Physicists specializing in fluid dynamics research are interested in a wide range of problems, from the morphodynamics of rivers and microfluidic arrays to the collective swimming patterns of microorganisms and how certain insects can walk on water. Those were just a few of the technical highlights at the annual meeting of the APS Division of Fluid Dynamics, held November 18-20 in Salt Lake City, Utah.
Microfluidic Shock Waves. An important technique for micrlofluidic lab-on-a-chip assays is capillary electrophoresis, and researchers are always seeking to improve the sensitivity of this technique. The best way to do this is to include an online sample preconcentration method. At Stanford University, Juan Santiago and his colleagues are developing methods to concentrate ions into small volumes using isotachophoresis (ITP), in which sample ions are injected between the high-mobility co-ions of a leading electrolye and the low mobility co-ions of a trailing electrolyte.
Applying an electric field causes sample species to segregate and focus into a series of narrow zones. “We use ITP to create sample ion concentration ‘shock waves’ in microchannels,” says Santiago. “These concentration waves can be integrated with on-chip electrophoresis for high-sensitivity assays.” The ultimate goal is to develop novel on-chip ITP assays which expand the design space of microfluidic devices.
Flowing Like a River. The flow of water over sediment or bedrock can create a wide range of beautiful patterns, from dunes and sand bars to alluvial fans and canyons. According to Gary Parker (University of Illinois), the key to the formation of these morphologies is an interaction between the fluid flow and the erodible boundary of the sediment and bedrock. “The flow changes the boundary via differential erosion/deposition, and the boundary changes the flow by offering a modified bed boundary condition,” he says.
Parker studied turbidity currents: bottom-hugging currents driven by the presence of sediment in suspension, which makes the water in the flow heavier than the ambient water. By simplifying the fluid mechanics by ignoring all temporal terms except the one describing the evolution of the boundary, he found that a single mathematical formulation provides an explanation of the features formed by swift fluid flow in mountain bedrock streams, gullies, steep alluvial river flows, and in the deep ocean.
A Bug’s Life. Researchers in MIT’s Department of Mathematics are looking to the world of arthropods–insects and spiders –for insights into better, biologically-inspired approaches to water repellency and fluid transport on a very small scale. MIT’s John Bush described his group’s work on water-walking arthropods and their ability to survive when submerged by virtue of a thin air bubble wrapped along their rough exteriors. “The diffusion of dissolved oxygen from the water into the bubble allows it to function as an external lung, and enables certain species to remain underwater indefinitely,” he said. They have also explored how such arthropods use their tilted flexible leg hairs to generate thrust, glide, and leap along a free surface, like water.
Swimming With Microorganisms. Since the 1980s, there has been much interest among fluid dynamicists in the collective behavior of swimming microorganisms in suspension. The cells are denser than the water in which they swim, giving rise to unusual bioconvection patterns. Even more interesting structures form in concentrated suspensions of bacteria, for example, and the prevailing hypothesis is that such structures emerge from purely hydrodynamic interactions between cells. Timothy Pedley of the University of Cambridge described one such model “in which cells are represented as inertia-free ‘spherical squirmers,’ whose behavior is dominated by near-field hydrodynamics.”
Fluids in the Classroom. A critical element in introductory fluid mechanics courses is teaching students to realize that mathematical models don’t always model the real world very well. According to Arizona State University’s Ronald Adrian, one effective way to teach them the difference is to have them model a simple experiment, then run it and compare the results of the model with the experimental results. “It would be even better if these experiments were simple enough that students could do them at home, rather than have a canned two-hour lab course,” he says. He is collecting such experiments for use in undergraduate or even K-12 classes, in hopes of building “a community of educators that want to move beyond the traditional mathematical exercises for homework.”