Multiscale Microstructure, Composition, and Stability of Surfactant/Polymer Foams

Structure formation of PNIPAM microgels in foams and foam films

Responsive aqueous foams are very interesting from a fundamental point of view and for various applications like foam flooding or foam flotation. In this study thermoresponsive microgels (MGs) made from poly(N-isopropyl-acrylamide) (PNIPAM) with varying cross-linker content, are used as foam stabilisers. The foams obtained are thermoresponsive and can be destabilised by increasing the temperature. The structuring of MGs inside the foam films is investigated with small-angle neutron scattering and in a thin film pressure balance. The foam films are inhomogeneous and form a network-like structure, in which thin and MG depleted zones with a thickness of ca. 30 nm are interspersed in a continuous network of thick MG containing areas with a thickness of several 100 nm. The thickness of this continuous network is related to the elastic modulus of the individual MGs, which was determined by atomic force microscopy indentation experiments. Both, the elastic moduli and foam film thicknesses, indicate a correlation to the network elasticity of the MGs predicted by the affine network model.

Probing foams from the nanometer to the millimeter scale by coupling small-angle neutron scattering, imaging, and electrical conductivity measurements

Liquid foams are multi-scale structures whose structural characterization requires the combination of very different techniques. This inherently complex task is made more difficult by the fact that foams are also intrinsically unstable systems and that their properties are highly dependent on the production protocol and sample container. To tackle these issues, a new device has been developed that enables the simultaneous time-resolved investigation of foams by small-angle neutron scattering (SANS), electrical conductivity, and bubbles imaging. This device allows the characterization of the foam and its aging from nanometer up to centimeter scale in a single experiment. A specific SANS model was developed to quantitatively adjust the scattering intensity from the dry foam. Structural features such as the liquid fraction, specific surface area of the Plateau borders and inter-bubble films, and thin film thickness were deduced from this analysis, and some of these values were compared with values extracted from the other applied techniques. This approach has been applied to a surfactant-stabilized liquid foam under free drainage and the underlying foam destabilization mechanisms were discussed with unprecedented detail. For example, the information extracted from the image analysis and SANS data allows for the first time to determine the disjoining pressure vs. thickness isotherm in a real, draining foam.

A new model to describe small-angle neutron scattering from foams

The modelling of scattering data from foams is very challenging due to the complex structure of foams and is therefore often reduced to the fitting of single peak positions or feature mimicking. This article presents a more elaborate model to describe the small-angle neutron scattering (SANS) data from foams. The model takes into account the geometry of the foam bubbles and is based on an incoherent superposition of the reflectivity curves arising from the foam films and the small-angle scattering (SAS) contribution from the plateau borders. The model is capable of describing the complete scattering curve of a foam stabilized by the standard cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) with different water contents, i.e. different drainage states, and provides information on the thickness distribution of liquid films inside the foam. The mean film thickness decreases with decreasing water content because of drainage, from 28 to 22 nm, while the polydispersity increases. These results are in good agreement with the film thicknesses of individual horizontal foam films studied with a thin-film pressure balance.

Numerical Modeling of Flow through Foam Nodes within the Dry and Wet Limits

We present a numerical simulation using three-dimensional microscale models to illustrate flow dynamics through different foam geometries. These were designed to represent the flow with various liquid volume fractions throughout the Plateau border (PB) and node system within the "dry" limit and throughout the two nodes and PB system within the "wet" limit. Most surfaces in the models involve a gas-liquid interface. Here, the stress-balance boundary condition was applied to achieve coupling between the surface and bulk. The three-dimensional Navier-Stokes equation along with the continuity equation was solved using the finite volume approach, and a qualitative evaluation of flow velocities in different foam geometries was obtained. The numerical results were validated against the available experimental results for foam permeabilities in the nodes and PBs. Discrepancies were expected between the simulated and empirical values as the latter values were obtained by considering only the viscous losses in the PBs. Furthermore, the scaled resistance to flow for varying gas-liquid interface mobilities and liquid fractions was studied. The individual geometrical characteristics of the node and PB components were compared to investigate the PB- and node-dominated flow regimes numerically. Additionally, more accurate information was obtained for comparing the average flow velocities within the node-PB and the two-node-PB systems, providing a better understanding of the effect of a gas-liquid interface on foam flow. We eventually show that when the foam geometry is correctly described, the relative effect of the geometrical factors of the PB and node components of system models can be evaluated, allowing a numerical flow simulation with a unique parameter-the Boussinesq number (B_o).