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Hot ice, measuring depression, perfect invisibility and flu vaccine incentives

Hot Ice to Lubricate Artificial Joints
Alexander D. Wissner-Gross and Efthimios Kaxiras
Physical Review E (August 2007)

A recent simulation has shown that thin layers of ice could persist on specially treated diamond coatings at temperatures well above body temperature, which could make ice-coated-diamond films an ideal coating for artificial heart valves, joint replacements, and wear-resistant prosthetics.

Physicists Alexander D. Wissner-Gross and Efthimios Kaxiras of Harvard modeled water ice on top of a diamond surface coated with sodium ions. They found that ice layers should persist on the treated diamond up to temperatures of 108 degrees Fahrenheit (42 Celsius), and in some circumstances could remain frozen beyond the boiling point of water.

Because of the gem's strength and other unusual characteristics, artificially grown diamond films are among the most promising candidates for applications ranging from medical implants to solar cells. Adding a layer of high-temperature ice could make the diamond even more suitable for medical devices by reducing its abrasiveness and inhibiting protein build-up.

Among other promising applications, the physicists believe that the ice layer could enhance the efficiency of diamond film-based solar collectors, while being much more environmentally friendly than lithium-ion batteries and other energy storage devices.

A short film that the researchers made from of some of their simulations was a finalist in the 2007 Materials Research Film Festival ( The work also earned Wissner-Gross the 2007 Dan David Prize Scholarship from Tel Aviv University and the 2007 Graduate Student Silver Award from the Materials Research Society. -JR

Measuring Depression
T. Nakamura, K. Kiyono, K. Yoshiuchi, R. Nakahara, Z. R. Struzik, and Y. Yamamoto
Physical Review Letters (forthcoming)

It's hardly surprising that clinically depressed people act differently than healthy people. Quantifying the difference, however, can be difficult. Now a collaboration of physicists and psychiatrists in Japan has found a way to clearly and objectively measure depression.

The researchers outfitted both healthy control subjects and depressed patients with accelerometers to continuously measure their motions over 5-day periods. Although activity levels in all of the subjects followed power-law patterns (a type of distribution that often turns up in physics studies of natural systems) the activity levels of depressed patients were clearly distinguished from healthy subjects by a number known as the scaling parameter. For patients with major depression, the scaling parameter is significantly smaller than it is for healthy subjects.

It can be a challenge to spot differences in behavior between depressed and healthy individuals via simple observation, and self-reported depression assessments are often unreliable. Applying instrumentation and statistical analyses common in physics research could dramatically improve the reliability and accuracy in measurements of depression, and may help in tailoring appropriate treatments for the debilitating ailment.- JR

Pillar of Invisibility
Zhichao Ruan, Min Yan, Curtis W. Neff, and Min Qiu
Physical Review Letters (forthcoming)

While we are a long way off from the lightweight, high-performance, magical cloak of Harry Potter, Muggle physicists have been busy designing ways to make invisibility possible. A recent theoretical analysis of a column-shaped invisibility cloak, by a collaboration of researchers from Sweden and China, showed that a cloak made to ideal specifications could render an object (or wizard) hidden inside perfectly invisible. However, even slight deviations from these specifications will cause the invisibility to break down.

The researchers analyzed the properties of a simulated tube of special metamaterials (manmade materials with intricate, microscopic structures) that can force light to follow a specified path. With the ideal wall thickness, the tube would flawlessly guide light around the inner chamber, rendering the wizard inside invisible. You could walk about, unaware of the cloaked wizard's presence, unless you unceremoniously slam yourself into a pillar that looks like nothing but empty air.

The wizard, who would be unable to see anything outside of the invisibility cloak, could reveal himself by deconstructing the cloak a layer at a time. Imagine if he could wave his wand and remove a layer from the inside of the column, leaving it the same diameter on the outside but making that inner chamber a little larger. With the inner layer removed, the wizard would appear as a thin line, and the background would be slightly distorted. As more of the inside is removed, the wizard would become more apparent and the background would become more distorted. Physicists haven't yet worked out exactly how these distortions would appear to human eyes.

In any case, the collaboration's theoretical study affirms that the ideal column design will allow for perfect invisibility, if metamaterials can be made to the right specifications.. -KM

Incentives to Prevent Flu Epidemics
Romulus Breban, Raffaele Vardavas, and Sally Blower
Physical Review E (forthcoming)

Recent findings from study of incentives, decision-making, and influenza epidemics may offer guidance on public health policy. A team of three researchers at the University of California in Los Angeles, two of them physicists, have found that while family-based vaccination incentives fail to prevent severe epidemics. A program that allows individuals three free annual vaccinations once they pay for the first may prove effective.

The study modeled three scenarios in which individuals were assumed to make their decisions on whether to vaccinate based on past experience. In the first, no incentives were offered. Vaccination rates would increase following years of epidemic and abruptly drop as people began to skip vaccinations, trusting that their peers would remain healthy. When the proportion of the population vaccinated against influenza is sufficiently low, severe epidemics occur, driving the vaccination rate up again the next year.

In the second and third scenarios, an incentive factored into the decision-making process. One possible incentive offers free vaccination for family members once the head of the household paid for his or hers. Counter-intuitively, the family incentive resulted in more frequent epidemics than the incentive-free case.

The third scenario offered an incentive to individuals, allowing them some number of free vaccinations after they paid for the first year. Above a certain threshold number of free years, the researchers found that severe epidemics do not occur. Using parameters that they believe to be realistic, they found that a minimum of three free years of vaccination may prevent influenza epidemics. -KM


Contact: James Riordon
American Physical Society

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