Double helical formation of cobalt silicate tubes under magnetic fields

Magnetic field effect on the growth of cobalt silicate tubes in aqueous solution of sodium silicate is studied. Cobalt silicate tubes grown from CoCl2 powders in 10wt.% sodium silicate solution under a horizontal magnetic field of 2.2 T and a field gradient of 20.3 T/m are found to tilt toward the higher field direction, which is attributed to the paramagnetic property of the Co2+ion. In addition, twisted structures are recognized on the tube walls. The direction of the twist is right-handed in the case that the higher field direction is parallel to the field direction and left-handed in the anti-parallel case. Observing the growth of the tubes in situ under magnetic fields has revealed that two cusps of silicate proceed revolving around at the growing end to form a tube wall double-helically. We have proposed a model for the mechanism of the helix formation that Lorentz force exerted on anions (Cl-) flowing through and out of the tubes would cause vortex around the growing ends and the vortex could strand the two cusps, which explains the twist direction with respect to the field direction reasonably