D.A.M. de Winter

Frictional properties and microstructure of calcite-rich fault gouges sheared at sub-seismic sliding velocities

We report an experimental and microstructural study of the frictional properties of simulated fault gouges prepared from natural limestone (96xa0% CaCO3) and pure calcite. Our experiments consisted of direct shear tests performed, under dry and wet conditions, at an effective normal stress of 50xa0MPa, at 18–150xa0°C and sliding velocities of 0.1–10xa0μm/s. Wet experiments used a pore water pressure of 10xa0MPa. Wet gouges typically showed a lower steady-state frictional strength (μxa0=xa00.6) than dry gouges (μxa0=xa00.7–0.8), particularly in the case of the pure calcite samples. All runs showed a transition from stable velocity strengthening to (potentially) unstable velocity weakening slip above 80–100xa0°C. All recovered samples showed patchy, mirror-like surfaces marking boundary shear planes. Optical study of sections cut normal to the shear plane and parallel to the shear direction showed both boundary and inclined shear bands, characterized by extreme grain comminution and a crystallographic preferred orientation. Cross-sections of boundary shears, cut normal to the shear direction using focused ion beam—SEM, from pure calcite gouges sheared at 18 and 150xa0°C, revealed dense arrays of rounded, ~0.3xa0μm-sized particles in the shear band core. Transmission electron microscopy showed that these particles consist of 5–20xa0nm sized calcite nanocrystals. All samples showed evidence for cataclasis and crystal plasticity. Comparing our results with previous models for gouge friction, we suggest that frictional behaviour was controlled by competition between crystal plastic and granular flow processes active in the shear bands, with water facilitating pressure solution, subcritical cracking and intergranular lubrication. Our data have important implications for the depth of the seismogenic zone in tectonically active limestone terrains. Contrary to recent claims, our data also demonstrate that nanocrystalline mirror-like slip surfaces in calcite(-rich) faults are not necessarily indicative of seismic slip rates.

Focused ion beam‐scanning electron microscope: exploring large volumes of atherosclerotic tissue

Atherogenesis is a pathological condition in which changes in the ultrastructure and in the localization of proteins occur within the vasculature during all stages of the disease. To gain insight in those changes, high‐resolution imaging is necessary. Some of these changes will only be present in a small number of cells, positioned in a ‘sea’ of non‐affected cells. To localize this relatively small number of cells, there is a need to first navigate through a large area of the sample and subsequently zoom in onto the area of interest. This approach enables the study of specific cells within their in vivo environment and enables the study of (possible) interactions of these cells with their surrounding cells/environment. The study of a sample in a correlative way using light and electron microscopy is a promising approach to achieve this; however, it is very laborious and additional ultrastructural techniques might be very valuable to find the places of interest.

Gridded Aclar: preparation methods and use for correlative light and electron microscopy of cell monolayers, by TEM and FIB–SEM

Aclar, a copolymer film with properties very similar to those of tissue culture plastic, is a versatile substrate to grow cells for light (including fluorescence) and electron microscopic applications in combination with both chemical fixation and cryoimmobilization. In this paper, we describe complete procedures to perform correlative light and electron microscopy using Aclar as substrate for the culture of cell monolayers to be finally embedded in plastic. First, we developed straightforward, efficient and flexible ways to mark the surface of the Aclar to create substrates to locate cells first at the light microscopy and then the electron microscopy level. All the methods enable the user to self‐design gridded Aclar pieces, according to the purpose of the experiments, and create a large number of substrates in a short time. Second, we confirmed that marked Aclar supports the normal growth and morphology of cells. Third, we validated the correlative light and electron microscopy procedure using Aclar. This validation was done for the high‐resolution analysis of endothelial cells using transmission electron microscopy and focused ion beam–scanning electron microscopy in combination with the use of fluorescence, phase contrast and/or bright field microscopy to map areas of interest at low resolution. The methods that we present are diverse, easy to implement and highly reproducible, and emphasize the versatility of Aclar as a cell growth substrate for diverse microscopic applications.

Cryogenic EBSD on ice: preserving a stable surface in a low pressure SEM

Naturally deformed ice contains subgrains with characteristic geometries that have recently been identified in etched surfaces using high‐resolution light microscopy (LM). The probable slip systems responsible for these subgrain boundary types can be determined using electron backscattered diffraction (EBSD), providing the etch features imaged with reflected LM can be retained during EBSD data acquisition in a scanning electron microscope (SEM). Retention of the etch features requires that the ice surface is stable. Depending on the pressure and temperature, sublimation of ice can occur. The equilibrium temperature for a low pressure SEM operating at 1 × 10−6 hPa is about −112°C and operating at higher temperatures causes sublimation. Although charging of uncoated ice samples is reduced by sublimation, important information contained in the etch features are removed as the surface sublimes. We developed a method for collecting EBSD data on stable ice surfaces in a low pressure SEM. We found that operating at temperatures of <–112°C reduced sublimation so that the original etch surface features were retained. Charging, which occurred at low pressures (<1.5 × 10−6 to 2.8 × 10−5 hPa) was reduced by defocusing the beam. At very low pressures (<1.5 × 10−6 hPa) the spatial resolution with a defocused beam at 10 kV was about 3 μm in the x‐direction at −150°C and 0.5 μm at −120°C, because at higher temperature charging was less and only a small defocus was needed to compensate it. Angular resolution was better than 0.7° after orientation averaging. Excellent agreement was obtained between LM etch features and EBSD mapped microstructures. First results are shown, which indicate subgrain boundary types comprised of basal (tilt and twist) and nonbasal dislocations (tilt boundaries).

FIB-SEM cathodoluminescence tomography: practical and theoretical considerations

Focused ion beam–scanning electron microscope (FIB‐SEM) tomography is a powerful application in obtaining three‐dimensional (3D) information. The FIB creates a cross section and subsequently removes thin slices. The SEM takes images using secondary or backscattered electrons, or maps every slice using X‐rays and/or electron backscatter diffraction patterns. The objective of this study is to assess the possibilities of combining FIB‐SEM tomography with cathodoluminescence (CL) imaging. The intensity of CL emission is related to variations in defect or impurity concentrations. A potential problem with FIB‐SEM CL tomography is that ion milling may change the defect state of the material and the CL emission. In addition the conventional tilted sample geometry used in FIB‐SEM tomography is not compatible with conventional CL detectors.

Microstructure of the Shaly Facies of Opalinus Clay on the Mm-nm Scale

Opalinus Clay is one of the fine-grained sedimentary formations investigated as a possible geological repository for the long-term storage of radioactive waste. Porosity, pore size, pore shape and connectivity of pores will help to define sealing capacities of a host rock. Here a combination of FIB-SEM, BIB-SEM and µ-CT has been used in order to investigate the nm-mm scale microstructure of the Shaly facies of Opalinus Clay in 2D and 3D. The µ-CT measurements gave a 3D overview of the microstructure of a 2 mm core down to a 2.6 µm voxel size. Which showed cracks, larger bivalve fragments and pyrite nodules embedded in a matrix, which was investigated down to the nm scale (enabling to image the pores) in 2D using the BIB-SEM method. FIB-SEM measurements were afterwards performed on selected areas to investigate the pore shape and connectivity in 3D down to the nm scale.