Redox Control of Particle Deposition from Drying Drops

Area of Science:

• Colloid and Surface Science
• Materials Science
• Physical Chemistry

Background:

• The coffee-ring effect (CRE) is a common phenomenon in drying drops, leading to particle accumulation at the edge.
• Controlling CRE is vital for applications like particle patterning and diagnostics, but current methods are limited.
• Understanding the factors influencing CRE, such as surfactant properties, is crucial for manipulation.

Purpose of the Study:

• To investigate the use of redox stimuli to control the coffee-ring effect (CRE) in drying sessile drops.
• To demonstrate tunable particle deposition patterns by manipulating surfactant redox states.
• To explore the potential of this method for advanced particle patterning and diagnostic applications.

Main Methods:

• Incorporation of redox-sensitive ferrocenyl cationic surfactants into drying drops containing anionic colloids.
• Systematic variation of surfactant hydrophobicity, concentration, and redox state.
• Chemical oxidation and electrochemical potential application to induce and control redox changes.

Main Results:

• Redox stimuli effectively canceled or harnessed the CRE, enabling tunable particle deposition.
• Particle deposition patterns could be precisely controlled, ranging from polycrystalline rings to uniform disks, by adjusting the oxidation rate.
• Microscopic organization of deposited particles was modified based on surfactant properties and redox state.
• Additive-free and reversible control was achieved through electrochemical actuation.

Conclusions:

• Redox-sensitive surfactants offer a robust and tunable method for controlling particle deposition in drying drops.
• This approach provides a pathway for redox-programmable particle patterning and visual redox state determination for diagnostics.
• The study clarifies the role of surfactant charge and amphiphilicity in directing particle deposition.