A double rigidity transition rules the fate of drying colloidal drops

Area of Science:

• Colloid and Surface Science
• Materials Science
• Fluid Dynamics

Background:

• Colloidal suspension evaporation is crucial for applications like powder production and understanding pathogen survival.
• Evaporating drops form shells that can become unstable, but the mechanisms are not fully understood, especially at lower evaporation rates.

Purpose of the Study:

• To investigate the shape instabilities of drying colloidal drops on hydrophobic surfaces across different evaporation rates.
• To elucidate the underlying mechanisms of these instabilities, particularly the role of particle aggregation and dynamics.

Main Methods:

• Combined macroscopic imaging with space- and time-resolved measurements of nanoparticle dynamics.
• Monitored shell thickness, particle distribution, and mobility during drop drying.
• Analyzed drops on hydrophobic surfaces under varying evaporation conditions.

Main Results:

• Identified two distinct shape instabilities occurring above a threshold evaporation rate: invagination and cracking.
• Observed permanent nanoparticle aggregation linked to the second instability (cracking).
• Discovered a novel, reversible glass transition of the shell preceding the first instability (invagination).

Conclusions:

• The study provides a unified state diagram for drying colloidal drops on hydrophobic surfaces.
• Distinguishes between reversible and irreversible particle aggregation mechanisms based on evaporation rate.
• Offers new insights into controlling colloidal shell formation for various applications.