Soft Matter (in the press)

Limblessness is no barrier to locomotion. Indeed, living systems that have remarkably simple topologies find little difficulty in converting chemical energy directly into mechanical energy. However, our best efforts to design machines with comparable efficiency are often far from simple.

Dipabali Hore and colleagues have succeeded in this respect by devising a novel means of inciting elastomeric microcylinders to roll up an inclined plane. The idea centres around an asymmetric swelling cycle induced by the application of an organic solvent. The solvent accumulates preferentially on the downhill side of the cylinder, causing it to swell differentially and creating a torque that sets the cylinder in motion — rolling up the hill.

The authors found that increasing the plane's inclination could actually increase the velocity of the cylinder, and developed a scaling relation that describes the dependence of this velocity on the material properties of both solvent and cylinder. That the cylinder was even able to bear the weight of cargo on its ascent raises the possibility of applying differential swelling to transport design on the microscale.