New mathematical model explains patterns of human movement by considering the costs

ScienceDaily (Oct. 13, 2011) — Using previously published data on the time-stamped locations of 100,000 anonymous cell-phone users, a researcher from Duke University has identified three distinct patterns of human mobility for short, medium, and long distance trips. In 2008, a separate research team that was not involved in the current study published a paper in which they had plotted data on cell-phone users' movements, and then fitted the data with a single, downward-sloping curve. The curve captured an intuitive relationship: the longer a trip, the less likely it was to occur.

Nicola Scafetta, however, thought deeper patterns might be hidden by the simple curve. In the AIP's journal Chaos, Scafetta proposes a finer-resolution analysis of the cell-phone data. He divided the data set up into three separate sections, one each for short (from 1 to 10 km), medium (from 10 to 300 km), and long (above 300 km) distance trips. He then fit each chunk of data with a separate curve. Surprisingly, the exponents from the three separate curve fits were simple numbers -- 1, 2 and 3 -- that illustrated a different relationship between distance and trip frequency for each zone. For all three zones, the likelihood of a trip decreases with increased distance, but the rate of decrease is faster in the higher-numbered zones.

Scafetta offers a physical and statistical explanation for this pattern. In zone one, people are running short-distance errands within an urban area, and may just consider one cost mechanism, like the time or the fuel cost of the trips, when deciding where and when to go. In the more distant zone two people are, for example, taking day-trips to nearby towns of specific interest. These trips might require travelers to consider both time and fuel costs in their decisions. And in zone three, people take multi-day trips and may consider time and fuel costs, as well as additional overnight lodging costs.

The increase in the number of considered costs for each zone could help explain the increase in the curve-fit exponent for each zone. Scafetta also rescaled the model and found that it could be used to interpret data gathered on the movements of volunteers who walked to their destinations, either in zone one (within 200 m) or zone two (from 200 to 1000 m).

The critical benefit of the alternative fitting method, Scafetta writes, is that it suggests clear physical and geographical mechanisms to explain the observations. Accurate models of human displacement have applications in traffic forecasting, urban planning, and in the study of social networks and the spread of diseases.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by American Institute of Physics, via EurekAlert!, a service of AAAS.

Journal Reference:

Nicola Scafetta. Understanding the complexity of the Lévy-walk nature of human mobility with a multi-scale cost/benefit model. Chaos: An Interdisciplinary Journal of Nonlinear Science, 2011

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Copper film could lower touch screen, LED and solar cell costs

ScienceDaily (Sep. 28, 2011) — Copper nanowires may be coming to a little screen near you. These new nanostructures have the potential to drive down the costs of displaying information on cell phones, e-readers and iPads, and they could also help engineers build foldable electronics and improved solar cells, according to new research.

Duke chemist Ben Wiley and his graduate student have developed a technique to organize copper atoms in water to form long, thin, non-clumped nanowires. The nanowires are then transformed into transparent, conductive films and coated onto glass or plastic.

The new research shows that the copper nanowire films have the same properties as those currently used in electronic devices and solar cells, but are less expensive to manufacture. The results were published online Sept. 23 in Advanced Materials.

The films that currently connect pixels in electronic screens are made of indium tin oxide, or ITO. It is highly transparent, which transmits the information well. But the ITO film must be deposited from a vapor in a process that is a thousand times slower than newspaper printing, and, once the ITO is in the device, it cracks easily. Indium is also an expensive rare earth element, costing as much as $800 per kilogram.

These problems have driven worldwide efforts to find less expensive materials that can be coated or printed like ink at much faster speeds to make low-cost, transparent conducting films, Wiley said.

One alternative to an ITO film is to use inks containing silver nanowires. The first cell phone with a screen made from silver nanowires will be on the market this year. But silver, like indium, is still relatively expensive at $1400 per kilogram.

Copper, on the other hand, is a thousand times more abundant than indium or silver, and about 100 times less expensive, costing only $9 per kilogram.

In 2010, Wiley and his graduate student Aaron Rathmell showed that it was possible to form a layer of copper nanowires on glass to make a transparent conducting film.

