Alejandro Mora

Discrete model for laser driven etching and microstructuring of metallic surfaces.

We present a unidimensional discrete solid-on-solid model evolving in time using a kinetic Monte Carlo method to simulate microstructuring of kerfs on metallic surfaces by means of laser-induced jet-chemical etching. The precise control of the passivation layer achieved by this technique is responsible for the high resolution of the structures. However, within a certain range of experimental parameters, the microstructuring of kerfs on stainless steel surfaces with a solution of H3PO4 shows periodic ripples, which are considered to originate from intrinsic dynamics. The model mimics a few of the various physical and chemical processes involved and within certain parameter ranges reproduces some morphological aspects of the structures, in particular ripple regimes. We analyze the range of values of laser beam power for the appearance of ripples in both experimental and simulated kerfs. The discrete model is an extension of one that has been used previously in the context of ion sputtering and is related to a noisy version of the Kuramoto-Sivashinsky equation used extensively in the field of pattern formation.

MINIMIZING STOCHASTICITY IN THE NAO INDEX

We show that the monthly North Atlantic Oscillation index routinely used in climatology as an indicator for global climate variability is not an optimal choice. A critical Markov analysis of the two pressure time-series for both, monthly and daily NAO index, indicates that the monthly index, due to its low sampling rate, contains higher stochastic terms than the daily index. Applying a recently developed variationally optimized Markov analysis leads to a new NAO index with minimal stochasticity.

Selforganized Structure Formation in Organized Microstructuring by Laser-Jet Etching

Laser-jet induced wet etching of stainless steel in 5M H3PO4 has been investigated. By this method, it is possible to cut and microstructure metals and alloys that form passive layers in strong etchants. Due to the laser heating of the metal and the adjacent layers of the etchant, the passive layer is removed and an active dissolution of the base metal together with the formation of hydrogen is observed. The reactions are limited by the transport of fresh acid and the removal of dissolved metal. A jet of etchant reduces the transport limitations. For definite ranges of the laser power, the feed velocity and the etchant jet velocity, a regime of periodic structure formation of the kerf, often called ripples, has been found. The ripple length depends on all three parameters. The ripple formation can be brought into correlation with a periodic change of the intensity of the reflected light as well as oscillations of the potential workpiece. It could be shown that the periodic structure formation is connected...

A simple discrete stochastic model for laser-induced jet-chemical etching

Recently developed processes based on laser-induced liquid jet-chemical etching provide efficient methods for high resolution microstructuring of metals. Like in other abrasive techniques (water-jet cutting, laser cutting, ion sputtering etc.) a spontaneous formation of ripples in the surface morphology has been observed depending upon the choice of system parameters. In this paper we present a discrete stochastic model describing the joint action of removal of material by chemical etching and thermally activated diffusion initiated by a moving laser leading to structure formation of a surface. Depending on scan speed and laser power different surface morphologies are observed ranging from rough surface structures to the formation of ripples. The continuum equation associated to the discrete model is shown to be a modified Kuramoto-Sivashinsky equation in a frame comoving with the laser beam. Fourier and wavelet techniques as well as large deviation spectra are used for a characterization of the surfaces.