Review article “Liquid Foams: New Insights and Perspectives from Neutron and Synchrotron Scattering Experiments”

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Small-angle scattering experiments for liquid foam characterisation have been experiencing renewed interest in recent years, with novel developments in both measuring cells and analysis formalism. There is work to do in making sure that the foam research and innovation community, and in particular industrial partners, make full use of the new tools available. In a newly published review, Leonardo Chiappisi (ILL) highlights recent advances in the field, discusses possibilities opened up by the SANS foam cell coupled with conductivity measurements and macrophotography developed within the ILL/ESRF Partnership for Soft Condensed Matter (PSCM), and identifies open challenges that could be addressed using neutron and synchrotron radiation.

Liquid foams are omnipresent in our daily lives, visibly in everything from shampoos to cappuccinos, and less visibly in the production of lightweight, soundproofing or insulating materials. They are also set to play a crucial role in the transition to a more sustainable economy, for example in advanced separation and purification procedures for rare earth elements.

Foams are also incredibly complex systems. Their properties, which depend on structural features spanning several orders of magnitude in size, together with their intrinsic instability, make them highly challenging and interesting to study. Understanding the structure of foam and its evolution over time is both paramount for the development of applications and of fundamental interest.

A new level of understanding of liquid foams can be achieved in the coming years thanks to the synergistic use of multiple techniques, focusing on the different relevant length scales. Scattering methods are indispensable tools in this array of techniques. At sub-micrometre length scales in particular, small-angle scattering presents unique advantages in resolving foam structure, offering profound insights into foam behaviour. Thanks to advanced analysis techniques, it also enables the simultaneous determination of several structural parameters with a time resolution that allows foam aging mechanisms to be monitored with exceptional temporal precision.

Today, a substantial segment of the foam research community, primarily with an interest in addressing applied research questions, has yet to fully utilise these tools. With the ongoing efforts of large-scale facilities to broaden the user community and make scattering experiments and subsequent data analysis more accessible, we can expect an increase in the use of scattering methods to address applied research questions in the future.

This is part of the motivation for this review, as explained by the author: "I hope that this work, which highlights the potential of large-scale facilities to address critical societal issues related to liquid foams, will encourage scientists and industry around the world to collaborate with us.”

A good example is the SANS foam cell coupled with conductivity measurements and macrophotography developed within the ILL/ESRF Partnership for Soft Condensed Matter (PSCM). The foam analyser is a custom-made version of a commercial instrument which makes foam characterisation possible on a macroscopic scale. The same cell can be used in-beam as well as off-beam in the lab. The equipment is very recent and has been used by five research groups this year, including three external user groups. Besides structural studies with SANS, spin-echo techniques allow the study of foam dynamics, while reflectometry can be used for studies at the air/water interface. "This innovative setup, which enables us to investigate foam at both macroscopic and microscopic scales, combined with an enhanced analytical framework, advances the scientific community's ability to characterise liquid foams," stresses Leonardo Chiappisi.

The range of questions that can be addressed is vast, in particular as far as applications are concerned, covering everything from cleaning agents and the recovery of electronic and nuclear waste to vegetable proteins and insulation materials. Producing stable but destructible foams is a challenge in several applications. Future developments range from further progress in data analysis to the type of scientific questions addressed using neutron and synchrotron scattering on liquid foams, with more complex colloidal systems, including protein-stabilised foams (relevant namely in food products), coming to the fore.

Fig 1. Foam setup during a neutron scattering experiment at ILL. Exactly the same foam column, which is coupled with conductivity measurements and macrophotography, can be used on the experiment and in the lab.

The Partnership for Soft Condensed Matter (PSCM)

The ILL/ESRF Partnership for Soft Condensed Matter offers support to both in-house scientists and users of the two facilities. The PSCM provides access to a broad suite of equipment for preparing and characterising soft matter samples. From volumetry and thermal analysis to light scattering and optical spectroscopy, 30 instruments are available. The PSCM is part of the exceptional set of user laboratories and support facilities present on the EPN campus, offering a level of expertise and range of equipment that is uncommon in large-scale facilities. About 30 to 40% of ILL’s scientific experiment proposals in the field of soft and biological matter are supported by the PSCM.

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Figure 2. Schematic representation of a typical SAXS experiment on single foam films and a SANS experiment on a three-dimensional foam. Small-angle scattering can effectively probe the structure of liquid foam at characteristic lengths up to 100 nm, and both neutron and synchrotron radiation can be used to probe the structure of single liquid films or three-dimensional foams.

In many cases, the choice between the two types of radiation is dictated by contrast conditions, with X-rays being sensitive to the electron density within the sample, while the scattering cross-section of neutrons varies irregularly with atomic number. This peculiarity of neutrons is particularly valuable for investigating soft matter in biological systems, where replacing hydrogen with deuterium allows us to highlight certain portions of the sample. For foams and thin liquid films, experiments performed on single, thin liquid films are mostly conducted using high brilliance synchrotron radiation, while structural investigations of three-dimensional foams at the nanometre scale are generally carried out using neutron radiation. The advantage lies in the fact that the neutron beam typically measures a few tens of millimetres, making it better suited to probe the spatially averaged, three-dimensional structure of liquid foams. This makes it possible to examine several hundred foam bubbles and films, resulting in sufficient statistical data.

The article:

Leonardo Chippisi, Liquid Foams: New Insights and Perspectives from Neutron and Synchrotron Scattering Experiments, Current Opinion in Colloid & Interface Science,

2024, 101823, ISSN 1359-0294,