Electrostatic suppression of the "coffee stain effect".

Area of Science:

• Fluid dynamics
• Colloid science
• Electromagnetism

Background:

• Droplet evaporation and particle self-assembly are crucial in various industrial processes.
• Understanding the influence of external fields on these phenomena is key for process optimization.
• Existing models often simplify complex interactions within evaporating droplets.

Purpose of the Study:

• To investigate the dynamics of slender, evaporating, particle-laden droplets under electric fields.
• To model the formation of inhomogeneous ring-like structures and explore methods for their suppression.
• To analyze the interplay of various physical forces including capillarity, viscous stress, and electrostatic forces.

Main Methods:

• Utilized lubrication theory to simplify governing equations for interfacial position and particle concentration.
• Developed a coupled system of evolution equations incorporating capillarity, viscous stress, Marangoni stress, Maxwell stress, van der Waals forces, and evaporation.
• Performed a parametric numerical study of the one-dimensional model.

Main Results:

• Recovered inhomogeneous ring-like structures in particle concentration due to enhanced evaporation and capillarity-induced flow.
• Demonstrated that carefully chosen electric fields can significantly suppress these ring-like structures.
• Briefly examined the three-dimensional behavior of the film and particle concentration.

Conclusions:

• Electric fields offer a viable mechanism to control particle distribution in evaporating droplets.
• The study provides insights into suppressing unwanted particle aggregation, crucial for microfluidics and materials science.
• Further investigation into 3D dynamics can refine control strategies for particle-laden evaporating systems.