Fig. 2.

Snapshots of a polyacrylamide hydrogel rod (≈2-mm radius and ≈2-cm length) on a thin elastomeric film of polydimethylsiloxane, which is subject to harmonic longitudinal vibrations (frequency, 100 Hz; amplitude, 0.27 mm). (a) Two “scales” or incisions are scored on the elastic layer, with opposite orientations making an angle of 60° with the vertical, which serve to rectify the motion of the gel as it is preferentially lubricated only in one direction. Thus, the two gel rods move toward each other even though the applied wave form is symmetric. (b) As the number of scales is increased, the velocity of the gel increases.