Hydrodynamics of writing with ink
Jungchul Kim, Myoung-Woon Moon, Kwang-Ryeol Lee, L. Mahadevan, and Ho-Young Kim
Physical Review Letters (forthcoming)
For millennia, writing has been the preferred way to convey information and knowledge from one generation to another. We first developed the ability to write on clay tablets with a point, and then settled on a reed pen, as preserved from 3000 BC in Egypt when it was used with papyrus. This device consisted of a hollow straw that served as an ink reservoir and allowed ink to flow to its tip by capillary action. A quill pen using a similar mechanism served as the instrument of choice for scholars in medieval times, while modern times have seen the evolution of variants of these early writing instruments to a nib pen, a ballpoint pen, and a roller ball pen. However, the fundamental action of the pen, to deliver liquid ink to an absorbent surface has remained unchanged for five thousand years.
Writing with ink involves the supply of liquid from a pen on to a porous hydrophilic solid surface, paper. The resulting line width depends on the pen speed and the physicochemical properties of the ink and of paper. Here we quantify the dynamics of this process using a combination of experiment and theory. Our experiments are carried out using a minimal pen: a long narrow tube that serves as a reservoir of liquid, which can write on a model of paper: a hydrophilic micropillar array. A minimal theory for the rate of wicking or spreading of the liquid is given by balancing capillary force that drives the liquid flow and viscous force exerted by the substrate. This allows us to quantitatively predict the shape of the front and the width of the line laid out by the pen, the results corroborated by experiments.
Faster-than-light neutrinos may leave trails at the LHC
Hooman Davoudiasl and Thomas G. Rizzo
Physical Review D (forthcoming)
Is Einstein's venerated theory of special relativity challenged by neutrinos? The LHC may help provide the answer. The OPERA experiment, at the Gran Sasso Laboratory in Italy, has reported observation of neutrinos that travel faster than light. This result, if confirmed, would violate one of the defining laws of special relativity -- a pillar of fundamental physics for over a century-- that forbids faster-than-light (FTL) travel. If neutrinos, which are extremely elusive by nature, can travel at FTL speeds it has been predicted that they would emit easy-to-detect particles, such as electron anti-electron pairs, along their paths. In our paper, we suggest looking for these conspicuous trails that would be left in the wake of neutrinos -- if they traversed the LHC detectors faster than light -- as a way of testing the FTL neutrino hypothesis implied by the OPERA results. The requisite neutrinos can originate from decays of top quarks that are copiously produced at the LHC.
Why it's hard to see Schroedinger's cat
Sadegh Raeisi, Pavel Sekatski, and Christoph Simon
Physical Review Letters (forthcoming)
Why do we not see quantum physical effects in our daily lives? This question was raised by Schroedinger in his famous cat paradox. One answer is that quantum superposition states, such as the cat being both dead and alive at the same time, are very fragile. When the cat interacts with its environment even just a tiny bit, the superposition is destroyed. This effect is known as decoherence, and it has been studied intensively over the last few decades. But it turns out that decoherence is not the only reason why quantum effects are hard to see. In a recent paper in Physical Review Letters, researchers point out an even more basic difficulty: seeing quantum effects requires extremely precise measurements. Studying a concrete example for such a "cat" motivated by recent experiments, a particular quantum state involving a large number of photons, they show that in order to see the quantum nature of this state, one has to be able to count the number of photons in it perfectly. This becomes more and more difficult as the total number of photons is increased. Distinguishing one photon from two photons is within reach of current technology, but distinguishing a million photons from a million plus one is not. This shows that seeing Schroedinger's cat is hard also because it would require exceptionally good eyesight.