Active Model H: Scalar Active Matter in a Momentum-Conserving Fluid

Area of Science:

• Physics
• Soft Matter Physics
• Fluid Dynamics

Background:

• Self-propelled particles (SPPs) in solvents can exhibit complex emergent behaviors, including phase separation.
• Hydrodynamic interactions and activity are key drivers of collective phenomena in active matter systems.

Purpose of the Study:

• To develop a continuum theory for phase separation in systems of SPPs without alignment interactions.
• To investigate the role of contractile activity and hydrodynamics in domain growth and coarsening.

Main Methods:

• Formulation of a continuum theory incorporating a concentration field with advective-diffusive dynamics.
• Introduction of an activity-induced stress contribution to the deviatoric stress tensor.
• Numerical confirmation of theoretical predictions regarding domain growth cessation.

Main Results:

• Activity generates an effective interfacial tension, which is negative for contractile swimmers.
• A balance between diffusive coarsening and active interface stretching leads to a cessation of domain growth.
• The predicted length scale for domain growth arrest was confirmed through numerical simulations.

Conclusions:

• A continuum theory successfully describes phase separation in non-aligning SPP systems.
• Contractile activity and hydrodynamics play a crucial role in regulating domain morphology and dynamics.
• The study highlights a subtle interplay between activity and hydrodynamics, even in the absence of alignment interactions.