Fluid Dynamics in a Cup

Scientists puzzle out when and why coffee spills

At a recent math conference, Rouslan Krechetnikov watched his colleagues gingerly carry cups of coffee. Why, he wondered, did the coffee sometimes spill and sometimes not? A research project was born.

Although the problem of why coffee spills might seem trivial, it actually brings together a variety of fundamental scientific issues. These include fluid mechanics, the stability of fluid surfaces, interactions between fluids and structures, and the complex biology of walking, explains Krechetnikov, a fluid dynamicist at the University of California, Santa Barbara.

In experiments, he and a graduate student monitored high-speed video of the complex motions of coffee-filled cups people carried, investigating the effects of walking speed and variability among those individuals. Using a frame-by-frame analysis, the researchers found that after people reached their desired walking speed, motions of the cup consisted of large, regular oscillations caused by walking, as well as smaller, irregular and more frequent motions caused by fluctuations from stride to stride, and environmental factors such as uneven floors and distractions.

Coffee spilling depends in large part on the natural oscillation frequency of the beverage—that is, the rate at which it prefers to oscillate, much as every pendulum swings at a precise frequency given its length and the gravitational pull it experiences. When the frequency of the large, regular motions that a cuppa joe experiences is comparable to this natural oscillation frequency, a state of resonance develops: the oscillations reinforce one another, much as pushing on a playground swing at the right point makes it go higher and higher, and the chances of coffee sloshing its way over the edge rise. The small, irregular movements a cup sees can also amplify liquid motion and thus spilling. These findings were to be detailed at a November meeting of the American Physical Society in Baltimore.

Once the key relations between coffee motion and human behavior are understood, it might be possible to develop strategies to control spilling, “such as using a flexible container to act as a sloshing absorber,” Krechetnikov says. A series of rings arranged up and down the inner wall of a container might also impede the liquid oscillations.

Source of Information : Scientific American Magazine

Logic That Feels the Noise

As microchips shrink, the inescapable electronic buzz that emerges from thermal fluctuations, cross talk between wires and other sources can endanger their proper function. A way around that problem could be stochastic resonance, a phenomenon in which noise can boost a weak signal and improve a system’s performance. Certain kinds of structures, such as a sensory nerve, will output a signal only when background noise is sufficiently high. Researchers at Arizona State University constructed logic gates—circuit elements that perform logic functions— that behave in a similar way. When noise levels are low, the gates perform unreliably; however, at the kinds of noise levels expected for the smallest transistors, they work correctly. Such unusual, nonlinear behavior could help microchips get smaller. Moreover, altering certain applied voltages in the circuit can reconfigure the gate on the fly, thereby creating a morphing processor. Tune in to the March 13 issue of Physical Review Letters for more details. —Charles Q. Choi

Source of Information : Scientific American(2009-05)

Cloaking Made Simpler

Invisibility without sophisticated metamaterials BY JOHN MATSON

In recent years optics researchers have come up with numerous concepts for invisibility cloaks— camoufl aging that would effectively reroute light around an object to be concealed. Most of these approaches have relied on so-called metamaterials, which are carefully engineered structures that have bizarre optical properties. A much simpler cloaking apparatus could do away with the need for metamaterials entirely. Researchers at BAE Systems in Washington, D.C., Towson University and Purdue University have devised a cloaking device based on two gold surfaces, one coated on a curved lens and one on a flat piece of glass. Stacked together, they can conceal an area in between by forming what is known as a tapered waveguide. The trick lies in the gradient of the material’s refractive index, which allows light shining parallel into the stack to bend around a central area “like water fl owing around a stone,” says study co-author Vladimir M. Shalaev, a professor of electrical and computer engineering at Purdue. Shalaev was part of a group that in 2007 designed a visible-light cloak using metamaterials. But that cloak worked only at a predefined wavelength of light and concealed a very small area. In contrast, the waveguide appears to work for multiple wavelengths of visible light and can hide a bigger area. “From the very beginning we realized the huge challenge” of making such a cloak, Shalaev says. “It’s not fundamentally impossible, but it’s really, really hard.” John Pendry, a physicist at Imperial College London, says that the tapered waveguide strategy is “a very clever idea.” Physicist Ulf Leonhardt of the University of Saint Andrews in Scotland agrees, calling the paper, in the May 29 Physical Review Letters, “a brilliant piece of work, a wonderfully simple idea.” Both researchers point out, however, that the new approach conceals a two-dimensional area rather than a three-dimensional one. “The things you might want to cloak are probably not confined to two dimensions,” Pendry remarks. Still, the system could find use in optical communications.

CLOAKING DEVICE can be made from a goldcoated
lens and glass. Laser light shining
in edgewise bends around a central spot,
effectively rendering that area invisible.

Source of Information : Scientific American August 2009