Knudsen number and universal behavior of collective flows in conformal and non-conformal systems

We investigate the role of the Knudsen number (Kn) as a scaling parameter governing the emergence of collective behavior in relativistic heavy-ion collisions. Using the Relativistic Boltzmann Transport approach, we explore different initial conditions for both conformal (massless) and non-conformal (massive) systems with a constant specific shear viscosity η / s . Observables such as the time evolution of anisotropic flow coefficients collapse onto universal curves for fixed classes of Knudsen number, when using a scaled time variable accounting for the system size and the speed of sound c s . More differential quantities, such as v n ( p T /⟨ E T ⟩), show a larger sensitivity to c s . We also study events with fluctuating initial profiles from the TRENTo model, simulating collision systems from O-O to Pb-Pb at RHIC and LHC energies. Universal scaling at a given Kn value also holds in these event-by-event simulations, suggesting that the Knudsen number provides a unified criterion for classifying collectivity across different systems, including small systems where thermalisation may not be fully realised.