Self-Generated Ions Modify the Pair Interaction and the Phase Separation of Chemically Active Colloids

Area of Science:

• Colloid and Interface Science
• Active Matter Physics
• Chemical Physics

Background:

• Chemically active colloids generate or consume ions, driving collective behaviors like phase separation and swarming.
• Interactions are typically mediated by ion concentration gradients, often simplified to a 1/r^2 force.
• Existing models may not fully capture the complexities of ion-mediated interactions in all active colloid systems.

Purpose of the Study:

• To investigate the interaction dynamics of isotropic, immotile active colloids with net ion production.
• To determine how self-generated ions influence pair-wise forces and collective behaviors.
• To challenge the simplified 1/r^2 interaction model for specific active colloid systems.

Main Methods:

• Utilized finite element simulations to model ion concentration gradients.
• Employed Brownian dynamics simulations to observe colloid interactions and collective motion.
• Analyzed the scaling of interaction forces with interparticle distance and ion concentration.

Main Results:

• Demonstrated that ion production by colloids (e.g., Ag in H2O2) alters interaction forces, deviating from the 1/r^2 simplification.
• Showed that the pair-wise attractive force decays as 1/r or 1/r^2 depending on interparticle distance.
• Observed that medium-sized clusters grow fastest, and cluster coarsening slows over time due to these ion-mediated interactions.

Conclusions:

• Self-generated ions play a critical role in shaping the collective behavior of chemically active colloids.
• The simplified 1/r^2 interaction model is insufficient for colloids with net ion production.
• Understanding these ion-driven dynamics is crucial for predicting and controlling active matter systems.