In this Letter we report a unique fractal aggregation behavior of NH4Cl crystallites grown in agarose gel. The fractal branches consist of faceted crystallites with remarkable correlations in their crystallographic orientations, which are formed by successive heterogeneous nucleations. Unlike the conventional fractal growth, this aggregation process occurs at a low growth driving force with strong crystallographic anisotropy, which leads to the ordering in the fractal branches. [S0031-9007 (98) 05675-0]

A surface-tension-gradient-induced growth phenomenon is reported in lipid monolayer films. The sur-face tension gradient is introduced to the monolayer film by local, subtle heating using the microscope light. This surface tension gradient is responsible for compressing the illuminated area continuously and induces the growth of domains of liquid condensed phase. The evolution of morphology from fractal to faceted and then to dendritic patterns is investigated by in situ observations. The mechanism of pattern formation in this system is discussed.

Alternating morphology transitions between dendrite and dense-branching morphology have been studied for the first time in electrochemical deposition of FeSO4 aqueous solution film. We suggest that the observed alternating transition is caused by the interchange of anisotropy in interfacial growth, which arises from the periodic accumulation and depletion of impurities in front of the growing interface in our experimental system. The selection problem In pattern formation is discussed.

We report a hydrodynamic instability related to the periodic growth of needlelike crystals observed in thin aqueous-solution films of Ba (N03)2 with the free surface in isothermal conditions. The oscilla-tion of the aqueous-solution film. in which the solution film separates and recontacts with the needle-like crystal periodically, has been investigated using in situ observations with interference-contrast mi-croscopy. Our experiments indicate that this phenomenon derives from the interplay of the Marangoni effect and the wetting effect.

In our previous work we demonstrated an unusual crystallite aggregate in which the crystallites correlate in crystallographic orientation and form a fractal pattern with strong anisotropy (Wang, M.; et al. Phys. Rev. Lett. 1998, 80, 3089. Liu, X. Y.; et al. J. Cryst. Growth 2000, 208, 687.). Yet it remains unanswered why each crystallite appears with specific orientation and obeys a strict order. Here we report an in-depth study of the origin of the long-range correlation of the crystallographic orientations in the aggregate investigated by means of micro-X-ray-diffraction, atomic force microscopy, and in-situ optical observation. The experimental data suggest that the topographic regularity of the aggregate arises from the consecutive rotation of the crystallographic orientation in the nucleation-mediated growth. This effect may occur when nucleation takes place in a region with inhomogeneous surface tension, and may help us to understand the long-range ordering effect in aggregating crystallites.