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Abstract

Active fluids flow on their own, driven internally by their constituents. Examples of active fluids include bacterial suspensions, cell layers, and mixtures of cytoskeletal filaments and molecular motors. These fluids exhibit chaotic flows, which, given their visual similarity with turbulence, have been called active turbulence. However, the extent of this analogy remains debated. As first predicted by Kolmogorov, classical turbulence is characterized by scaling laws with universal exponents. Here, we report that active turbulence also follows universal scaling laws.

We measure the flow field in an active liquid-crystal film made of microtubules and kinesin motors. We experimentally verify that the energy spectrum exhibits two theoretically predicted scaling regimes characterized by universal exponents, and we find a new scaling regime that arises from the coupling of the active film with the surrounding passive fluids, which provide a source of external dissipation. By fitting the experiments to a new theoretical framework that explains all the scaling regimes, we also extract elusive properties of the active fluid, such as its viscosity.

Overall, our work experimentally demonstrates scaling laws with universal exponents in active turbulence, and it shows how these flows are affected by the coupling to the surrounding fluids.