A double rigidity transition rules the fate of drying colloidal drops

• Published in: Soft matter 2023-09, Vol.19 (36), p.6968-6977
• Main Authors: Milani, Matteo, Phou, Ty, Ligoure, Christian, Cipelletti, Luca, Ramos, Laurence
• Format: Article
• Language: English
• Subjects: Colloids; Dairies; Drying; Evaporation; Evaporation rate; Glass transition; Hydrophobic surfaces; Hydrophobicity; Instability; Nanoparticles; Physics; Rigidity; Spatial distribution; Viruses
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/d3sm00625e

The evaporation of drops of colloidal suspensions plays an important role in numerous contexts, such as the production of powdered dairies, the synthesis of functional supraparticles, and virus and bacteria survival in aerosols or drops on surfaces. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. Previous works propose that, for drops evaporating very fast, the instability occurs when the particles form a rigid porous solid, constituted of permanently aggregated particles at random close packing. To date, however, no measurements could directly test this scenario and assess whether it also applies to drops drying at lower evaporation rates, severely limiting our understanding of this phenomenon and the possibility of harnessing it in applications. Here, we combine macroscopic imaging and space- and time-resolved measurements of the microscopic dynamics of colloidal nanoparticles in drying drops sitting on a hydrophobic surface, measuring the evolution of the thickness of the shell and the spatial distribution and mobility of the nanoparticles. We find that, above a threshold evaporation rate, the drop undergoes successively two distinct shape instabilities, invagination and cracking. While permanent aggregation of nanoparticles accompanies the second instability, as hypothesized in previous works on fast-evaporating drops, we show that the first one results from a reversible glass transition of the shell, unreported so far. We rationalize our findings and discuss their implications in the framework of a unified state diagram for the drying of colloidal drops sitting on a hydrophobic surface. Time laps of a drop of nanoparticles suspension drying on top of a hydrophobic surface. A first regime with a homogeneous shrinkage is followed by a regime where an air bubble nucleates and grows. The pictures combine standard and speckle imaging.