Analysis of empirical data on the tumbling of microrods in a shear

Abstract

This thesis builds upon an existing experiment [1] investigating the tumbling of microrods in a shear flow. The orientational dynamics of microrods in simple shear flows and in turbulent flows is a subject of great importance. This is because the orientational dynamics of particles strongly affects the bulk properties of the suspension [2]. There is now a large number of studies theoretically investigating the tumbling of microrods in flows [2, 3, 4, 5, 6], but there comparatively little experimental data [1, 7, 8]. The laboratory setup [1] allows for recording of large amounts of data for these rods in the form of grayscale movies at high framerates. The primary aim of this thesis is to analyse this data and to compare the results to existing theory. There are three observables in this experiment, the projected position of the rod in the channel, the projected length of the rod, and the projected orientation vector of the rod, all of these in the image plane of the recorded movies. The recorded movies are analysed using Matlab [9]. Each frame is analysed and the Canny edge-detection algorithm [10] is used to find the particle edges. The particle edges are subjected to an elliptic fit in order to approximate the position, orientation and length of the projected particle with the center, orientation, and length of the fitted ellipse. These properties are translated into the three dimensional components of the rod orientation vector n(t). Computer simulations of the equations of motion in the form given in [2, 4] were performed. The results are compared to the trajectories observed in the experiment. There is qualitative agreement between the experimental and the theoretical results.