What is it about?
Drying usually concentrates materials, but in complex fluids it also creates strong spatial gradients. In this work we followed, inside a single drying film, how composition, structure, thermodynamics and transport evolve from the bulk liquid to the air interface. We used a unidirectional drying geometry connected to an infinite reservoir and combined confocal Raman microscopy and small-angle X-ray scattering to obtain local measurements of water content and microgel organization with micrometre resolution. This allowed us to directly measure how the distance between neighbouring microgels changes along the drying direction, showing their progressive interpenetration, deformation and deswelling. Close to the air interface, the particles completely lose their colloidal identity and form a continuous polymer network. To understand the molecular origin of this behaviour, we compared three reference systems: hard colloids, soft microgels, and a linear PNIPAM polymer solution. We also measured independently the relationship between water activity and water content using sorption experiments. Together, these measurements provide a complete, spatially resolved description of a drying complex fluid, linking local structure, local composition and macroscopic drying kinetics.
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Why is it important?
Drying controls the formation of coatings, membranes, functional films, biomedical formulations and many natural barriers. Yet most descriptions rely on either colloidal or polymer models and lack local experimental validation. Our results show that microgels dry in a regime that cannot be understood from particle size alone. Because they can interpenetrate and expel water, they pack far beyond hard-sphere close packing and create a steep water gradient inside the film. This leads to: a diffusive growth of the drying front (√t law) a surprisingly weak influence of air humidity on drying kinetics a direct link between cracking and the thermodynamic state of water By measuring water activity as a function of water content, we demonstrate that the drying film explores the full thermodynamic curve of the polymer–water mixture. This explains why evaporation becomes controlled by molecular interactions rather than by external humidity. More broadly, the methodology shows that it is possible to predict transport in drying complex fluids from independently measured local quantities: structure, composition and thermodynamics. This opens the way to quantitative, multiscale models for real formulations containing soft particles, polymers or mixtures of both.
Perspectives
What I find most exciting in this work is that we could follow a drying system simultaneously at the colloidal and molecular scales and connect them through direct measurements. We did not infer the mechanism from global kinetics alone — we measured locally how water content, particle spacing, and thermodynamic state evolve inside the film. A key moment was realizing that the microgels eventually form a continuous polymer network near the air interface. At that point, the system is no longer a colloidal packing but a molecular mixture, and this completely changes how evaporation responds to humidity. The comparison with a linear polymer solution was essential. It showed that the unusual drying kinetics do not come from particle size, but from polymer–water interactions and from the collapse of water transport at low water content. This experimental framework can now be applied to many practical systems where soft particles, polymers and small molecules coexist. Because all relevant quantities can be measured independently, it provides a route toward predictive control of drying in complex formulations rather than empirical optimization.
Kevin Roger
Centre National de la Recherche Scientifique
Read the Original
This page is a summary of: How the interplay of molecular and colloidal scales controls drying of microgel dispersions, Proceedings of the National Academy of Sciences, November 2021, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2105530118.
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