Observation of a backward sliding motion for rollers on surfaces in viscoelastic fluid.

When wheels roll on the ground, they move forward via the contact friction. This propulsion mechanism is also frequently used to create artificial microswimmers in viscous liquid environment. In such cases hydrodynamic lubrication at the contact point greatly reduces the driving forces, nevertheless forward motion can be still remarkable under rapid rotation. Interestingly, here we demonstrate that when rolling in viscoelastic fluids, a roller can slide backwards even though its rotational direction suggests forward motion. These fluids exhibit along sheared flow lines a non-zero elastic tension due to the stretch of the viscoelastic media. When the roller rolls on a surface within such fluid, a rear-front flow-field asymmetry develops, which leads to a net backward viscoelastic stress that forces the roller to slide backwards. In addition, the viscoelastic flow field results in an effective attraction between the roller and the surface. This allows the assembling of a microscale gearing system which transmit motion from a rotating colloid to a much larger object. Our findings opens up new directions for the fabrication of micro scale actuation systems relevant for active matter design and cargo delivery in complex liquid environment.