Symmetry-breaking phase transitions in highly concentrated semen

Area of Science:

• Biophysics
• Fluid Dynamics
• Soft Matter Physics

Background:

• Active suspensions, composed of self-motile particles, exhibit complex emergent behaviors.
• Understanding collective motion in biological systems like sperm is crucial for reproductive biology and micro-robotics.
• Previous models like the Vicsek model describe local alignment leading to macroscopic order.

Purpose of the Study:

• To experimentally investigate the self-motion dynamics of a confined active suspension of spermatozoa.
• To identify the conditions and mechanisms driving collective motion and vortex formation in semen.
• To validate theoretical models of self-organized hydrodynamics in biological active matter.

Main Methods:

• Utilizing a custom-designed annular microfluidic chip to confine fresh semen samples.
• Systematic dilution of semen to vary sperm concentration and observe phase transitions.
• Microscopic observation and analysis of fluid dynamics, including vortex formation and rotation.
• Comparison of experimental results with predictions from self-organized hydrodynamics theories.

Main Results:

• Sufficiently high sperm concentration induces a spontaneous, stable vortex state in the fluid.
• Vortex rotation direction (clockwise/counterclockwise) is unpredictable but robust.
• Richer dynamics, including oscillations, are observed at higher concentrations.
• A clear phase transition to collective motion, driven by sperm alignment, is evidenced.
• Experimental findings align with predictions from adapted hydrodynamic models.

Conclusions:

• Confined spermatozoa suspensions exhibit a phase transition to collective rotational motion.
• The observed phenomenon is analogous to the Vicsek model, demonstrating self-organization.
• Self-organized hydrodynamics provides a valid theoretical framework for understanding sperm collective dynamics.