Laser scanning confocal microscopy and fluorescence microscopy - New insights in granular structures and mechanics

Abstract:  Granular matter, also known as bulk solids, consist of discrete particles with sizes between micrometers and meters. When handling granular matter in industries or process engineering, the relation between single particle motion and macroscopic behavior is still not well understood and experimentally challenging. For exploring the microscopic properties on a single particle level, 3D imaging techniques are required. Laser scanning confocal microscopy of fluorescently labelled particles is a versatile method getting insights into 3D structures of bulk solids. To satisfy the requirements for confocal microscopy, new granular model systems in the wet and dry state were designed and prepared. In this work two different model systems, one for dry and one for wet granulates, were examined in 3D while a mechanical load was applied. In dry granulates the particle rotation plays a crucial role, when the bulk solid is sheared. To explore their entire particle motion with all degrees of freedom, a technique to visualize the rotation of spherical micrometer sized particles in 3D was developed. In wet granulates a binding liquid drives the local bulk structure and also the bulk mechanics. To examine the 3D structure of the binding liquid on the micrometer scale independently from the particles, the existing confocal microscope setup was further developed. A second illumination and detection beam path for the simultaneous observation of both phases was implemented. Confocal microscopy in combination with nanoindentation gave new insights into the single particle motions and dynamics of granular systems under a mechanical load. These novel experimental results can help to improve the understanding of the relationship between bulk properties of granular matter, such as volume fraction or yield stress and the dynamics on a single particle level.