How to fall infinitely deep in sand or grain

When you drop something onto sand you are used to seeing it crater the sand a little and stop with a thud. New research shows that if an object dropping is dense enough then it will reach a terminal velocity in the granular material and keep going forever. It’s a highly unexpected and nonintuitive result but has been shown by dropping metal balls into polystyrene beads.

A computer simulation of metal balls dropping into a granular material. The simulations matched the actual experiments conducted to show that a sufficiently dense ball can fall infinitely deeply in a granular material.

A computer simulation of metal balls dropping into a granular material. The simulations matched the actual experiments conducted to show that a sufficiently dense ball can fall infinitely deeply in a granular material.

The researchers dropped ping pong balls filled with metal of different masses into a 5 meter deep silo of polystyrene balls. They attached a thread with marks along it to the ping pong ball so that a high speed camera could capture the movement as the ping pong ball dropped. With this setup, the researchers could achieve 2 millimeter precision with their depth measurements.

Above a certain mass, the ping pong balls continued to fall all the way to the bottom of the tube and had reached a constant (terminal) velocity. In this regime, the polystyrene beads seemed to be acting just like a fluid.

The researchers answer the question of how massive/dense a ping pong ball sized object would need to be to continue falling infinitely deeply in sand and they get an answer of about 14 kg or a density of 400 g/cm^3. That is about 400 times the density of water and there is nothing on Earth known of that density. So unless you discover some crazy new material, have no fear about dropping your keys at the beach and having them continue to sink until they hit bedrock.

Ref: Phys. Rev. Lett. 106, 218001 (2011)

Hot bodies have less drag

If you’ve ever watched water droplets skitter about on the surface of a hot skillet, you’ve been entranced by the Leidenfrost effect in action. It can occur when you have a hot object in contact with a colder liquid. If the temperatures are right, the heat from the hot object will vaporize the liquid causing a think layer of gas between them. In the case of a skillet, the hot pan creates a thin layer of water vapor that the drop of water then floats on. The vapor layer is also usually a much better insulator of heat than the liquid so it causes the drop of water to last much longer than if it was just in contact with the pan.

Now researchers have shown the Leidenfrost effect works very well in reverse. They dropped metal balls heated to different temperatures into a liquid and watched how fast they fell. The chose a room temperature ball, a heated ball that wasn’t enough to make the Leidenfrost effect occur, and a ball heated above the Leidenfrost temperature.

If you click on this image, you’ll see the results.

Combined video showing the fall of a 20 mm steel sphere at 25 degrees C, 110 degrees C, and 180 degrees C. For 110 C there is an intensive bubble release and for 180 C there is a continuous vapor film. The frame rate used was 1000 fps and the video playback speed is 30 fps.

Combined video showing the fall of a 20 mm steel sphere at 25 degrees C, 110 degrees C, and 180 degrees C. For 110 C there is an intensive bubble release and for 180 C there is a continuous vapor film. The frame rate used was 1000 fps and the video playback speed is 30 fps.

The Leidenfrost effect not only creates a vapor layer but that then reduces the drag on the falling spheres. The researchers found that the drag was reduced by as much as 85% between a room temperature ball and a ball above the Leidenfrost temperature. Close-up photographs of the ball above the Leidenfrost temperature clearly show the vapor layer.

(a) Digital camera snapshot of a heated 15 mm steel sphere held stationary in fluorinated liquid with sphere temperature above the Leidenfrost temperature. A thin vapor layer streaming around the sphere can be observed by the ripples moving along the sphere surface. (b) Snapshot at the instant when the sphere has cooled to the Leidenfrost temperature that is marked by an explosive release of bubbles.

(a) Digital camera snapshot of a heated 15 mm steel sphere held stationary in fluorinated liquid with sphere temperature above the Leidenfrost temperature. A thin vapor layer streaming around the sphere can be observed by the ripples moving along the sphere surface. (b) Snapshot at the instant when the sphere has cooled to the Leidenfrost temperature that is marked by an explosive release of bubbles.

Ref: Phys. Rev. Lett. 106, 214501 (2011)

New cosmic ray data still allows for dark matter possibility

New measurements of cosmic ray electrons by the PAMELA spacecraft have left open the door for an observation of dark matter but are still consistent with no dark matter signal. Although the measurements of the cosmic ray electrons are consistent with no new sources of electrons, such as dark matter annihilation, the electron and positron data is better fit with some kind of additional source. What that is remains unclear at this time.

The electron energy spectrum measured by PAMELA and other modern instruments. Some of the wiggles in the curves have been interpreted as signs of dark matter by some scientists. However, that interpretation is not conclusive.

The electron energy spectrum measured by PAMELA and other modern instruments. Some of the wiggles in the curves have been interpreted as signs of dark matter by some scientists. However, that interpretation is not conclusive.

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Rogue waves replicated in lab

In the 2000 film The Perfect Storm, the ship Andrea Gail is capsized just when the storm seems to be dying down by a huge wave that seems to come out of nowhere. In the 2006 film Poseiden, an ocean liner is capsized by a similar type of wave. Although these waves might look like poetic license on the part of the filmmakers, they really exist and have, for the first time, been re-created in the laboratory.

