Multistable interaction between a spherical Brownian particle and an air-water interface

Area of Science:

• Colloid and Surface Science
• Soft Matter Physics
• Interfacial Phenomena

Background:

• Understanding particle-interface interactions is crucial for fields like nanotechnology and materials science.
• Existing models often simplify particle surfaces and neglect specific interfacial phenomena.
• Brownian particles at interfaces exhibit complex behaviors influenced by multiple forces.

Purpose of the Study:

• To quantitatively measure the interaction energy between charged polystyrene particles and the air-water interface.
• To investigate the origins of unexpected equilibrium positions and dynamics observed at the interface.
• To explore the role of particle surface properties and adsorbed nanobubbles in interfacial interactions.

Main Methods:

• Utilized a specifically designed Dual-Wave Reflection Interference Microscopy (DW-RIM) setup to track particle-interface distances.
• Applied Boltzmann equation analysis to derive interaction potentials from experimental data.
• Observed and analyzed particle equilibrium positions and rotational diffusion dynamics.

Main Results:

• Identified two distinct equilibrium positions for the particles, hundreds of nanometers from the interface.
• The farthest position was explained by the DLVO model including gravity; the closest position remains unexplained by current theories.
• Frozen rotational diffusion dynamics at the closest position suggest orientation-dependent interactions and highlight surface heterogeneity.

Conclusions:

• Particle-interface interactions are more complex than current models predict, especially at high ionic concentrations.
• Air nanobubbles adsorbed on particle surfaces play a significant role in determining interfacial behavior and interaction energies.
• Particle surface heterogeneities are critical factors influencing interfacial dynamics and potential energy landscapes.