Title: [NCTS IAC Lecture] Active Turbulence
Speaker: Prof. ‪Jean-François Joanny (Collège de France)
Time: 2023/6/19 (Mon.) 14:30-15:30
Place: NCTS Physics Lecture Hall, 4F, Chee-Chun Leung Cosmology Hall, NTU
Online: https://nationaltaiwanuniversity-zbh.my.webex.com/nationaltaiwanuniversity-zbh.my/j.php?MTID=mc135c800857b3a7bc46676f8931459cf

Abstract:
Active materials are non-equilibriun systems that consume energy at the scale of their constituents. Quite generally they are unstable and flow spontaneously even in the absence of pressrure gradients. At very large values of the active stress, the spontaneous flow becomes chaotic both in time and space, in a regime called active turbulence. We studied numerically in two dimensions the active turbulent regime for an incompressible active nematic fluid, in the limit of very large activity and when the nematic order is defect free. When the flow alignement parameter, which measures the coupling between the flow and the nematic alignement, vanishes, we show that there is no energy transfer between scales but that the velocity power spectrum has a universal scaling behavior with the wave vector. When the flow alignment parameter is finite, the universal scaling regime still exists but we find two distinct and surprising dynamical scenarios. For contractile aligning nematics, the flow instability is non-linear and leads to strongly fluctuating turbulent patterns and intermittency. For extensile aligning nematics, the turbulence is drastically arrested and the flow pattern organizes in a self-avoiding tree-like structure. We show that this labyrinth structure of the flow is at the origin of the dynamical arrest and that it provides a plausible explanation of the wave length selection.