STEP BUNCHING ON A VICINAL FACE OF A CRYSTAL GROWING IN A FLOWING SOLUTION

The effect of a parallel shear flow and anisotropic interface kinetics on the onset of (linear) instability during growth from a supersatura ted solution is analyzed including perturbations in the flow velocity. The model used for anisotropy is based on the microscopic picture of step motion. A shear flow (linear Couette flow or asymptotic suction p rofile) parallel to the crystal-solution interface in the same directi on as the step motion (negative shear) decreases interface stability. For large wavenumbers k(x), the perturbed flow field can be neglected and a simple analytic approximation for the stability-instability dema rcation is found. A shear flow counter to the step motion (positive sh ear) enhances stability and for sufficiently large shear rates (on the order of 1 s(-1)) the interface is morphologically stable. Alternativ ely, the approximate analysis predicts that the system is unstable if the solution flow velocity in the direction of the step motion at a di stance (2k(x))(-1) from the interface exceeds the propagation rate ups ilon(x) of step bunches induced by the interface perturbations. The ap proximate results are applied to the growth of ADP and lysozyme. For s ufficiently low supersaturations, the interface is stable for positive shear and unstable for negative shear. More generally, there is a cri tical negative shear rate for which the interface becomes unstable as the magnitude of the shear rate increases. For a range of growth condi tions for ADP, the magnitude of this critical shear rate is 2k(x) upsi lon(x). Even shear rates due to natural convection may be sufficient t o affect stability for typical growth conditions.