Wet chemical etching of single-bore microstructured silicon dioxide fibers

Abstract

We model the process of wet chemical etching of the external surface of a single-bore microstructured silicon dioxide fiber in hydrofluoric acid (HFA) while water is pumped through the internal channel to prevent etching of it. The model uses the Stokes flow for the velocity throughout the system and the advection–diffusion equation for the concentration of HFA. We determine the etch rate as a function of HFA concentration using data from experiments designed for this purpose, from which we calculate the change in the fiber surface. We solve our equations using a time-stepping finite-element method and verify our model by comparing to results found experimentally. We investigate the effects of different water flow rates, diffusivity, buoyancy, and bore radius. We find the water being pumped through the bore does not fully protect it and there is some etching of the internal channel, which is difficult to see in experimental images. We also obtain an estimate of the diffusivity of high-concentration HFA in water.