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Active systems exhibit unique fluid-fluid phase separation due to non-equilibrium dynamics. This review explores how broken detailed balance and novel interfacial tensions drive these phenomena, differing from equilibrium systems.

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Area of Science:

  • Soft Matter Physics
  • Non-equilibrium Statistical Mechanics
  • Active Matter Theory

Background:

  • Fluid-fluid phase separation is common in active systems with local non-equilibrium dynamics, seen in biological and synthetic systems.
  • Traditional understanding of phase separation from equilibrium systems assumes detailed balance, which is absent in active systems.
  • The absence of detailed balance fundamentally alters phase separation mechanisms, leading to phenomena not observed in equilibrium.

Purpose of the Study:

  • To review recent advancements in understanding the role of activity in fluid-fluid phase separation.
  • To highlight novel phenomena driven by non-equilibrium dynamics, such as reverse Ostwald ripening.
  • To discuss the theoretical frameworks and experimental comparisons for active phase separation.

Main Methods:

  • Focus on continuum field theories, particularly those with a single conserved scalar order parameter and supplemented velocity fields.
  • Review analytical and numerical studies of these continuum theories.
  • Compare theoretical predictions with particle-based models and experimental observations.

Main Results:

  • Active phase separation can exhibit distinct interfacial tensions, including negative values, leading to unique phenomena like reverse Ostwald ripening.
  • The phenomenology of phase separation is radically altered by the absence of detailed balance in active systems.
  • Continuum theories provide a framework to understand these complex behaviors, with qualitative agreement from particle models and experiments.

Conclusions:

  • Activity introduces novel mechanisms and phenomena in fluid-fluid phase separation beyond equilibrium predictions.
  • Continuum theories are powerful tools for studying active matter, though experimental validation faces challenges due to system complexity.
  • Further research is needed to fully elucidate the diverse regimes and open questions in active phase separation.