Anomalous pull-off forces between surfactant-free emulsion drops in different aqueous electrolytes

Area of Science:

• Colloid and Interface Science
• Fluid Dynamics
• Physical Chemistry

Background:

• Understanding drop-drop interactions is crucial for processes like emulsion stability and microfluidics.
• Surfactants are typically used to control these interactions, but their absence presents unique challenges.
• Continuum models are widely used but their applicability to surfactant-free systems needs validation.

Purpose of the Study:

• To determine the conditions under which continuum models accurately describe thin-film interactions between surfactant-free organic drops.
• To investigate the influence of electrolyte concentration and collision velocity on drop-drop collision dynamics.
• To identify deviations from continuum model predictions and explore underlying mechanisms.

Main Methods:

• Systematic experimental study of organic drop collisions in aqueous electrolyte solutions.
• Varying electrolyte concentrations (e.g., 50 mM, 500 mM) and collision velocities (~1 μm/s).
• Analysis of thin-film interactions, coalescence, and pull-off forces using advanced imaging and force measurements.

Main Results:

• Continuum models accurately predict drop-drop interactions and coalescence at collision velocities above ~1 μm/s, even without surfactants.
• At intermediate salt concentrations (50 mM) and lower velocities (<1 μm/s), significant pull-off forces were observed.
• The magnitude of these pull-off forces varied systematically, indicating ion-specific and time-dependent effects.

Conclusions:

• Continuum hydrodynamics and surface force models are sufficient for describing surfactant-free drop collisions above a critical velocity threshold.
• Deviations at lower velocities and intermediate salt concentrations reveal complex phenomena related to ion-specificity and interaction time.
• These findings are critical for understanding and controlling kinetic stability in systems with marginal stability.