A systematic study of hydrodynamic attractors in periodically driven systems at weak and strong couplings

Abstract

We study equilibration and hydrodynamic response in conformal systems undergoing periodic expansion and contraction along one spatial direction. Using strongly coupled holography (AdS/CFT), weakly coupled kinetic theories (relaxation-time approximation, scalar $ϕ^4$ theory and the Yang--Mills sector of QCD effective kinetic theory) and hydrodynamics (Navier--Stokes, Müller--Israel--Stewart (MIS) and rBRSSS), we investigate the shear response to a periodic drive of varying amplitude $A$ and frequency $ω$, both analytically in linear response and numerically. We find that, at the linear level, $A\ll 1$, periodically driven systems approach a cyclic attractor that in general differs from Navier--Stokes behaviour. This attractor is universal among kinetic theories and accurately described by MIS theory: for slow driving, $ωτ_π\ll 1$, where $τ_π$ is the shear relaxation time, it approaches the Navier--Stokes limit, whereas for fast driving the shear stress saturates at its non-interacting (free-streaming) value. In strongly coupled holographic systems, the periodic drive excites non-hydrodynamic quasinormal modes, which produce a characteristic ringing in the approach to the cyclic attractor, in contrast to the purely exponential approach found in kinetic theories. This difference is a direct consequence of the distinct non-hydrodynamic spectra of the two classes of theories. Moreover, unlike in kinetic theory, the holographic response is sensitive to the curvature terms of second-order hydrodynamics. Over longer times, non-linear viscous heating, which is quadratic in the drive amplitude, raises the temperature and thereby lowers $ωτ_π$, since $τ_π\sim 1/T$. This induces a slow drift of the cyclic attractor, which eventually converges towards Navier--Stokes hydrodynamics at asymptotically late times.