The puzzling success of an empirical model
DOI: 10.1063/PT.5.7307
When snakes and lizards, like the sandfish skink shown in the photograph, wriggle on or in granular environments, their movements often resemble swimming

First proposed in the 1950s to calculate speeds of sea-urchin spermatozoa swimming in seawater (J. Gray, G. H. Hancock, J. Exp. Biol. 32, 802, 1955), RFT approximates locomotion in viscous fluids relatively well. But for granular media, it’s bafflingly spot on. To figure out why, Ken Kamrin
The predicted force profiles and other properties that Kamrin and Askari derived from their continuum formulation turned out to be strikingly similar to what RFT produces. Furthermore, their analysis revealed that RFT’s success is a natural consequence of how granular media deform in response to intruder motion. Kamrin and Askari suggest their method could identify other materials ripe for RFT—for instance, they predict that it should work for mud, paste, and other cohesive media. (H. Askari, K. Kamrin, Nat. Mater., in press