New microfluidic explosive detectors, an all-fluidic logic family based on bubbles, and robots that mimic the movement of snails, slugs, and jellyfish were among the many fascinating highlights of the 2006 fall meeting of the APS Division of Fluid Dynamics (DFD), held November 19-21 at the University of Florida in Tampa Bay. The conference also featured a special US-Mexico Mini-Symposium on geophysical fluid dynamics, as well as mini-symposia on quantum turbulence, connections between fluid dynamics and plasma physics, and education.
Much Ado About Microfluidics.
Saffman-Taylor instability in a Hele-Shaw cell. This image was a winner in the 2004 Gallery of Fluid Motion.
A collaboration between scientists at Philips Research and Eindhoven University of Technology has developed “artificial cilia”: polymer micro-actuator devices, made with standard micro-technology processing techniques, which respond to an applied electrical or magnetic field by changing their shape. The size and shape of the polymer actuators mimics that of the beating cilia covering the external surface of micro-organisms such as paramecium. The team hopes to eventually apply this new method of microfluidic actuation to the building of biosensors.
At the same session, researchers from Stanford University reported on their development of a novel microfluidic, remote-sensing chemical detection platform for real-time sensing of airborne explosive agents. The key enabling technology is a newly developed concept termed Free-Surface Fluidics (FSF), where one or more fluidic surfaces, confined by surface tension forces, are exposed to the surrounding atmosphere. Combining the FSF architecture with surface-enhanced Raman spectroscopy allows real-time profiling of atmospheric species and detection of airborne agents–most notably of 4-aminobenzenethiol, a chemical species similar in size and structure to TNT. Bubble Logic.
MIT’s Neil Gershenfel and Manu Prakash presented their concept of microfluidic bubble logic: specifically, a new all-fluidic logic family based on two-phase flow in micro-scale geometries that exploits hydrodynamic interactions as a primary mechanism to introduce non-linearity. For instance, the presence or absence of a bubble would represent a bit, so a bubble could carry both information and a material payload at the same time. The researchers presented rudimentary microfluidic bubble logic gates (AND/OR/NOT), memory, and cascaded boolean circuits, which they believe could one day be applied as a control scheme to large-scale integrate biochemical processors.
Controlling bubbles is a critical aspect of many applications involving fluid systems, such as ink-jet printing. Researchers continually seek to improve their understanding of bubble formation and interactions to better control such systems. Detlef Lohse of the University of Twente in The Netherlands reported on a new technique that enables him to quantitatively study bubble-bubble and bubble-surface interactions. He found that in piezo-acoustic ink-jet printing, bubbles can become entrained, grow by rectified diffusion and eventually seriously disturb the jetting process by counteracting the pressure build-up at the nozzle. He also demonstrated that bubble nucleation at surfaces–commonly associated with randomness–can in fact be perfectly controlled in both space and time. Locomotive Robosnails
. Snails and slugs have a very energy-efficient means of adhesive locomotion, producing muscular waves of shear stress on a viscoelastic mucus to propel themselves along a solid substrate. MIT’s Anette Hosoi described recent work on applying a simple mechanical model to derive criteria for favorable fluid material properties to lower the energetic cost of locomotion. She and her MIT colleagues, Brian Chan and Theresa Guo, have designed robotic machines–dubbed “Robosnail 1” and “Robosnail 2”–that use a waving foot to propel themselves over viscous fluid. Robosnail 2 can climb walls and move upside down on a layer of Carbopol, a gel-like water-based polymer solution. They presented new 3D modeling of finite-width snails and a design for future snails capable of moving faster than their own waving velocity.
Drawing on similar locomotive examples in nature, scientists from Tokyo University proposed building a micro-robot out of soft material to resemble a jellyfish. The robot would propel itself much like its biological counterpart, so a greater understanding of the creature’s swimming motion is desired –particularly how it produces thrust in its “expanding phase” of its swimming motion. The Tokyo team studied those motions via a motion-capture camera and measured the vector field of flow around the jellyfish. It is known that the jellyfish is principally propelled by a vortex ring ejected at the contracting phase, and the researchers found that a similar vortex ring with an opposite vorticity seems to be at work in the expanding phase. Booming Sand Dunes.
For centuries desert explorers have heard the
Photo by Melany Hunt, Caltech
Booming sand dunes
booming sounds of the desert–low frequency sustained tones that accompany the avalanching of sand on large dunes. These desert travelers, including Marco Polo, attributed the sounds to beating drums or harps, voices of spirits, lost horsemen or other superstitions. Melany Hunt of Caltech discussed her recent work involving field and laboratory measurements of the booming sound at several locations and on different days as part of the US-Mexico mini-symposium on geophysical fluid dynamics.
It is not a noise composed of many frequencies but instead contains a dominant audible frequency and several higher harmonics. The sound can be heard after a naturally occurring slumping event or triggered by forcing sand down the leeward face of a large dune. In the later case, the dune will continue to boom and vibrate even after the sand has visibly stopped moving. Hunt’s field measurements show that the frequency ranges from 75 to 110 Hz depending on the desert location and time of the year. Her measurements suggest that the physical features (such as a moisture barrier) of the sand dune plus the characteristics of the shearing on the surface may contribute to a wave-guide phenomena that results in a resonate behavior at a characteristic frequency.
Also featured at the US-Mexico mini-symposium was a talk on granular flows in volcanic environments –multi-phase system flows that involve some combination of solid, liquid and air in response to applied shear stress–by Lucia Capra (National University of Mexico, Juriquilla), drawing on examples from several active Mexican volcanoes. Other UNAM scientists reported on their proposal to flush a polluted lagoon in Cancun using a wave and tide driven seawater pump. They believe their approach could improve the lagoon’s natural “flushing time” form two to four years, down to six months, so the ecosystem could better cope with the large amount of waste and thick layer of accumulated organic matter on the lagoon bed–the result of decades of Cancun’s thriving tourist industry. Reading Einstein’s Tea Leaves.
Among Albert Einstein’s lesser-known interests was his paradoxical observation of tea leaves centrally accumulating at the base of a stirred teacup. A team of scientists from Monash University has applied this basic concept to the fluid flow patterns observed when they applied a voltage to a sharp electrode tip above the liquid surface of a microfluidic chamber. This generates an electrohydrodynamic air thrust that shears the liquid surface and induces liquid recirculation. The recirculation sweeps colloidal particles suspended within the liquid in a helical swirling motion and deposits them at a stagnation point located centrally at the bottom of the chamber. The scientists believe the phenomenon can be exploited for bioparticle trapping and concentration. At the DFD meeting, they demonstrated the rapid separation of red blood cells from blood plasma, for possible application in miniaturized blood diagnostic kits. Inside a Bamboo Flute.
Wind instruments produce sound from the vibration of the air flow inside the wind instrument. Trumpets or clarinets use a mouth or reed to produce variable sounds, but there is no mechanical vibration mechanism in a flute. A team of researchers at the University of Tokyo are working to measure in greater detail the air flow and vibration inside and outside the flute to improve the manufacture of quality instruments. They used a traditional Japanese bamboo flute in their experiments. First they measured the argon gas flow at 5000 Hz using a high-frequency pulse laser, employing oil mist as tracer particles. They also tried to measure the flow when a human played the instrument using a CW laser. They succeeded n measuring the oscillating flow, finding that near the hole of the bamboo flute, the air went out from ad came into the instrument at about 500 Hz depending on the tone.