Hyperuniformity in phase ordering: the roles of activity, noise, and non-constant mobility.
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Hyperuniformity emerges generically in the coarsening regime of phase-separating fluids. Numerical studies of active and passive systems have shown that the structure factorS(q) behaves asqςforq → 0, with hyperuniformity exponentς = 4. For passive systems, this result was explained in 1991 by a qualitative scaling analysis of Tomita, exploiting isotropy at scales much larger than the coarsening length. Here we reconsider and extend Tomita's argument to address cases of active phase separation and of non-constant mobility, again findingς = 4. We further show that dynamical noise of varianceDcreates a transientς = 2 regime forq^≪q^∗∼Dt[1-(d+2)ν]/2, crossing over toς = 4 at largerq^. Here,νis the coarsening exponent for the domain sizeℓ, such thatℓ(t)∼tν, andq^∝qℓis the rescaled wavenumber. In diffusive coarseningν=1/3, so the rescaled crossover wavevectorq^∗vanishes at large times whend⩾2. The slowness of this decay suggests a natural explanation for experiments that observe a long-livedς = 2 scaling in phase-separatingactivefluids (where noise is typically large). Conversely, ind = 1, we demonstrate that with noise theς = 2 regime survives ast→∞, withq^∗∼D5/6. (The structure factor is not then determined by the zero-temperature fixed point.) We confirm our analytical predictions by numerical simulations of continuum theories for active and passive phase separation in the deterministic case and of Model B for the stochastic case. We also compare them with related findings for a system near an absorbing-state transition rather than undergoing phase separation. A central role is played throughout by the presence or absence of a conservation law for the centre of mass positionRof the order parameter field.
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Acknowledgements: We thank Ronojoy Adhikari, Balázs Németh, Sriram Ramaswamy and Johannes Pausch for useful discussions. F D L acknowledges the support of the University of Cambridge Harding Distinguished Postgraduate Scholars Programme. X M thanks the Cambridge Commonwealth, European and International Trust and China Scholarship Council for a joint studentship. C N acknowledges the support of the Institut National de Physique (INP) through the International Research Program (IRP) ‘IFAM’. Work funded in part by the ANR grant ‘PSAM’.
Funder: China Scholarship Council; doi: http://dx.doi.org/10.13039/501100004543
Funder: University of Cambridge Harding Distinguished Postgraduate Scholars Programme
Funder: Cambridge Trust; doi: http://dx.doi.org/10.13039/501100003343
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1361-648X
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Agence Nationale de la Recherche (PSAM)