Archive for the ‘Physics’ Category
A link to a paper in European Journal of Physics as to whether you should walk or run in a rain — in Bombay, where, you get drenched even if you are under the umbrella, it might be a good idea to learn to enjoy the warm, big fat drops hitting you!
Two interesting papers from PNAS:
Take a look!
The mysterious stability of nanobubbles on surfaces is a puzzle baffling soft matter and colloid scientists. Bubbles inside a fluid tend to be spherical, but surface bubbles have the appearance of blisters with typical widths of 1000 nanometers (nm) and heights of 20 nm. The existence of surface bubbles was proposed to explain the extremely long range and the magnitude of the strongly attractive forces observed between hydrophobic surfaces in water [...]. Nanobubbles are of interest because they are easily produced and are stable, and as such, their presence may be altering many aqueous interfaces and exerting influence on processes as diverse as froth flotation to the transportation of anticancer drugs across membranes. Classically, bubbles will deflate, leading to an increase in Laplace pressure (the pressure differential inside and outside a bubble) and a positive feedback loop that results in their rapid disappearance. However, surface nanobubbles, seemingly unaware of the rules, can remain stable for days. Now, writing in Physical Review Letters, James Seddon and coauthors [...] at the University of Twente, the Netherlands, have proposed an explanation for this stability, whereby the properties of the gas within a nanobubble generate a recirculation of the surrounding liquid, which effectively ensures that the gas escaping the bubble through diffusion is recaptured and the bubble lifetime is extended.
Take a look!
Nagel’s befuddlement, which he shared with his colleagues, could be stated this way: Why does all the material suspended in a drop of coffee end up at the edge when the drop evaporates, considering that it started out dispersed across the whole drop? The effect was common to all droplets of dispersed colloidal objects, including milk, blood, ink, and paint, evaporating on a wide variety of surfaces, suggesting there should be a general explanation.
The physical picture that emerged was beautiful and simple: As the droplet dries, the liquid evaporating from the thinning outer edge, where the contact angle θ is shrinking to zero, must be replenished by liquid from the drop’s interior. This sets up a strong outward flow in the solvent, which carries most of the solute to the contact line. Pre-existing surface roughness can provide the force to pin the contact line, but the contact line further pins itself through a feedback loop between flow and patterning: the outward flow increases the deposition of solute, which serves to anchor the fluid and reinforce the outward flow.
The simplicity of this picture carries some caveats—the suppression of counterflows that are due to gradients in surface tension (Marangoni flows) is one example. However, in the years since Nagel’s observation, the coffee ring has taken on a life of its own.
Inverse problems are generally known to be hard. Here is a commentary on a couple of papers published in PRL which discusses one such problem – namely, finding a potential that gives rise to a given type of lattice. One of the papers referred to in the commentary linked above has this to say:
In general, proving that a certain configuration is the ground state of a given potential is a very hard problem. In fact, the exact nature of the ground state is not rigorously known even for simple interactions such as the Lennard-Jones potential . In this Letter we have described
a direct method to design potentials for targeted self-assembly of lattices, a problem usually approached using iterative methods involving repeated relaxations of the system [2,3]. From our construction follows the somewhat counterintuitive observation that it is actually simpler to find a potential with a given configuration as a ground state than to determine the ground state(s) of a given potential.
Another thing about these papers is the use of the concept of the reciprocal space. May be I can use these papers to tell students the power of reciprocal space based techniques when I teach my mathematical methods course next semester.
Oh! Just for that title I am writing this post about this piece in PNAS:
Hummingbird tongues pick up a liquid, calorie-dense food that cannot be grasped, a physical challenge that has long inspired the study of nectar-transport mechanics. Existing biophysical models predict optimal hummingbird foraging on the basis of equations that assume that fluid rises through the tongue in the same way as through capillary tubes. We demonstrate that the hummingbird tongue does not function like a pair of tiny, static tubes drawing up floral nectar via capillary action. Instead, we show that the tongue tip is a dynamic liquid-trapping device that changes configuration and shape dramatically as it moves in and out of fluids. We also show that the tongue–fluid interactions are identical in both living and dead birds, demonstrating that this mechanism is a function of the tongue structure itself, and therefore highly efficient because no energy expenditure by the bird is required to drive the opening and closing of the trap. Our results rule out previous conclusions from capillarity-based models of nectar feeding and highlight the necessity of developing a new biophysical model for nectar intake in hummingbirds. Our findings have ramifications for the study of feeding mechanics in other nectarivorous birds, and for the understanding of the evolution of nectarivory in general. We propose a conceptual mechanical explanation for this unique fluid-trapping capacity, with far-reaching practical applications (e.g., biomimetics).