Boundary disorder

 Disorder on boundaries is surprisingly sufficient to destroy phase separation (order) in the bulk!

yellow ball Local random asymmetries at the boundary acts as pumps that stir up the interior of the active fluid over long distances.

(a)  Simulation setup                                             (b) Density fluctuations            (c) Current fluctuations (y-direction)

yellow ball Imry-Ma argument can be generalized to include the effect long-range fields sourced at a boundary

The ordered phase in the bulk is unstable to boundary field induced flips of large enough domains for

  Passive flat wall:     Active flat wall:    

Passive rough wall:   Active rough wall: 

yellow ball Turning on boundary disorder in a phase separated state:

Ydan Ben Dor, S. Ro, Y. Kafri, M. Kardar, J. Tailleur, Phys. Rev. E 105, 044603 (2022)