27 research outputs found
Can phoretic particles swim in two dimensions?
Artificial phoretic particles swim using self-generated gradients in chemical species (self-diffusiophoresis) or charges and currents (self-electrophoresis). These particles can be used to study the physics of collective motion in active matter and might have promising applications in bioengineering. In the case of self-diffusiophoresis, the classical physical model relies on a steady solution of the diffusion equation, from which chemical gradients, phoretic flows, and ultimately the swimming velocity may be derived. Motivated by disk-shaped particles in thin films and under confinement, we examine the extension to two dimensions. Because the two-dimensional diffusion equation lacks a steady state with the correct boundary conditions, Laplace transforms must be used to study the long-time behavior of the problem and determine the swimming velocity. For fixed chemical fluxes on the particle surface, we find that the swimming velocity ultimately always decays logarithmically in time. In the case of finite Péclet numbers, we solve the full advection-diffusion equation numerically and show that this decay can be avoided by the particle moving to regions of unconsumed reactant. Finite advection thus regularizes the two-dimensional phoretic problem.The research was supported by NSF Grants DMS-1109315 and DMS-1147523 (Madison) and by the European Union through a CIG grant (Cambridge)
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Use of Louvers For Guiding Fish
Late in 1951, the U. S. Bureau of Reclamation completed construction of the world\u27s second largest pumping plant at Tracy, California. In connection with this project, it was found that facilities would have to be installed to prevent fish of several species and as short as 10 millimeters from passing into the pump intakes.
Because of the magnitude of the project and lack of knowledge as to the best protective facility, a test structure with several types of fish screens was installed. Fish stopped by the screens were collected by a port and riser system. Tests revealed that none of these facilities adequately met fish protection requirements at Tracy. It was therefore necessary to consider other methods. Research by members of a team from the U. S. Fish and Wildlife Service and the Bureau of Reclamation developed a louver system for guiding fish.
The louver system consists of a fence-like series of vertical steel slats set with their broad faces at right-angles to the direction of flow and placed across the channel in a diagonal line terminating at a narrow by-pass. When fish approach the louver slats, particularly when water velocities are high, they tend to avoid them and swim away at an angle of about 90 degrees. In maintaining this position, they place themselves somewhat broadside to the approach flow and are carried downstream, through the by-pass and either into the main stream or a holding pond.
Efficiency studies of louvers in a test flume and in the pilot structure indicated that striped bass, salmon, shad, catfish, and other species can be successfully diverted and collected by this system