Drying of Droplets of Colloidal Suspensions on Rough Substrates

Area of Science:

• Physics
• Materials Science
• Fluid Dynamics

Background:

• Droplet drying is crucial for depositing solutes onto substrates in various technological applications.
• Substrate roughness, whether intended or unintended, can significantly influence droplet behavior during drying.
• Understanding droplet dynamics on textured surfaces is essential for controlling deposition patterns.

Purpose of the Study:

• To develop a lubrication-theory-based model for studying the drying of colloidal suspension droplets on substrates with topographical defects.
• To investigate the influence of substrate defects and colloidal particles on droplet drying dynamics and particle deposition patterns.
• To elucidate the role of contact-line pinning in controlling the coffee-ring effect.

Main Methods:

• A one-dimensional partial differential equation system was formulated to model droplet shape and particle concentration.
• Lubrication theory, precursor film, disjoining pressure, and a one-sided evaporation model were employed.
• Finite-difference numerical solutions were used to simulate the drying process.

Main Results:

• Without particles, droplet contact lines can pin to defects, balancing capillary and disjoining pressure gradients.
• The model reproduces experimentally observed constant-radius and constant-contact-angle drying stages.
• With particles and a defect, contact-line pinning accelerates solidification, leading to coffee-ring deposition patterns, contrasting with central deposition without defects.

Conclusions:

• Contact-line pinning to substrate defects plays a critical role in controlling the dynamics of drying colloidal droplets.
• The presence of surface defects can alter particle deposition from a central cone to an edge coffee-ring pattern.
• The developed model accurately predicts experimental observations, highlighting the importance of surface topography in drying phenomena.