Microstructures of a feather lock together

Airplane wings are a compromise between stability and efficiency. Their shape is rigid except for the flaps and slats that pilots can tune during takeoffs and landings to attain the desired level of lift and drag. That system works reasonably well, but birds take off, cruise, turn, and land more efficiently and nimbly than airplanes by morphing their flexible wings continuously and independently of the local atmospheric conditions (see Physics Today, June 2007, page 28 ). To uncover the biophysical mechanism that makes morphing possible, a group of biologists and engineers recently teamed up. David Lentink, graduate student Laura Matloff (both at Stanford University ), and their colleagues determined that adjacent feathers on the wings of rock pigeons contain microstructures that fasten together to prevent gaps that would hinder flight.

To figure out how the feathers stick to each other, Matloff designed a setup in which a motor pressed two overlapping feathers together with a constant normal force and then slid them apart. The results indicated that one feather initially passed across the other with a small opposing force. But before the feathers completely slid past each other, the force abruptly increased, which caused them to lock and resist separation.

If the resistance was a function of friction alone, the researchers calculated that the coefficient of friction for even the lowest measured normal force would be far greater than the theoretical limit of the feather’s material properties. When two of the coauthors, Stanford graduate student Lindsie Jeffries and Smithsonian researcher Teresa Feo , used scanning electron and x-ray microscopy to look at the feathers in more detail, they found microstructures, shown in the lower part of the figure above. They interlock when overlapping feathers (P5 and P6) extend, and they quickly unlock when the wings bend in a different configuration. (Black scale bars are 10 mm; m is the middle, and b is the base.)

The researchers used their findings to build PigeonBot , a biohybrid robot with real feathers. During test flights, PigeonBot executed turns by changing the position of each wing independently of the other, like birds do. Matloff and her colleagues hope that the insights from the experiments can help designers improve the maneuverability of aircraft. (L. Y. Matloff et al., Science 367, 293, 2020 .)