqueous foams are interior Plateau borders, exterior Plateau borders, nodes, and films. Furthermore, these elements' contribution to the drainage of foam and hydraulic resistance are studied. The Marangoni phenomena that can happen in aqueous foams are listed as Marangoni recirculation in the transition region, Marangoni-driven flow from Plateau border towards the film in the foam fractionation process, and Marangoni flow caused by exposure of foam containing photosurfactants under UV. Then, the flow analysis of combined elements of foam such as PB-film along with Marangoni flow and PB-node are studied. Next, we contrast the behavior of foams in different conditions. These various conditions can be perturbation in the foam structure caused by injected water droplets or waves or using a non-Newtonian fluid to make the foam. Further review is about the effect of oil droplets and particles on the characteristics of foam such as drainage, stability and interfacial mobility.

Preparation and Investigation of Foaming Amphiphilic Fluorinated Nanoparticles for Enhanced Oil Recovery

Amphiphilic nanoparticles have attracted increasing interest as Pickering emulsifiers owing to the combined advantages of both traditional surfactants and homogeneous particles. Here, foaming amphiphilic fluorinated nanoparticles were prepared for enhanced oil recovery by the toposelective surface modification method. The structure and properties of amphiphilic nanoparticles were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, a laser diffraction method, fluorescence microscopy, a pendant drop tensiometer, and foamscan. It was found that the amphiphilic fluorinated nanoparticles exhibited significant interfacial activity at the air-water interface and generated stabilized aqueous foams against coalescence and drainage even in the absence of surfactants. When the particle concentration reached 0.6 wt %, the adsorption of the amphiphilic nanoparticles at the interface was saturated and the equilibrium surface tension dropped to around 32.7 mN/m. When the particle concentration reached 0.4 wt %, the Gibbs stability criterion was fulfilled. The amphiphilic nanoparticles foam system has a better plugging capacity and enhanced oil recovery capacity. The results obtained provide fundamental insights into the understanding of the self-assembly behavior and foam properties of amphiphilic fluorinated nanoparticles and further demonstrate the future potential of the amphiphilic nanoparticles used as colloid surfactants for enhanced oil recovery applications.

Elasticity of particle-loaded liquid foams

Mixing solid particles with liquid foam is a common process used in industry for manufacturing aerated materials. Desire for improvement of involved industrial processes and optimization of resulting foamed materials stimulates fundamental research on those complex mixtures of grains, bubbles and liquid. In this paper, we generate well-controlled particle-loaded liquid foams and we determine their elastic behavior as a function of particle size (6-3000 μm) and particle volume fraction (0-6%). We focus on both the elastic modulus exhibited by the material at small strain and the strain marking the end of the linear elastic regime. Results reveal the existence of a critical particle-to-bubble size ratio triggering a sharp transition between two well-defined regimes. For small size ratios, the behavior is governed by the mechanical properties of the solid grains, which have been proved to pack in the shape of a foam-embedded granular skeleton. In contrast, bubbles elasticity prevails in the second regime, where isolated large particles contribute only weakly to the rheological behavior of the foamed material. The modeling of elasticity for each regime allows for this transition to be normalized and compared with previously reported particle size-induced effects for foam drainage (Haffner et al. J. Colloid Interface Sci., 2015, 458, 200-208) and solid foam mechanics (Khidas et al., Compos. Sci. Technol., 2015, 119, 62-67). This highlights that rheology and the other properties of particle-loaded foams are subjected to the same size-induced morphological transition.

Foamed emulsion drainage: flow and trapping of drops

Foamed emulsions are ubiquitous in our daily life but the ageing of such systems is still poorly understood. In this study we investigate foam drainage and measure the evolution of the gas, liquid and oil volume fractions inside the foam. We evidence three regimes of ageing. During an initial period of fast drainage, both bubbles and drops are very mobile. As the foam stabilises drainage proceeds leading to a gradual decrease of the liquid fraction and slowing down of drainage. Clusters of oil drops are less sheared, their dynamic viscosity increases and drainage slows down even further, until the drops become blocked. At this point the oil fraction starts to increase in the continuous phase. The foam ageing leads to an increase of the capillary pressure until the oil acts as an antifoaming agent and the foam collapses.

