Turbulent flows consist of many eddies of various sizes. This variety of eddies results from a cascade process, whereby smaller eddies are generated from larger ones. This generation of small-scale is, in fact, a fundamental property of turbulent flows, and is expected to be the source of the irreversibility of turbulent flows, which, as recently observed, manifests in the apparently random motion of fluid particles transported by a turbulent flow, e.g., in the atmosphere or in the ocean. When looking carefully, one finds that fluid particles transported in the flow gain kinetic energy slowly and lose it quickly, which results in an asymmetry in the probability of kinetic energy change along fluid particles: over a given short period, the chance for a fluid particle to lose a large amount of kinetic energy is greater than the chance for a fluid particle to gain the same amount of kinetic energy, even when statistically the total energy of flow is not changing with time.

In a recently published work (Phys. Rev. Lett., 116:124502, 2016), Prof. Haitao Xu and co-workers in France and Germany show that this asymmetry is a signature of vortex stretching, a key element in the process of small-scale generation in turbulence. Therefore, the properties of the motion of a single tracer are related, in an unexpected way, to the internal dynamics and structure of turbulence, providing new insight into the study of fluid turbulence, especially on its Lagrangian properties, i.e., the view of turbulence from the perspective of tracer particles in the flow.