Active particles crossing sharp viscosity gradients.

Area of Science:

• Physics of active matter
• Fluid dynamics
• Biophysics

Background:

• Active particles exhibit directed motion (taxis) in response to environmental gradients.
• Rheological properties, specifically viscosity gradients, are increasingly recognized as a mechanical driver for particle taxis.
• Previous experiments show microorganisms like Chlamydomonas Reinhardtii are reoriented by sharp viscosity changes.

Purpose of the Study:

• To develop a theoretical model explaining the reorientation of active particles in viscosity gradients.
• To provide a mechanistic understanding of observed algal scattering at viscosity interfaces.
• To derive a quantitative law governing particle orientation across viscosity boundaries.

Main Methods:

• Modeling active swimmers as spherical squirmers.
• Focusing on small, sharp changes in fluid viscosity.
• Deriving a theoretical law analogous to Snell's law for particle reorientation.

Main Results:

• A theoretical law was derived that governs the orientation of active particles at a viscosity interface.
• The model's predictions show good agreement with experimental observations of algal behavior.
• The study elucidates the role of asymmetric viscous forces in particle redirection.

Conclusions:

• Viscosity gradients provide a mechanical mechanism for active particle taxis.
• The derived Snell's-law-analog accurately predicts particle reorientation at viscosity interfaces.
• This work offers a fundamental understanding of how active particles navigate complex fluid environments.