Interactions between micro-scale oil droplets in aqueous surfactant solution determined using optical tweezers

Area of Science:

• Colloid and Surface Science
• Physical Chemistry
• Materials Science

Background:

• Emulsion stability is critical and depends on droplet interactions.
• Previous studies primarily focused on larger droplets (20-200 µm).
• Micro-scale droplets (<10 µm) are prevalent in applications but less understood.

Purpose of the Study:

• To investigate the dynamic interaction forces between micro-scale oil droplets (5.0 µm).
• To compare interaction mechanisms of micro-scale droplets with those of larger droplets.
• To provide insights into the stabilization mechanisms of oil-in-water (O/W) emulsions at a relevant size scale.

Main Methods:

• Directly measured interaction forces between 5.0 µm tetradecane droplets in water using optical tweezers.
• Determined force-distance curves and analyzed hydrodynamic effects.
• Measured zeta potential using Zetasizer Nano ZSP and studied the influence of salt and surfactant concentrations.

Main Results:

• Zeta potential decreased with increasing salt concentration and increased with surfactant concentration.
• Repulsive forces diminished with higher salt concentrations due to suppressed electrostatic double-layer forces.
• Hydrodynamic suction effects intensified with increased droplet approach velocity; micro-droplet deformation was negligible, invalidating models for larger droplets.

Conclusions:

• Developed a pN-resolution method for studying micro-scale emulsion droplet interactions.
• Demonstrated that micro-droplet deformation can be ignored in interaction studies.
• Provided crucial insights into O/W emulsion stabilization relevant to food, cosmetics, medicine, and materials science.