  We study spacing selection in directional solidification of Al-Cu alloys
under transient growth conditions. New experimental results are presented which
reveal that dendritic spacing versus solidification rate evolves in an almost
step-wise fashion, consistent with previous theoretical predictions of Langer
and co-workers. Phase field simulations of directional solidification with
dynamical growth conditions approximating those in the experiments confirm this
behavior. Changes in dendrite arm spacing is shown to be consistent with
dendrite instabilities confined, initially, to sub-domains, rather than the
entire system. This is due to the rapid variation in growth conditions, which
prevent the system from adapting as a whole but, rather, in a succession of
quasi-isolated domains.
