Viscoelastic Particle-Laden Interface Inhibits Coffee-Ring Formation

Area of Science:

• Colloid and Surface Science
• Materials Science
• Fluid Dynamics

Background:

• Drying drops of colloidal suspensions commonly form coffee-ring patterns due to outward capillary flow.
• Polystyrene particles are widely used in research but tend to form these undesirable ring patterns upon drying.
• Understanding and controlling particle deposition patterns is crucial for applications like thin-film fabrication.

Purpose of the Study:

• To investigate the effect of soft microgel particles on the evaporation-driven pattern formation of polystyrene particles in drying drops.
• To determine the mechanism by which soft colloids influence particle deposition and coffee-ring formation.
• To explore the potential for creating uniform particulate deposits by suppressing the coffee-ring effect.

Main Methods:

• Mixtures of polystyrene and soft microgel particles were dried under controlled conditions.
• Time-resolved video microscopy was employed to observe particle behavior during drying.
• Analysis of particle displacement was performed to assess their mobility at the interface.

Main Results:

• The addition of a small amount of soft microgel particles completely suppressed the coffee-ring effect typically observed with polystyrene particles.
• Soft colloids promoted the adsorption of polystyrene particles to the water-vapor interface, increasing their concentration.
• A viscoelastic, gel-like particle-laden interface was formed, confirmed by immobile polystyrene particles.
• This interface significantly reduced bulk flow-assisted migration of polystyrene particles to the drop edge.

Conclusions:

• Soft microgel particles can effectively eliminate the coffee-ring effect in drying polystyrene suspensions.
• The formation of a viscoelastic interface by soft colloids is key to achieving uniform particle deposition.
• This approach offers a method for controlling particle assembly and creating homogeneous thin films from colloidal mixtures.