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A new theoretical model of hair explains how the curliness and elasticity of hair fibers produce the characteristic shape of a ponytail.
iStockphoto/Darren Mower

Ponytailed physics professors may now be able to use their coif as a lecture prop. Researchers have for the first time disentangled the factors that determine how hairs hang together, as reported in Physical Review Letters . The analytical model, which was based on a statistical characterization of ponytail shapes, treats the forces on individual hair fibers as continuous quantities inside a hair bundle. The resulting hair “equation of state” could apply to other fibers in biology and industry.

Many scientists (and even more hairdressers) have wondered about the meanderings of hair. Leonardo da Vinci thought hair flowed like water, and this sort of fluid analogy has guided computer animators trying to recreate hair and fur on the movie screen. However, no model has yet addressed one of the most basic hair problems: what shape is a ponytail?

For their part, Raymond Goldstein of the University of Cambridge, UK, and his colleagues obtained human hair switches (a type of commercially available hairpiece) and measured the random curvature (or curliness) of a sample of individual hairs. They then assembled different ponytails—all about 25 centimeters long—and recorded the average shape. This data helped in the formulation of an equation of state that balanced four competing effects: gravity, tension, an elastic restoring force, and a “swelling pressure” coming from the curliness. The model correctly predicted the shape of ponytails as the lengths of the switches were progressively shortened. The authors believe their surprisingly simple equation of state could be used to study other hair “styles,” as well as dynamic problems like a swinging ponytail. – Michael Schirber

Shape of a Ponytail and the Statistical Physics of Hair Fiber Bundles

Raymond E. Goldstein, Patrick B. Warren, and Robin C. Ball

Phys. Rev. Lett. , 078101 (2012)

Published February 13, 2012

Brian Swingle believes that quantum entanglement could explain the nature of spacetime—an idea that could lead to a quantum theory of gravity.

Researchers demonstrated the simplest possible chemical reaction by using optical tweezers to bring together two atoms to form a molecule.

At an annual physics meeting in the Alps, WIMPs appeared to lose their foothold as the favored dark matter candidate, making room for a slew of new ideas.

More Features »

For our ten-year anniversary, the editors of Physics look back at some of the past research we have covered and ask: What’s become of it?

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Issue 11, 2018
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Lab on a Chip

A microengineered human corneal epithelium-on-a-chip for eye drops mass transport evaluation

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Zachary Estlack , Ted Reid and Jungkyu Kim *

* Corresponding authors

Department of Mechanical Engineering, Texas Tech University, Lubbock, USA (806) 834 6106

Department of Ophthalmology, Texas Tech University Health Science Center, Lubbock, USA

Animals are commonly used for pharmacokinetic studies which are the most frequent events tested during ocular drug development and preclinical evaluation. Inaccuracy, cost, and ethical criticism in these tests have created a need to construct an model for studying corneal constraints. In this work, a porous membrane embedded microfluidic platform is fabricated that separates a chip into an apical and basal side. After functionalizing the membrane surface with fibronectin, the membrane's mechanical and surface properties are measured to ensure correct modeling of characteristics. Immortalized human corneal epithelial cells are cultured on the membrane to create a microengineered corneal epithelium-on-a-chip (cornea chip) that is validated with experiments designed to test the barrier properties of the human corneal epithelium construct using model drugs. A pulsatile flow model is used that closely mimics the ocular precorneal constraints and is reasonable for permeability analysis that models conditions. This model can be used for preclinical evaluations of potential therapeutic drugs and to mimic the environment of the human cornea.

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Publication details

The article was received on 08 Feb 2018, accepted on 29 Apr 2018 and first published on 02 May 2018

Article type: Paper
DOI: 10.1039/C8LC00158H
Citation: , 2018, 18 , 1539-1551
Lab Chip

Pao : There’s this idea that, “Yes, they can use this information to manipulate other people, but I’m not gonna fall for that, so I’m protected from being manipulated.” Slowly, over time, you become addicted to the interactions, so it’s hard to opt out. And they just keep taking more and more of your time and pushing more and more fake news. It becomes easy just to go about your life and assume that things are being taken care of.

A Propaganda Engine ‘Unlike Any in History’: QA With Early Facebook Investor

McNamee : If you go back to the early days of propaganda theory , Edward Bernays had a hypothesis that to implant an idea and make it universally acceptable, you needed to have the same message appearing in every medium all the time for a really long period of time. The notion was it could only be done by a government. Then Facebook came along, and it had this ability to personalize for every single user. Instead of being a broadcast model, it was now 2.2 billion individualized channels. It was the most effective product ever created to revolve around human emotions.


García : If you pulled the plug on Facebook, there would literally be riots in the streets. So in the back of Facebook’s mind, they know that they’re stepping on people’s toes. But in the end, people are happy to have the product, so why not step on toes? This is where they just wade into the whole cesspit of human psychology. The algorithm, by default, placates you by shielding you from the things you don’t want to hear about. That, to me, is the scary part. The real problem isn’t Facebook — it’s humans.

McNamee : They’re basically trying to trigger fear and anger to get the outrage cycle going, because outrage is what makes you be more deeply engaged. You spend more time on the site and you share more stuff. Therefore, you’re going to be exposed to more ads, and that makes you more valuable. In 2008, when they put their first app on the iPhone, the whole ballgame changed. Suddenly Bernays’s dream of the universal platform reaching everybody through every medium at the same time was achieved by a single device. You marry the social triggers to personalized content on a device that most people check on their way to pee in the morning and as the last thing they do before they turn the light out at night. You literally have a persuasion engine unlike any created in history.

Step 7

No one in the government — or, for that matter, in the tech industry’s user base — seemed interested in bringing such a wealthy, dynamic sector to heel.

García : The real issue is that people don’t assign moral agency to algorithms. When shit goes sideways, you want someone to fucking shake a finger at and scream at. But Facebook just says, “Don’t look at us. Look at this pile of code.” Somehow, the human sense of justice isn’t placated.

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