How the interplay of molecular and colloidal scales controls drying of microgel dispersions

Abstract

Bringing an aqueous dispersion or solution into open air leads to water evaporation. The resulting drying process initiates the buildup of spatial heterogeneities, as nonvolatile solutes and colloids concentrate. Such composition gradients associate with mesostructure gradients, which, in turn, impact flows within these multicomponent systems. In this work, we investigate the drying of microgel dispersions in respect to two reference systems, a colloidal dispersion and a polymer solution, which, respectively, involve colloidal and molecular length scales. We evidence an intermediate behavior in which a film forms at the air/liquid interface and is clearly separated from bulk by a sharp drying front. However, complex composition and mesostructure gradients develop throughout the drying film, as evidenced by Raman and small-angle X-ray scattering mapping. We show that this results from the soft colloidal structure of microgel, which allows them to interpenetrate, deform, and deswell. As a result, water activity and water transport are drastically decreased in the vicinity of the air/liquid interface. This notably leads to diffusional drying kinetics that are nearly independent on the air relative humidity. The interplay between water fraction, water activity, and mesostructure on water transport is generic and, thus, shown to be pivotal in order to master evaporation in drying complex fluids.

Microfluidic Evaporation, Pervaporation, and Osmosis: From Passive Pumping to Solute Concentration

“…The kinetics of directional drying have also been recently studied for the case of microgel dispersions using the same setup shown in Figure . Directional drying concentrates these microgels up to volume fractions higher than the close-packing of hard spheres due to their ability to deform and interpenetrate.…”

“…The kinetics of directional drying have also been recently studied for the case of microgel dispersions using the same setup shown in Figure 44. 305 Directional drying concentrates these microgels up to volume fractions higher than the close-packing of hard spheres due to their ability to deform and interpenetrate. For these highly deformable particles, the mechanisms described in section 4.5.2 for molecular mixtures 279,280 again apply and lead to drying kinetics that are almost independent of the ambient relative humidity, however with the presence of a drying front (at low volume fraction) due to their colloidal nature.…”

How Softness Matters in Soft Nanogels and Nanogel Assemblies

“…Nanoparticles’ softness further concerns their behavior during filtration , and drying processes . Indeed, the knowledge on the nanogel characteristics and their solvent activity are essential to understand transport phenomena through dense soft and permeable nanoparticles dispersions.…”

“…Nanoparticles' softness further concerns their behavior during filtration 252,253 and drying processes. 254 Indeed, the knowledge on the nanogel characteristics and their solvent activity are essential to understand transport phenomena through dense soft and permeable nanoparticles dispersions. After the peculiar drying behavior of PNIPAM nanogels was recently revealed, 254 further studies focusing on the interplay between solvent and nanogels are necessary to model such phenomena both at the colloidal and molecular scales.…”

“…254 Indeed, the knowledge on the nanogel characteristics and their solvent activity are essential to understand transport phenomena through dense soft and permeable nanoparticles dispersions. After the peculiar drying behavior of PNIPAM nanogels was recently revealed, 254 further studies focusing on the interplay between solvent and nanogels are necessary to model such phenomena both at the colloidal and molecular scales. As such, we envision that the investigation of the nanogel permeability will become central in the future.…”