But at that time, the performance of the film was not good enough for practical applications because the wires clumped together. The new way of growing the copper nanowires and coating them on glass surfaces eliminates the clumping problem, Wiley said.

He and Rathmell also created the new copper nanowires to maintain their conductivity and form when bent back and forth 1,000 times. In contrast, ITO films' conduction and structure break after a few bends.

Wiley said the low-cost, high-performance, and flexibility of copper nanowires make them a natural choice for use in the next generation of displays and solar cells. He co-founded a company called NanoForge Corp in 2010 to manufacture copper nanowires for commercial applications.

In early 2011, NanoForge received a $45,000 North Carolina IDEA grant for refinement and scale-up of the manufacturing process of copper nanowires, and it is now filling orders.

With continuing development, copper nanowires could be in screens and solar cells in the next few years, which could lead to lighter and more reliable displays and also to making solar energy more competitive with fossil fuels, Wiley said.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Duke University.

Journal Reference:

Rathmell, A. R. and Wiley, B. J. he Synthesis and Coating of Long, Thin Copper Nanowires to Make Flexible, Transparent Conducting Films on Plastic Substrates. Advanced Materials, 2011 DOI: 10.1002/adma.201102284

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Copper film could lower touch screen, LED and solar cell costs

ScienceDaily (Sep. 28, 2011) — Copper nanowires may be coming to a little screen near you. These new nanostructures have the potential to drive down the costs of displaying information on cell phones, e-readers and iPads, and they could also help engineers build foldable electronics and improved solar cells, according to new research.

Duke chemist Ben Wiley and his graduate student have developed a technique to organize copper atoms in water to form long, thin, non-clumped nanowires. The nanowires are then transformed into transparent, conductive films and coated onto glass or plastic.

The new research shows that the copper nanowire films have the same properties as those currently used in electronic devices and solar cells, but are less expensive to manufacture. The results were published online Sept. 23 in Advanced Materials.

The films that currently connect pixels in electronic screens are made of indium tin oxide, or ITO. It is highly transparent, which transmits the information well. But the ITO film must be deposited from a vapor in a process that is a thousand times slower than newspaper printing, and, once the ITO is in the device, it cracks easily. Indium is also an expensive rare earth element, costing as much as $800 per kilogram.

These problems have driven worldwide efforts to find less expensive materials that can be coated or printed like ink at much faster speeds to make low-cost, transparent conducting films, Wiley said.

One alternative to an ITO film is to use inks containing silver nanowires. The first cell phone with a screen made from silver nanowires will be on the market this year. But silver, like indium, is still relatively expensive at $1400 per kilogram.

Copper, on the other hand, is a thousand times more abundant than indium or silver, and about 100 times less expensive, costing only $9 per kilogram.

In 2010, Wiley and his graduate student Aaron Rathmell showed that it was possible to form a layer of copper nanowires on glass to make a transparent conducting film.

But at that time, the performance of the film was not good enough for practical applications because the wires clumped together. The new way of growing the copper nanowires and coating them on glass surfaces eliminates the clumping problem, Wiley said.

He and Rathmell also created the new copper nanowires to maintain their conductivity and form when bent back and forth 1,000 times. In contrast, ITO films' conduction and structure break after a few bends.

Wiley said the low-cost, high-performance, and flexibility of copper nanowires make them a natural choice for use in the next generation of displays and solar cells. He co-founded a company called NanoForge Corp in 2010 to manufacture copper nanowires for commercial applications.

In early 2011, NanoForge received a $45,000 North Carolina IDEA grant for refinement and scale-up of the manufacturing process of copper nanowires, and it is now filling orders.

With continuing development, copper nanowires could be in screens and solar cells in the next few years, which could lead to lighter and more reliable displays and also to making solar energy more competitive with fossil fuels, Wiley said.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Duke University.

Journal Reference:

Rathmell, A. R. and Wiley, B. J. he Synthesis and Coating of Long, Thin Copper Nanowires to Make Flexible, Transparent Conducting Films on Plastic Substrates. Advanced Materials, 2011 DOI: 10.1002/adma.201102284

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here