In natural sedimentation, many particles of interest are both large and nonspherical. Some common particle types (e.g. naturally occurring aggregates) do not have a uniform mass distribution. As a result, the centers of mass and buoyancy are not co-located, leading to complex settling dynamics. Here we investigated the orientation and terminal velocity of initially horizontal, freely falling cylinders, in which the mass distribution was either constant (uniform-density, UD) or bipartite, undergoing a step change halfway along the length (compound-density, CD). Cylinders had relatively low aspect ratios (1AR4), and fell at intermediate Reynolds numbers (of order 100). We recorded the velocity, orientation, and landing site of each cylinder in quiescent flow. Results showed significant differences between the settling characteristics of uniform- and. compound-density cylinders. For cylinders with AR=1, varying center of mass location (CD cylinders) led to a more predictable fall trajectory and orientation compared to UD cylinders. Varying the center of mass location for AR=2 cylinders resulted in two different settling modes, demarcated by a transitional Reynolds number regime. For AR=4 cylinders, varying the center of mass location did not produce as large an effect, but did lead to an angled settling trajectory with a pronounced off-horizontal orientation relative to the UD cylinders. All CD cylinders, regardless of aspect ratio, were biased to land on the side of the tank where the more-dense end of the cylinder was initially oriented. In general, cylinders with the smallest vertical projected area fell with the greatest terminal velocity; however, the mechanisms controlling orientation remain unclear. Our results have important implications for predicting the settling behavior of naturally-occurring particles, and lay the groundwork for further study of particles settling in complex flows such as turbulence. Given our results in still water, the net torque created by non-co-located center of mass and center of volume are likely to strongly impact particle motion in turbulence.

This open access book, published in the Soft and Biological Matter series, presents an introduction to selected research topics in the broad field of flowing matter, including the dynamics of fluids with a complex internal structure -from nematic fluids to soft glasses- as well as active matter and turbulent phenomena. Flowing matter is a subject at the crossroads between physics, mathematics, chemistry, engineering, biology and earth sciences, and relies on a multidisciplinary approach to describe the emergence of the macroscopic behaviours in a system from the coordinated dynamics of its microscopic constituents. Depending on the microscopic interactions, an assembly of molecules or of mesoscopic particles can flow like a simple Newtonian fluid, deform elastically like a solid or behave in a complex manner. When the internal constituents are active, as for biological entities, one generally observes complex large-scale collective motions. Phenomenology is further complicated by the invariable tendency of fluids to display chaos at the large scales or when stirred strongly enough. This volume presents several research topics that address these phenomena encompassing the traditional micro-, meso-, and macro-scales descriptions, and contributes to our understanding of the fundamentals of flowing matter. This book is the legacy of the COST Action MP1305 “Flowing Matter”.

This book provides the reader with an introduction to the physics of complex plasmas, a discussion of the specific scientific and technical challenges they present and an overview of their potential technological applications. Complex plasmas differ from conventional high-temperature plasmas in several ways: they may contain additional species, including nano meter- to micrometer-sized particles, negative ions, molecules and radicals and they may exhibit strong correlations or quantum effects. This book introduces the classical and quantum mechanical approaches used to describe and simulate complex plasmas. It also covers some key experimental techniques used in the analysis of these plasmas, including calorimetric probe methods, IR absorption techniques and X-ray absorption spectroscopy. The final part of the book reviews the emerging applications of microcavity and microchannel plasmas, the synthesis and assembly of nanomaterials through plasma electrochemistry, the large-scale generation of ozone using microplasmas and novel applications of atmospheric-pressure non-thermal plasmas in dentistry. Going beyond the scope of traditional plasma texts, the presentation is very well suited for senior undergraduate, graduate students and postdoctoral researchers specializing in plasma physics.