Séminaire Aurélien Goerlinger

Jet impacts and moving plants

In this presentation, I will discuss three distinct problems.

First, I will examine oscillating circular hydraulic jumps. Circular hydraulic jumps are ubiquitous phenomena that can be observed in everyday life. When a tap is opened and one looks at the bottom of a sink, two distinct flow regions can be identified. Near the jet impact point, the liquid layer is thin and fast, whereas farther away it becomes thicker and slower. The transition between these two regions is abrupt and is marked by a circular liquid wall: the circular hydraulic jump. According to the literature, for steady experimental parameters, the jump is a steady phenomenon as well. However, I will show that under specific conditions, the jump can exhibit unsteady behavior: it oscillates, periodically opening and closing in a regular manner.

In the second part, I will address the impact of a vertical water jet onto a heated surface. Although this problem has been extensively investigated in engineering contexts due to its cooling applications (e.g., quenching), its hydrodynamic aspects remain largely unexplored. I will show that two distinct hydrodynamic regimes can be identified depending on the jet Weber number. At low Weber numbers, a single droplet forms beneath the jet, grows, and eventually detaches, making way for a new droplet. At high Weber numbers, droplets are ejected radially after impact, at a well-defined angle relative to the horizontal. I will focus in particular on this second regime, as well as on the transition between the two regimes.

Finally, I will turn to biophysics and discuss signal transmission in motile plants. In particular, I will focus on Mimosa pudica, often regarded as an emblematic example of plant motility. When subjected to an external stimulus (such as touch, burning, or cutting), its leaves rapidly fold, a response commonly interpreted as a defense mechanism. The remarkable speed of this movement has long fascinated scientists seeking to understand what functional equivalent of muscles operates in this plant. Moreover, when a single leaf is burned, distant, unstimulated leaves also droop and fold. This observation implies the existence of a signal that propagates throughout the plant, in a manner analogous to electrical signals in animal nervous systems. I will discuss the hydrodynamic mechanism through which such signals may travel within Mimosa pudica.