Upstream Rheotaxis of Catalytic Janus Spheres.

Area of Science:

• Physics of Living Systems
• Soft Matter Physics
• Microfluidics

Background:

• Microorganisms inhabit fluid flows, necessitating responses to currents.
• Artificial microswimmers' behavior in flow depends on their swimming patterns.
• Rheotaxis, or navigating fluid flows, is a key response mechanism.

Purpose of the Study:

• To investigate the rheotactic response of a novel micromotor system (Cu@SiO2) with a puller-like flow pattern.
• To understand how externally imposed flows influence the directional swimming of these micromotors.
• To explore the effect of swimming pattern on microswimmer behavior in fluid dynamics.

Main Methods:

• Experimental observation of Cu@SiO2 micromotors in controlled fluid flows.
• Flow field calculations using a self-electrophoresis model.
• Theoretical modeling using a simple squirmer model for puller-type microswimmers.

Main Results:

• Cu@SiO2 micromotors consistently exhibited upstream (positive rheotaxis) directional response in applied flows.
• Experimental observations were well-reproduced by the puller-type squirmer model.
• A novel 'jumping' behavior was observed and captured at high flow speeds, where particles roll along the shear plane.

Conclusions:

• Cu@SiO2 micromotors effectively function as puller-type swimmers, demonstrating significant rheotaxis.
• The study validates theoretical predictions for puller-type microswimmers and introduces a new phenomenon at high flow speeds.
• Understanding microswimmer flow patterns is crucial for predicting their behavior in complex fluid environments.