Recently in Fluids & rheology Category

The tunable elasticity and porosity of colloidal gels lead to some interesting applications, among them tissue scaffolding and drug delivery. Conventionally, colloidal particles interact and assemble under entropic and electrostatic forces to form predictable structures. But greater control can be achieved from an approach developed by Paul Clegg, Michael Cates, and their collaborators at the University of Edinburgh in the UK. The researchers disperse silica particles in the single-phase region of two partially miscible solvents—water and the organic base 2,6-lutidine. When the solution is heated above a critical temperature, the solvents separate and the particles become trapped at the liquid–liquid interfaces. The bulky particle domains then jam together and arrest the solvent separation, forming a two-phase network the researchers call a bijel. But cool the solution and remix the solvents too soon and the distinct structure disappears, as shown in movie 1 and the two left images in which the colloids appear green, the water black, and the lutidine red. Now the researchers have discovered an approach to stabilize the bijel structure. When the phase-separated solution is allowed to sit for at least 24 hours before it is cooled, the bijel surprisingly keeps its shape, as shown in the two right images and movie 2. From Monte Carlo simulations, the researchers deduce how the resulting network of colloidal monolayers, or monogel, stays intact: the particles become compressed by capillary forces, remain attracted by van der Waals forces, and are kept from collapsing into each other by repulsive electrostatic forces. (E. Sanz et al., Phys. Rev. Lett., in press.) —Jermey N. A. Matthews

Movie 1

Movie 2

Turbulent flows of a liquid along a surface experience frictional drag, a macroscopic phenomenon that affects the speed and efficiency of marine vessels, the cost of pumping oil through a pipeline, and countless other engineering parameters. The drag arises from shear stress, the rate per unit area of momentum transfer from the flow to the surface. To reduce the flux, engineers could add polymers to the flow, inject bubbles against the surface, or combine the two methods, but those approaches bear their own cost. Jonathan Rothstein and colleagues at the University of Massachusetts Amherst now offer a proof-of-principle demonstration of a new, passive option for reducing drag in turbulent flow. They tailored the microscale structure of a hydrophobic material—polydimethylsiloxane, similar to the rubbery polymer used to caulk bathtubs—to create air pockets at the surface, as shown in the figure, that allow the flow to “slip” (shear free) at the liquid–air interface. The greater the area covered by air pockets, the greater the reduction in shear stress—up to 50%, they estimate, judging from particle-image velocimetry and pressure-drop experiments over a wide range of Reynolds numbers. The researchers found that the critical Reynolds number at which the onset of drag reduction occurs is related to the ratio of two length scales—one associated with the geometry of the hydrophobic surface corrugations, the other with the thickness of the viscous boundary layer there. (R. J. Daniello, N. E. Waterhouse, J. P. Rothstein, Phys. Fluids 21, 085103, 2009.) —R. Mark Wilson

In principle, setting a droplet in motion inside a microfluidic channel is straightforward: Apply pressure and the liquid flows. In practice, however, precise control of droplet flow simultaneously along multiple channels is technically challenging; conventional pressure pumps are not feasible for microfluidic systems. Inspired by the potential of finely tunable acoustic-pressure generators, a group of engineers at the University of Michigan set out to control droplet motion with music. First, they composed a computer-synthesized sequence of single notes and chords. That signal was then sent to four resonance cavities that were tuned according to their lengths to extract and amplify narrow, non-overlapping frequency bands from the input tones. As shown in the figure and the movie, unidirectional droplet flow was generated from the difference between positive air pressure in the oscillating cavity and relative negative pressure at vent ports near the cavity's outlet. Although the relatively high frequencies of the selected tones produced steady flow, the researchers adjusted the relative amplitudes of the input tones as needed to compensate for variations in average flow velocity. Maybe someday, conducting complex lab-on-chip microfluidic operations will be as simple as stringing together a few musical notes. (S. M. Langelier et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900043106.) — Jermey N. A. Matthews