Foam drainage in the presence of solid particles

We conducted forced drainage experiments to study the liquid flow within the foams stabilized by a cationic surfactant (CTAB) in the presence of partially hydrophobic silica particles. The results show that the presence of solid particles, even when present in small amounts (0.0932 g L(-1) foam), can significantly decrease the foam permeability. The scaling behaviour (power law) between the drainage velocity and the imposed flow rate indicates that the presence of solid particles in the foams triggers a transition of the foam drainage regime from a node-dominated regime to a Plateau border-dominated regime. We applied two foam drainage equations for aqueous foams to simulate the experimental data and interpret the transition. The simulation results show that the presence of solid particles in the foams increases the rigidity of the interfaces and the viscous losses in the channels (the Plateau borders) of the foams, and decreases the foam permeability. We also generalize the theory for the effects of unattached hydrophilic particles on foam drainage by considering the effects of hydrophobicity and concentration of solid particles on the confinement of foam networks. This study explores liquid drainage in three-phase foams and is relevant to the field of hydrophobic particle separation by froth flotation, in which the wash water is commonly applied to the froth layer to improve the product grade.

A critical review of the growth, drainage and collapse of foams

This review focuses on the current knowledge regarding (i) the mechanisms governing foamability and foam stability, and (ii) models for the foam column kinetics. Although different length scales of foam structure, such as air-water interface and liquid film, have been studied to elucidate the mechanisms that control the foamability and foam stability, many questions remain unanswered. It is due to the collective effects of different mechanisms involved and the complicated structures of foam sub-structures such as foam films, Plateau borders and nodes, and foam networks like soft porous materials. The current knowledge of the effects of solid particles on liquid film stability and foam drainage is also discussed to highlight gaps in our present level of understanding foam systems with solid particles. We also critically review and summarize the models that describe macroscopic foam behaviors, such as equilibrium foam height, foam growth and collapse, within the context of the mechanisms involved.

Flow and jamming of granular suspensions in foams

The drainage of particulate foams is studied under conditions where the particles are not trapped individually by constrictions of the interstitial pore space. The drainage velocity decreases continuously as the particle volume fraction φ(p) increases. The suspensions jam--and therefore drainage stops--for values φ*(p) which reveal a strong effect of the particle size. In accounting for the particular geometry of the foam, we show that φ*(p) accounts for unusual confinement effects when the particles pack into the foam network. We model quantitatively the overall behavior of the suspension--from flow to jamming--by taking into account explicitly the divergence of its effective viscosity at φ*(p). Beyond the scope of drainage, the reported jamming transition is expected to have a deep significance for all aspects related to particulate foams, from aging to mechanical properties.

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.

Capture of particles in soft porous media

We investigate the capture of particles in soft porous media. Liquid foam constitutes a model system for such a study, allowing the radii of passage in the pore space to be tuned over several orders of magnitude by adjusting the liquid volume fraction. We show how particle capture is determined by the coupling of interstitial liquid flow and network deformation, and present a simple model of the capture process that shows good agreement with our experimental data.

Shear induced drainage in foamy yield-stress fluids

Shear induced drainage of a foamy yield-stress fluid is investigated using MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes the system at rest, a fast drainage is observed when a horizontal shear is imposed. It is shown that the sheared interstitial material behaves as a viscous fluid in the direction of gravity, the effective viscosity of which is controlled by shear in transient foam films between bubbles. Results provided for several bubble sizes are not captured by the R2 scaling classically observed for foams. Furthermore, foam films are found to be responsible for the unexpected arrest of drainage, thus trapping irreversibly a significant amount of interstitial liquid.