Recent experiments and simulations are starting to answer some fundamental questions about how life came to be.
Image credit: Shutterstock/Radoslaw Lecyk.
Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved October 9, 2012 (received for review July 17, 2012)
Abstract
Erosion by flowing fluids carves striking landforms on Earth and also provides important clues to the past and present environments of other worlds. In these processes, solid boundaries both influence and are shaped by the surrounding fluid, but the emergence of morphology as a result of this interaction is not well understood. We study the coevolution of shape and flow in the context of erodible bodies molded from clay and immersed in a fast, unidirectional water flow. Although commonly viewed as a smoothing process, we find that erosion sculpts pointed and cornerlike features that persist as the solid shrinks. We explain these observations using flow visualization and a fluid mechanical model in which the surface shear stress dictates the rate of material removal. Experiments and simulations show that this interaction ultimately leads to self-similarly receding boundaries and a unique front surface characterized by nearly uniform shear stress. This tendency toward conformity of stress offers a principle for understanding erosion in more complex geometries and flows, such as those present in nature.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: ristroph{at}cims.nyu.edu.
Author contributions: L.R., M.N.J.M., S.C., M.J.S., and J.Z. designed research; L.R. and M.N.J.M. performed research; L.R., M.N.J.M., S.C., M.J.S., and J.Z. analyzed data; and L.R., M.N.J.M., S.C., M.J.S., and J.Z. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
Sculpting of an erodible body by flowing water
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