Textbooks depict the Gulf Stream, the Kuroshio, and other great ocean currents as smooth, river-like streams. Reality is messier. Gravitationally bound to the spinning globe, the oceans constitute a complex, turbulent system. How turbulence influences one particular ocean current is revealed in a new field and computational study led by Amy Bower of Woods Hole Oceanographic Institution. In 2003–05 her team released 76 floats off the Newfoundland coast at a rate of about six every three months. Her aim was to trace the southward flow of cold arctic water in the Deep Western Boundary Current (DWBC), which hugs the continental slope of the Eastern Seaboard and eventually meets the Gulf Stream off Cape Hatteras. The floats recorded their courses by triangulating signals from a set of moored sound beacons along the route. Like nuclear submarines, the floats surfaced at the end of their voyage and beamed up their recordings to a satellite. To their surprise, Bower and her colleagues found that only seven floats followed the DWBC's coastal route. Most took a wide, irregular path farther east in the Atlantic interior. The same behavior showed up in the team's simulation of the two-year field study. Emboldened by that resemblance, the researchers simulated a further 13 years of flow and found a richer, more complex pattern than appears in textbooks. Understanding such patterns in the present-day ocean, says Bower, is an essential ingredient for predicting the effects of global warming on Earth's future climate. (A. S. Bower, M. S. Lozier, S. F. Gary, C. W. Böning, Nature 459, 243, 2009.)—Charles Day

Traveling smoothly through a turbulent medium is no mean feat, as anyone who regularly flies in an airplane can attest. Scientists have investigated how fish navigate through turbulent currents, but until recently they had not addressed the analogous issue of animal flight through turbulent air. Now biologist Stacey Combes has filmed male orchid bees (genus Euglossa) flying in turbulent airstreams and, with colleague Robert Dudley, has described the effects of the turbulent air on the bee’s flight stability and maximum speed. Combes induced the bees to fly in a turbulent airstream by luring them with an attractive scent. As the airspeed increased, the bees found it increasingly difficult to avoid the rolling illustrated in the left image. When the airspeed was high enough and maintaining stable flight difficult enough, the bees extended their hind legs, as depicted in the right photograph.

That move increased the moment of inertia about the roll axis by roughly 50% and improved stability, but it also increased body drag and energy expenditure by about 30%. In a second experiment, Combes altered the turbulence of the stream by inserting different geometric grids. Bees flying in the lower-turbulence environment were able to reach higher speeds before instabilities caused them to be ejected from the air stream. (S. A. Combes, R. Dudley, Proc. Natl. Acad. Sci. USA, doi:10.1073/pnas.0902186106.) —Steven K. Blau

Related link: Dragonfly Flight, Z. Jane Wang, Physics Today October 2008, page 74.

In his "Milkdrop Coronet," strobe-photography pioneer Harold Edgerton famously captured the splash produced by a milk droplet falling into a saucer. But our understanding of the underlying physics remains poor. It's known that before a liquid droplet splashes upward from a surface, a thin sheet of liquid spreads out from the impact point. Four years ago experiments by Sidney Nagel and colleagues at the University of Chicago showed, surprisingly, that splashing on a dry surface can be suppressed by reducing the ambient air pressure. The researchers concluded that compressible effects in the air are responsible for the splashing (L. Xu, W. W. Zhang, S. R. Nagel, Phys. Rev. Lett. 94, 184505, 2005). Now Michael Brenner and coworkers at Harvard University have further looked into the air's role in how droplets splash on a dry surface. Taking into account the compressibility and viscosity of the gas and the surface tension of the liquid, they modeled the behavior of the approaching droplet as it reaches the surface. They find that instead of spreading out over the surface, the liquid spreads over a very thin film of air. When the droplet nears the surface, pressure builds beneath it and the bottom of the droplet deforms by flattening and then becoming dimpled. The droplet's bottom perimeter develops a kink that, still over a layer of air, moves out and creates capillary waves. The calculations don't, however, show any indications of splashing; the researchers suggest that other parameters, such as the droplet viscosity and thermal transfer, must become important after the initial spreading phase. (S. Mandre, M. Mani, M. P. Brenner, Phys. Rev. Lett., in press.) — Richard J. Fitzgerald

