13 Towards Measuring Intergranular Force Transmission Using Confocal Microscopy and Digital Volume Correlation 87 Fig. 13.2 2D projections of an image stack of (a) polyacrylamide specimen and (b) granular specimen Fig. 13.3 Displacement component u1 for subset sizes (a) [128,128 32] and (b) [32 32 32], (c) correlation coefficient map The model granular material shows great promise for image and volume correlation methods using fluorescence confocal microscopy imaging methods. As seen in Fig. 13.2b, individual fluorescent microspheres show strong speckle patterns allowing for both particle tracking and analysis of individual particles. Figure 13.3a, b show that correlation subset size greatly affects the displacement results for DVC analysis due to the large regions of black voxels between individual particles, which are more difficult to correlate. Partitioning of images will allow for greater control over the correlation, as indicated by the correlation coefficient plot in Fig. 13.3c. Both Figs. 13.2b and 13.3 show one of the major drawbacks to using optical microscopy methods: the depth of view is limited by the optical transparency of the specimen. Particles composed of a more optically transparent medium, lower fluorescent dye intensity, and use of an optically compatible liquid between particles can help to eliminate these effects.
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