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Physico-chemical systems driven away from thermal equilibrium usually undergo various types of instabilities leading to the formation of spatio-temporal patterns on macroscopic time and space scales. In two-dimensional geometries, patterns of different symmetries may be simultaneously stable. We study here the phase stability of hexagonal planforms in the framework of amplitude equations and reduced dynamical models close to the instability points.

The mechanical behavior of thin films subjected to laser irradiation is described by a dynamical model that is based on coupled evolution equations for the deformation and vacancy density fields. Lattice vacancies are generated in a thin layer as a result of shallow absorption of electromagnetic laser radiation. The strain field associated with lattice dilatation due to vacancies is shown to couple with bending and stretching mechanical deformation fields. The dynamical model developed here is an extension of the work of Emel'yanov in two respects: ͑1͒ the coupling between the diffusion and mechanical deformation fields is rigorously developed with additional cross-field contributions; ͑2͒ new equations for reduced dynamics are derived from this model, and are used to analyze the physical conditions for the onset of a deformational instability. For a given material, the threshold for this instability is correlated mainly with laser power. We also show that, although the instability threshold and critical wavelengths are given by the linear part of the dynamics, the selection and type of deformation patterns induced by this instability require a nonlinear formulation. Both numerical and analytical analysis are performed here. According to the relative importance of nonlinearities arising from the defect or from the bending dynamics, square or hexagonal planforms are shown to be selected. Furthermore, it appears that one-dimensional gratings are always unstable in isotropic systems. The results for square patterns are consistent with experimental observations, while those for hexagonal and one-dimensional gratings show the importance of anisotropies on their final selection. ͓S0163-1829͑97͒03847-2͔

The coupling between surface deformation and defect dynamics may be at the origin of deformation patterns in thin films under laser irradiation. We analyze a simple model describing the dynamics of such systems in the case of focused laser irradiation. We show, through linear, nonlinear, and numerical analysis, how rose deformation patterns, with the petal number increasing with laser intensity, naturally arise in this model, in agreement with experimental observations. [S0031-9007(97)04173-2]

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