Séance Séminaire

Séminaire des Doctorant·e·s

Drying drops of colloidal suspensions

A colloidal suspension is a binary system composed by solid colloidal particles dispersed in a solvent, which properties vary with the percentage of solid content. The slow evaporation of drops of colloidal suspensions plays a key role in numerous contexts, such as, virus survival in aerosols, production of powdered milk and preparation of functional supraparticles for applications like catalysis, photonics, or sensing. Depending on the evaporation speed, quantified by a Peclet number Pe comparing evaporation and diffusion, the nanoparticles may remain homogeneously distributed or accumulate at the periphery of the drop, forming a dense shell, may undergo a shape instability. Previous works proposed that the onset of buckling is caused by particles aggregation, forming a rigid porous solid, with the particles permanently aggregated particles at random close packing. However, these studies have been done only on fast evaporating drops (Pe>1000). To date, however, no measurements could directly test this scenario applies for drops drying at low evaporation rate (Pe< 100). To answer this question, we have developed a custom multispeckle light scattering set-up to measure with spatial and temporal resolution the microscopic dynamics of colloids confined in a drop. Interestingly, we find that, about a threshold evaporation rate, the colloids in the drop undergo two consecutive state transitions, each of them associated to a macroscopic shape instability. The first instability is characterized by an invagination of the drop, which is induced by the NPs forming a dense shell, that undergoes a dynamic glass transition dye to their high microscopic relaxation time. Subsequently, the NPs irreversibly aggregate, leading an abrupt macroscopic fracture of the drop. This phenomenon is depicted by a general state diagram that shows a unified picture for data gathered at different evaporation rates relating the microscopic dynamics of NPs confined in the drop and the macroscopic shape instability of the drying drops.