Dog owners no longer have to lay awake at night, wondering how rapidly a wet dog should oscillate its body to effectively shed water onto its favorite human. Presumably, dogs already know, and now, thanks to slo-mo photography, physicists know, too. Which means that everyone who is taking physics courses should pay particular attention to this video because your professor is going to ask you to explain this on your upcoming final exam.
Drying of wet fur is critical to mammalian heat regulation. In this fluid dynamics video, a Georgia Institute of Technology research team captures how rapidly hairy/furry mammals must oscillate their bodies when shaking to shed water droplets. High-speed videography and fur-particle tracking shows the angular position of the animal’s shoulder skin as a function of time. X-ray cinematography is used to track the motion of the skeleton.
The team found that hairy mammals shake their bodies to remove water at frequencies between 4 and 30 Hz, with smaller mammals shaking faster than larger ones: a mouse shakes at 27 Hz, a rat at 18 Hz, a labrador retriever at 4.3 Hz, while a grizzly bear shakes at 4Hz.
“Shake frequencies asymptotically approach 4Hz as animals grow in size,” they conclude.
Not surprisingly, they also found that the looseness of the animal’s skin increases the amplitude and speed of shaking.
“[It’s] nature’s analogy to the spin cycle of a washing machine,” the authors write.
Interestingly, the best fit for this data is not R^0.5 as they predicted. Instead the universal rule for shaken fur is that the frequency increases withR^0.75, suggesting that their model is missing an important correction factor.
“Perhaps the fur makes a difference?” the team proposes in their video.