Effect of an electric field on propagating Co(OH)2 Liesegang patterns

The effect of an applied DC electric field on the propagation of a Co(OH)2 Liesegang pattern from Co2+ and NH4OH is investigated. The field free pattern is known to propagate down the tube due to band dissolution at the top and band formation at the bottom. At a fixed concentration of Co2+ (0.134 M), the front propagation is accelerated by the field which is applied in the direction of wave propagation. The pattern propagates faster under a higher voltage. The dependence of wave velocity on field strength is non-linear. When the concentration of Co2+ is varied at constant voltage (6.00 V), two opposite trends are obtained. Below a characteristic time tc=1.7 days, the velocity of propagation increases with decreasing concentration and above tc, the velocity increases with increasing concentration. This latter behavior (above tc) completely reverses the field free trend. The effect of the field on the morphological appearance of the bands is discussed.