With their ability to manipulate microliter to nanoliter volumes of liquids, microfluidic devices have found increasing application in a variety of fields, from ink-jet technology to proteomics and DNA analysis. Most current microfluidic devices are made from glass or polymers, and advances in design and fabrication have opened the realm of three-dimensional, complex flow paths. George Whitesides and colleagues at Harvard University have recently demonstrated 3D devices made from stacked layers of ordinary paper and tape. Thanks to paper's wicking ability, the devices don't require external pumps to drive the liquids through. Indeed, the wicking property of paper is routinely exploited in medical tests such as those for blood glucose, pregnancy, and HIV. To define the microfluidic pathways in the paper-based microfluidic device, the team impregnated each paper layer with a common photoresist, a hydrophobic polymer that could be patterned with UV light. With their channels thus established, the layers of paper were alternated with layers of double-sided tape; holes cut in the tape connected channels in adjacent paper layers. The figure illustrates the complex routing that can be achieved: Four differently colored liquids deposited on the top of a 5 cm × 5 cm, nine-layer stack (left) are, within 5 minutes, wicked through horizontally and vertically to the array of 1024 detection zones on the bottom (right). With reagents or antibodies placed in detection zones prior to assembly, such devices would provide highly parallel, independent assays. The Harvard team sees particular potential for their paper-based devices in medical diagnostics in developing countries. (A. W. Martinez, S. T. Phillips, G. M. Whitesides, Proc. Natl. Acad. Sci. USA, in press.) — Richard J. Fitzgerald

At the very moment a droplet of water breaks away from a dripping faucet, a singularity is formed. The dynamics leading up to the singularity are governed by the competition between the water’s inertia and its surface tension. (Water’s viscosity is low enough that it does not play a role.) In the reverse setup—an air bubble breaking away from an underwater nozzle—the pinch-off process is driven instead by the difference in pressure between the air and the water. As a result, the bubble and droplet systems differ both in the shapes formed and in the dependence on time. Now, Justin Burton and Peter Taborek of the University of California, Irvine, have observed both bubble-like and droplet-like behavior in a single continuously variable system: xenon bubbles in water over a range of pressures (and hence xenon densities). At low pressures, xenon bubbles behave like air bubbles, as shown in the top row of the figure. At 68 atmospheres, the highest practical pressure for the system, the xenon bubbles are 70% as dense as water and look like upside-down water droplets, as shown in the bottom row. To quantify the behavior of the Xe–water system, the researchers measured the width of the pinch-off region’s neck as a function of time before pinch off. For water droplets, the neck width is proportional to time to the 2/3 power; for air bubbles, it is proportional to time to the 0.57 power. By that standard, the researchers observed a sharp boundary between the bubble-like and droplet-like regimes at a xenon density that is 25% of the density of water. (J. C. Burton, P. Taborek, Phys. Rev. Lett. 101, 214502, 2008.) — Johanna Miller

During an effusive volcanic eruption—one that produces flowing lava, as shown here, as opposed to projectile material or clouds of ash—civil authorities need to know which way and how far the lava will flow so that they can decide whether and when to order evacuations. But lava is a difficult fluid to model, because as it cools, it crystallizes and eventually stops flowing. Robert Wright and colleagues at the University of Hawaii in Honolulu have developed a new model for forecasting lava flows. Their model combines two previously published ones: FLOWGO, which simulates lava's heat loss to predict how far it will flow before solidifying, and DOWNFLOW, which takes a stochastic approach to predict the lava's direction. Since DOWNFLOW’s stochastic method is computationally simple—but still accurate—Wright and colleagues’ model yields results much more quickly than other forecasting techniques. Moreover, FLOWGO accounts for the effusion rate—the rate of lava coming out of the ground—which strongly affects the flow length and can change substantially over the course of an eruption. When combined with satellite monitoring of the effusion rate, Wright and colleagues’ model can potentially provide updated forecasts in near real time. (R. Wright, H. Garbeil, A. J. L. Harris, Geophys. Res. Lett., in press.) —Johanna L. Miller