DC electric fields produce periodic bending of polyelectrolyte gels

The periodic bending of polyelectrolyte hydrogels made of polyacrylamide, polyacrylic and polymethacrylic acids with 10% ionization under the DC electric field was observed in 0.8 mM CaCl2. When the field with intensity 13.5 V/cm was applied perpendicular to the longitudinal axis of the gel sample, the gel first bent to cathode, then more substantially to anode, then to cathode again and so on with damping amplitude. Within experimental error up to two periods of such oscillations were observed. The maximum amplitude of bending was approximately 30% of sample length. After 150–200 s, the gel sample finally reached a steady state slightly bent to anode. During these oscillations the volume of the sample gradually decreased by 10–30% depending on the gel. Kinetics of displacement of the free end of the gel sample could be fitted by a damped sine wave function, depending initial amplitude, final displacement, wave period, and decay time. All parameters except wave period depended on sample cross-section. The amplitude, wave period and decay time decrease with the increase of initial degree of gel swelling. The physical basis of the periodic bending is discussed based on the theoretical approach of Doi, Matsumoto and Hirose, although that theory can describe only initial cathodic and successive anodic displacement of gel.