Drop impacts on stalagmites
Stalagmites grow thanks to the tiny amounts of minerals brought by the drops that fall on it. How much of these marvellous shapes is rooted in the physics of impacting drops?
Drops that detach from stalactites are loaded in calcium and bicarbonate ions. Each drop impacts on the underlying stalagmite and covers it with a thin liquid film. The relatively low partial pressure of carbon dioxide in the cave induces its degassing from the film and the subsequent precipitation of calcium carbonate onto the stalagmite. In caves, we observe a variety of stalagmite shapes that must originate from differences in the distribution of calcium carbonate by the impacting drop. In this fundamental research project, we aim at understanding the influence of drop physics on stalagmite morphogenesis.
Our first study focused on what happens before drop impact: the free fall from the stalactite. Intuitively, one would say that drops simply fall on a vertical straight line from the stalactite so they should always impact at the same point on the stalagmite. Our high-speed movies taken in three french caves (Aven d'Orgnac, la Salamandre, Clamouse) revealed that it is not the case: drops originating from a single stalactite impact at random locations sometimes distributed over the whole stalagmite top. We showed that high falls result in a wide distribution and consequently in a wide stalagmite. The physical explanation is rooted in the aerodynamics of the free fall. The falling drop sheds asymmetric vortices in its wake, and each vortex imparts a tiny amount of horizontal momentum to the drop, in a chaotic direction. We designed a model of the free fall based on a Langevin-like equation that considers this momentum contribution from the vortices. The model captures well the increase of horizontal deviations with increasing fall heigth.
Our next step is to characterize and understand the mixing and subsequent distribution of fresh calcium from the impacting drop within the liquid film that covers the stalagmite. It yields wonderful flower-shaped patterns !
Related publications
- J. Parmentier, V. Terrapon and T. Gilet, Drop impact on thin film: Mixing, thickness variations, and ejections, Phys. Rev. Fluids 8, 053603 (2023)
- J. Parmentier, S. Lejeune, M. Maréchal, F. Bourges, D. Genty, V. Terrapon, J.-C. Maréchal and T. Gilet, A drop does not fall in a straight line: a rationale for the width of stalagmites, Proc. R. Soc. A 475, 20190556 (2019)
Press
- How stalagmites get their shape (New York Times)
- Pourquoi les stalagmites ont-elles des formes différentes? (La Croix) [French language]
Contact @ µFL: Justine Parmentier, Tristan Gilet
Collaborations: Vincent Terrapon (A&M, ULiège), Jean-Christophe Maréchal (BRGM), François Bourges (Géologie Environnement Conseil), Dominique Genty (CNRS 5805 EPOC)
