SPONTANEOUS MOTION OF AMPHOTERIC POLYMER GELS ON WATER

We have discovered that cross-linked hydrophobic-hydrophilic copolymer gels swollen in organic solvent undergo spontaneous motion when immersed in water. The mode of motion largely depends on the shape of the gel: a disc-shaped gel exhibits translational motion while a triangular or a square shaped one exhibits rotation. The velocity and duration of gel motion are strongly associated with its size and chemical structure. We consider that the gel, immersed in water, rapidly forms a partially organized structure on its outer surface and gradually shrinks, simultaneously producing high osmotic pressure and hydrostatic pressure. By virtue of these two pressures, the organic solvent is released from the gel and the gel obtains a net reaction force to induce motion, presumably due to its heterogeneous structure.