DENDRITIC SIDEBRANCHING WITH PERIODIC LOCALIZED PERTURBATIONS - DIRECTIONAL SOLIDIFICATION OF PIVALIC ACID COUMARIN 152 MIXTURES

We have studied the response of the sidebranches of pivalic acid dendr ites, growing by directional solidification, to localized periodic the rmal perturbations. The perturbations were generated by a laser beam f ocused near the tip of a single dendrite growing in a glass capillary, with the pulse duration, repetition rate, and intensity controlled se parately. The perturbation dramatically altered the sidebranch structu re, producing ordered sidebranches of well-defined wavelength, synchro nous with the perturbation, which were strongly correlated on the two sides of the dendrite. The dependences of the sidebranch amplitude on the frequency of the perturbation and on the distance from the dendrit e tip were compared to the predictions of Barber, Barbieri, and Langer [Phys. Rev. A 36, 3340 (1987)] and found to be in qualitative agreeme nt. The value of the selection parameter sigma found from these fits t o the theory is compared to a value, obtained from material parameters also determined in this experiment, and to a value deduced from the i nitial Mullins-Sekerka instability of the planar crystal-melt interfac e.