Solute-mediated interactions between active droplets.

Area of Science:

• Physical Chemistry
• Soft Matter Physics
• Chemical Engineering

Background:

• Concentration gradients are crucial for biological processes and active particle behavior.
• Particles generate gradients via dissolution, adsorption, or surface reactions, influencing their energy and interactions.
• These gradients can drive particle self-propulsion (autophoresis) and govern inter-particle forces.

Purpose of the Study:

• To investigate solute gradient-mediated interactions between slowly dissolving droplets.
• To decouple interaction forces from self-propulsion effects.
• To characterize the nature and scaling of the repulsive force.

Main Methods:

• Experimental measurement of steady-state forces between dissolving droplets.
• Theoretical modeling of dissolution and gradient formation.
• Analysis of force dependence on interparticle distance, droplet size, and solute concentration.

Main Results:

• A regime was identified where solute gradients mediate interactions without inducing autophoresis.
• A direct measurement of a steady-state repulsive force was achieved, scaling as F∼1/r^{2}.
• The dissolution process was found to be diffusion-limited, not reaction-limited.
• A theoretical model accurately described the interactions using a single fit parameter (l=16±3nm), representing a swollen micelle length scale.

Conclusions:

• Solute gradients can mediate inter-particle forces independently of self-propulsion in dissolving systems.
• The observed repulsive force is governed by diffusion-limited dissolution.
• The findings provide insights into the out-of-equilibrium behavior and interactions of reactive particles.