In situ nanoscale observations of gypsum dissolution by digital holographic microscopy

Abstract

Recent topography measurements of gypsum dissolution have not reported the absolute dissolution rates, but instead focus on the rates of formation and growth of etch pits. In this study, the in situ absolute retreat rates of gypsum (010) cleavage surfaces at etch pits, at cleavage steps, and at apparently defect-free portions of the surface are measured in flowing water by reflection digital holographic microscopy. Observations made on randomly sampled fields of view on seven different cleavage surfaces reveal a range of local dissolution rates, the local rate being determined by the topographical features at which material is removed. Four characteristic types of topographical activity are observed: 1) smooth regions, free of etch pits or other noticeable defects, where dissolution rates are relatively low; 2) shallow, wide etch pits bounded by faceted walls which grow gradually at rates somewhat greater than in smooth regions; 3) narrow, deep etch pits which form and grow throughout the observation period at rates that exceed those at the shallow etch pits; and 4) relatively few, submicrometer cleavage steps which move in a wave-like manner and yield local dissolution fluxes that are about five times greater than at etch pits. Molar dissolution rates at all topographical features except submicrometer steps can be aggregated into a continuous, mildly bimodal distribution with a mean of 3.0 µmolm−2 s−1 and a standard deviation of 0.7 µmolm−2 s−1.

Phase Uncertainty in Digital Holographic Microscopy Measurements in the Presence of Solution Flow Conditions

“…Recent research at NIST is utilizing the DHM to quantify mineral dissolution fluxes by observing in situ changes in surface topography over time [13,14], similar to what is performed in geochemical studies by measurements with vertical scanning interferometry (e.g., [54][55][56]). Through these measurements, the surface-normal dissolution flux, ks, is computed based on the surface-normal dissolution velocity, vs, which is simply the change in height over time of a pixel or set of pixels, Δh/Δt, and the molar volume, Vm [54]:…”

“…Though discussed for dissolution (i.e., when vs < 0 and ks < 0), since that was the primary focus of the recent studies [13,14], Eq. (7) applies equally to precipitation or growth studies (i.e., when vs > 0 and ks > 0).…”

“…Note that in these experiments, an inert reference mask was partially applied to the surface to serve as a reference height offset, so that relative measurements were determined. In experiments where only one reaction is occurring (i.e., only dissolution, precipitation, or growth), spatial averaging can be performed, which can provide details on the spatial variability of fluxes [13,14] and can also reduce the effects of noise [22,32]. Spatial averaging is conducted by averaging over an ROI and evaluating the mean change in height over time, so vs essentially becomes a mean height change over time rather than a height change per pixel over time.…”

“…It is a quantitative phase microscopy technique capable of being configured for reflection or transmission microscopy, and it has been used for a variety of applications [1], including static and dynamic surface metrology [2][3][4][5][6][7], particle tracking [8,9], tracking and monitoring of live biological cells [3,[10][11][12], and monitoring surface dissolution [13,14] or growth [15] kinetics. DHM and related quantitative phase imaging technologies have been extensively developed for the study of biological specimens [16][17][18].…”

“…However, one complication of the technique is the presence of inherent (intrinsic and extrinsic) noise [23][24][25][26][27][28][29], which has not undergone a rigorous analysis to relate it to uncertainty in the measured phase (and therefore height) data. Furthermore, recent studies have utilized reflection DHM 1 to track in situ surface topography changes of dissolving mineral phases in static and flowing water [13,14], but it is inconclusive from those studies how the presence of water (or any solution) or the use of flowing water conditions affects the noise and uncertainty in the measurement. Therefore, the objective of the present study is to further quantify the uncertainty in such measurements in order to better understand the limitations of the instrument, sampling statistics, and minimum measurable surface topography changes.…”