Ionic Effects in Ionic Diffusiophoresis in Chemically Driven Active Colloids.

Area of Science:

• Colloid science
• Physical chemistry
• Soft matter physics

Background:

• Chemically driven colloidal particles exhibit phoretic motion.
• Interactions between colloidal particles are influenced by their surrounding ionic environment.
• Understanding these interactions is crucial for controlling particle behavior.

Purpose of the Study:

• To experimentally investigate the impact of salt concentration on the phoretic motion of colloidal particles.
• To determine how ionic strength, zeta potential, and particle size influence particle interactions.
• To explore the reversal of self-propulsion in Janus colloids.

Main Methods:

• Experimental study of colloidal particle motion in varying salt concentrations.
• Measurement of particle interactions (attractive/repulsive) and self-propulsion.
• Development of an effective model incorporating colloid-ion interactions and Debye length.
• Quantitative comparison of model predictions with experimental results.

Main Results:

• The response of passive colloids to active colloids is dependent on ionic strength, zeta potential, and passive colloid size.
• The self-propulsion direction of Janus colloids can be reversed by lowering their zeta potential below a critical threshold.
• An effective model accurately captures particle responses and velocities under ionic diffusiophoresis theory.

Conclusions:

• Ionic strength significantly modulates phoretic motion and inter-colloid interactions.
• Zeta potential is a key factor in controlling the directionality of self-propelled Janus colloids.
• The developed model provides a quantitative framework for understanding colloidal behavior beyond the thin double-layer approximation.