Physics of the Wet-Dog Shake

If you haven't already, be sure to check out the highly entertaining (and informative) recent article in Wired Magazine about a team of researchers who studied the physics of why wet animals shake their fur. The researchers, led by Andrew Dickinson from Georgia Institute of Technology, built a mathematical model to explain why wet dogs (and other animals) shake their fur, and to determine the optimum shake speed to achieve this goal. Their research, published in the journal Fluid Dynamics, calculates that by shaking their fur, animals generate enough centrifugal force to overcome the surface tension that normally binds water to fur (see abstract, printed below, for details). Testing their model required filming various wet animals (including dogs, rats, mice, and a bear) shaking their fur dry; these have been edited into the below, frankly adorable, video.

The Wet-Dog Shake. Dickerson A, et al. Abstract: "The drying of wet fur is a critical to mammalian heat regulation. In this fluid dynamics video, we show a sequence of films demonstrating how hirsute animals to rapidly oscillate their bodies to shed water droplets, nature's analogy to the spin cycle of a washing machine. High-speed videography and fur-particle tracking is employed to determine 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. We determine conditions for drop ejection by considering the balance of surface tension and centripetal forces on drops adhering to the animal. Particular attention is paid to rationalizing the relationship between animal size and oscillation frequency required to self-dry."