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Area of Science:

  • Soft matter physics
  • Statistical mechanics
  • Non-equilibrium systems

Background:

  • Active particle systems, like bacterial colonies or synthetic microswimmers, exhibit complex collective behaviors.
  • Phase separation into dense and dilute regions is a common phenomenon in these systems.
  • Understanding the interfacial properties is crucial for predicting macroscopic behavior.

Purpose of the Study:

  • To numerically investigate phase separation in a model of active suspensions.
  • To evaluate the role of active swimming pressure on interfacial properties.
  • To challenge existing thermodynamic descriptions of active matter.

Main Methods:

  • Numerical simulations of self-propelled repulsive particles.
  • Exploitation of non-square box geometries to achieve stable slab configurations.
  • Evaluation of an intensive active swimming pressure model.

Main Results:

  • Stable phase separation into dilute and dense phases was observed.
  • A stable 'slab' configuration with interfaces aligned to box edges was achieved.
  • Negative excess stress (negative tension) was found at the interface.
  • The negative tension was rationalized by a positive stiffness, matching numerical data.

Conclusions:

  • Stable phase separation and negative interfacial tension are genuine non-equilibrium effects.
  • Results challenge effective thermodynamic descriptions and mappings to passive systems.
  • The study highlights the unique physics of active matter beyond equilibrium analogies.