Inferring non-equilibrium interactions from tracer response near confined active Janus particles.

Area of Science:

• Colloid and Interface Science
• Soft Matter Physics
• Chemical Engineering

Background:

• Chemically active Janus particles create local chemical gradients in solutions.
• These gradients induce hydrodynamic flows and self-propulsion, but direct field mapping is difficult.
• Indirect methods, like observing tracer particle responses, are needed to understand these active systems.

Purpose of the Study:

• To experimentally investigate the response of sedimented silica tracer particles to active Janus particles.
• To analyze how the configuration and motion state of active Janus particles influence tracer behavior.
• To develop a coarse-grained model for understanding activity-induced interactions.

Main Methods:

• Experimental setup involving sedimented silica tracer particles and active Pt/silica Janus particles.
• Varying the configuration of active Janus particles (immobilized with different orientations, or motile).
• Utilizing a coarse-grained model to interpret experimental observations of tracer particle dynamics.

Main Results:

• Observed complex effective interactions between active Janus particles and tracer particles.
• Demonstrated that tracer response depends significantly on the active particle's configuration and motion.
• Showed that activity-induced osmotic flows at the wall are crucial for modeling tracer behavior near immobilized Janus particles.

Conclusions:

• Indirect observation of tracer particle responses provides a viable method for studying active Janus particle fields.
• The configuration and motion of active particles critically dictate their interactions with surrounding tracers.
• Incorporating wall-induced osmotic flows improves the qualitative understanding of tracer dynamics in these active colloidal systems.