  A theoretical study of the emergence of helices in the wake of precipitation
fronts is presented. The precipitation dynamics is described by the
Cahn-Hilliard equation and the fronts are obtained by quenching the system into
a linearly unstable state. Confining the process onto the surface of a cylinder
and using the pulled-front formalism, our analytical calculations show that
there are front solutions that propagate into the unstable state and leave
behind a helical structure. We find that helical patterns emerge only if the
radius of the cylinder R is larger than a critical value R>R_c, in agreement
with recent experiments.
