Before the breach: Interactions between colloidal particles and liquid interfaces at nanoscale separations

Area of Science:

• Colloid and Interface Science
• Materials Science
• Physical Chemistry

Background:

• Particles at fluid-fluid interfaces are key for self-assembly and Pickering emulsions.
• Lateral particle interactions are well-studied, but normal interactions before contact are less understood.

Purpose of the Study:

• To investigate the normal interactions between colloidal particles and a fluid interface.
• To understand how salt concentration influences these interactions.
• To explore the implications for particle assembly at interfaces.

Main Methods:

• Digital holographic microscopy was used to track individual micrometer-size colloidal particles.
• Measurements were taken as particles approached an aqueous-oil interface.

Main Results:

• Particle-interface interactions showed nonmonotonic behavior with salt concentration: repulsive at 1 mM, attractive at tens of mM, negligible at 100 mM, and attractive again above 200 mM.
• Attractive regimes allowed particle binding at nanometer separations without interface breaching.
• Classical theories failed to explain observations, but a nonlinear screening theory succeeded.

Conclusions:

• Normal particle-interface interactions are complex and salt-dependent, challenging classical theories.
• These interactions influence lateral particle arrangements, suggesting out-of-equilibrium assembly in particle-laden interfaces.
• Understanding these forces is critical for designing Pickering emulsions and other interfacial materials.