Instabilities of stressed crystal surfaces in contact with a fluid

1. Instabilities of stressed crystal surfaces in contact with a fluid Evolution of surface structures AGU fall meeting 2002, paper no. NG12C-1039 We present three different experiments. All experiments were performed with NaClO3 crystals of height 3 to 4 mm, width 2 to 3 mm and thickness 2 mm in a saturated solution of NaClO3 at room temperature, 22±0.001°C. The crystals are stressed vertically with a piston. Experiment one takes place at the highest stress of about 8 MPa (a to i) with a crystal that has two notches on the sides. Experiment two takes place at a lower stress of about 4 MPa (j to l) and experiment three takes place without stress (m). The experiments take one to two weeks. In experiment one we lower the crystal in the saturated solution and leave it unstressed for 24 hours. During these first 24 hours we can observe that surface tension smoothens the edges of the crystal. After 24 hours (time 0.0 in a) the crystal is loaded vertically where upon surface patterns start to evolve. The evolution of patterns on the crystal surface takes place in three different stages. 1. The onset of the ATG-instability with mostly parallel and horizontal grooves on the crystal surface. 2. Upwards travel of grooves on the crystal surface and coarsening of the pattern. 3. One large groove travels upwards across the crystal surface leaving the surface flat again. In experiment two we use a crystal without notches and reduce the stress to 4 MPa (j to l). The general evolution of patterns in experiment one and two are similar. The main difference to experiment one is that the crystal in experiment two seems to develop two wavelengths, one small with grooves on the surface and one on the scale of the crystal. Experiment three takes place without stress (m). After a few days small waves travel up the crystal and it develops perfect crystal facets. Daniel Koehn*, Dag Kristian Dysthe, Bjørn Jamtveit Physics of Geological Processes, University of Oslo, P.O.box 1048 Blindern, N-0316 Oslo, Norway *Tectonophysics, Department of Geology, University of Mainz, Mainz, D Germany We present an experimental investigation on the dissolution of single stressed crystals of NaClO3 in contact with a reactive fluid. The crystals are immersed in a saturated fluid, stressed vertically by a piston and monitored constantly in situ with a CCD camera. The experiment is temperature-controlled and the dilation of the sample is measured with a capacitance dilatometer. Once the crystal is stressed it develops a roughness on its free surface with an almost constant wavelength in accordance to the Assaro-Tiller-Grinfeld [1,2] instability (ATG). The initial roughness is composed of parallel dissolution grooves perpendicular to the shortening direction. We observe for the first time a transient evolution of this roughness. The structures are not stable but grooves on the crystal surface start to migrate upwards (against gravity), grow in size and repel each other. This secondary instability results in a coarsening of the pattern, which switches from a one-dimensional geometry of parallel grooves to a two-dimensional geometry with horizontal and vertical grooves. At the end of the experiment one large groove travels across the crystal and the surface becomes flat again. Pressure solution creep between the piston and the top of the crystal only plays a role in the very beginning of the experiment. . Our experiments suggest that the crystal-fluid system is driven out of equilibrium when the crystal is stressed and that the system goes through transient processes involving surface energy and elastic energy effects to come back to a new equilibrium under stress. During our experiments the processes operating on the free surface seem to be faster and of greater importance than pressure solution at the top of the crystal. Transient patterns under different stress conditions Schematic summary of the surface patterns in the three performed experiments. In all three experiments one can observe short-term surface energy effects that round the crystal edges and long-term surface energy effects where waves travel up the crystal and the crystal becomes flat. The long-term effect of the surface energy seems to couple with the stress induced grooves once the roughness has coarsened up to the same scale as the surface energy waves in the range of the size of the crystal. The developing patterns on the crystal surface are a transition towards a new equilibrium of the system under stress. The patterns are more pronounced if the system is driven further out of equilibrium i.e. if the stress on the crystal is higher. Coarsening of the roughness Traveling of grooves Pressure Solution Creep The coarsening of the wavelength of the roughness is recorded for the first 90 hours of experiment one. During this time the pattern coarsens progressively with an almost constant velocity of about 9.1±0.4 mm per hour. A coarsening with the same velocity would lead to the wavelength spanning the crystal (3mm) after 300 hours. This fits well with the final evolution of the crystal surface where the grooves disappear and the surface becomes flat. Traveling of grooves is probably due to concentration gradients in the fluid that develop as an effect of gravity, higher concentration (and density) at the bottom of the cell. The upper edge of a groove on the crystal surface will therefore tend to dissolve and the lower edge will tend to grow. This effect will result in a velocity of grooves along the concentration gradient in the fluid towards the top of the crystal. Above the velocity of the superstructure in experiment one is shown. It travels in cycles with a period of about 25h and an amplitude of about 8mm/h except when the front crosses the notches on the middle of the crystal. Vertical shortening of the crystal versus time measured with the capacitance dilatometer. The first 6 hours the crystal is shortened by pressure solution creep that decays exponentially with time. Then there is a jump of 7 mm in crystal height which reinitiates the decay rate. The inset shows that there is no long time creep. References: [1] R.J. Asaro and W.A. Tiller, Metall. Trans. 3, 1789 (1972) [2] M.A. Grinfeld, Sov. Phys. Dokl. 31, (1986) Electronic address: koehn @mail.uni-mainz.de