Exploiting Heteroaggregation to Quantify the Contact Angle of Charged Colloids at Interfaces

Area of Science:

• Colloid and Interface Science
• Materials Science
• Physical Chemistry

Background:

• Determining the equilibrium position and contact angle of colloidal particles at fluid-water interfaces is crucial for understanding interfacial phenomena.
• Existing methods for visualizing the three-phase contact line are often complex or lack precision.

Purpose of the Study:

• To develop a straightforward and accurate method for visualizing and quantifying the equilibrium contact angle (θ) of charged colloidal particles at fluid-water interfaces.
• To leverage electrostatic interactions for precise visualization of the three-phase contact line.

Main Methods:

• Utilized charge-stabilized nanoparticles as the aqueous phase to create oil-water and air-water interfaces.
• Exploited heteroaggregation between oppositely charged particles for selective nanoparticle deposition on the aqueous-facing surface of larger colloidal particles.
• Employed dip-coating to transfer particle assemblies to solid substrates for electron microscopy visualization and contact angle measurement.

Main Results:

• Achieved distinct visualization of the three-phase contact line around colloidal particles at fluid-water interfaces through selective nanoparticle deposition.
• Successfully preserved nanoparticle assemblies on larger particles after transfer to solid substrates.
• Demonstrated the method's suitability by measuring the contact angle (θ) of various sized polystyrene particles (220 nm to 8.71 μm).

Conclusions:

• The developed method provides an effective means to visualize and quantify the equilibrium contact angle (θ) of charged colloidal particles at fluid-water interfaces.
• The accuracy of the contact angle measurement is influenced by the size ratio between deposited nanoparticles and interfacial colloidal particles.
• This technique offers a robust approach for interfacial characterization in colloid science.