Merchant ship labouring in heavy seas as a huge wave looms astern. Huge waves are common near the 100-fathom line in the Bay of Biscay. Published in Fall 1993 issue of Mariner's Weather Log. Credits: NOAA Photo Library

Merchant ship labouring in heavy seas as a huge wave looms astern. Huge waves are common near the 100-fathom line in the Bay of Biscay. Published in Fall 1993 issue of Mariner's Weather Log. Credits: NOAA Photo Library

Thought to be mythical for a long time, the first rogue wave to be scientifically measured was observed at the Draupner oil platform in the North Sea on January 1, 1995. There have been other likely observations of rogue waves and some might have even been responsible for the sinking of ships. Satellite observations confirm the existence of rogue waves.

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Build a white hole in your kitchen sink

A white hole is the opposite of a black hole in that matter can only come out of a white hole and never penetrate it’s event horizon from the outside. It turns out that you can make a simple hydrodynamic analogy in your sink with exactly the same mathematical description as a white hole. Experimenters have shown for the first time that this truly is a white hole in the sense that nothing can penetrate from the outside.

You can make a white hole, the reverse of a black hole, in your kitchen sink. The needle in this picture is creating a Mach cone downstream of it.

You can make a white hole, the reverse of a black hole, in your kitchen sink. The needle in this picture is creating a Mach cone downstream of it.

To make a white hole in your sink all you need to do is run water at a high enough speed that it carves out a circle around where it hits the sink. The circle is a shockwave where water from the outside is moving too slowly to be able to penetrate any further upstream.

In the photo, there is a needle stuck into the stream, showing what is called a Mach cone. Mach cones form behind anything moving at supersonic speeds, and it is the Mach cone hitting you that you hear as a sonic boom when a supersonic plane flies overhead.

One of the predictions about a white hole is that Mach cones should form around objects inside the hole with the angle of the Mach cone getting greater until you get to the very edge where it forms a right angle. Testing these angles is the process the experimenters used to show that the white hole analogy holds up exactly.

As the needle is placed further out, the Mach cones get wider until the reach a right angle at the event horizon and then disappear outside the event horizon.

As the needle is placed further out, the Mach cones get wider until the reach a right angle at the event horizon and then disappear outside the event horizon.

In this series of images, the needle is placed at different distances out from the source of water and you can see the angle getting larger as it moves toward the boundary and then outside the boundary there is no Mach cone, as predicted.

Knowing that the mathematics and properties of white holes really do stand up, it seems that these hydrodynamic systems could be more useful than previously thought in understanding the behavior of black holes, which are just white holes run in reverse.

Ref: Phys. Rev. E 83, 056312 (2011)

A one-way gate for cells

Researchers have created a microfunnel that only allows cells to move through it in one direction, potentially useful for both cell transport and cell selection mechanisms.

 

Deformation of a single cell through microscale funnel constrictions in forward and reverse directions. Trying to enter through the narrow end requires a higher pressure than through the wide end.

Deformation of a single cell through microscale funnel constrictions in forward and reverse directions. Trying to enter through the narrow end requires a higher pressure than through the wide end.

As the cell hits the funnel it deforms to try to squeeze through the funnel but it takes a certain pressure to force it through. From the wide end, that pressure is much lower than trying to pass first through the narrow end. That means the gate is pressure dependent and raising the pressure above some threshold will allow cells to pass both directions but below a certain pressure the funnel is strictly one way.

 

Design of a two-layer microfluidic device for measuring the pressure required to deform single cells through microscale funnel constrictions. An upper flow layer comprises two sets of 10 funnels arranged in opposite polarity and decreasing in pore size toward the center of the chain. Single cells are introduced at (e) and (f), and then deformed through the funnel chain using the pressure difference between (c) and (d).

Design of a two-layer microfluidic device for measuring the pressure required to deform single cells through microscale funnel constrictions. An upper flow layer comprises two sets of 10 funnels arranged in opposite polarity and decreasing in pore size toward the center of the chain. Single cells are introduced at (e) and (f), and then deformed through the funnel chain using the pressure difference between (c) and (d).

This phenomenon depends on the structure of the cell inside so the cutoff pressure for cells going each direction is different for different cell types. That could allow the one-way gate to also act as a filter to select out certain types of cells and not others.

But perhaps the best way to understand the mechanism is just to look at a few pictures of the setup from the paper itself.

Ref: Phys. Rev. E 83, 051910 (2011)

How reptiles deliver venom

Rattlesnakes inject venom into their prey through tubular fangs but they are an exception among snakes. Most other venomous snakes and other reptiles inject venom along grooves in their fangs. Now researchers have shown how this process, combined with the fluid properties of venom, is well-suited to injecting venom below the surface of the skin of prey animals.

Scanning electron micrographs showing the prominent grooves (horizontal arrows) on the fangs of (a) a banded snake (Bothryum lentiginosum), a lizard eater, and (b) a mangrove snake (Boiga dendrophila), a generalist feeding on both birds and lizards. The Boiga specimen was prepared with the fang imbedded in prey tissue, so only the base of the fang is visible; the prey tissue has separated slightly from the fang forming a clear venom tube (vertical arrow).

Scanning electron micrographs showing the prominent grooves (horizontal arrows) on the fangs of (a) a banded snake (Bothryum lentiginosum), a lizard eater, and (b) a mangrove snake (Boiga dendrophila), a generalist feeding on both birds and lizards. The Boiga specimen was prepared with the fang imbedded in prey tissue, so only the base of the fang is visible; the prey tissue has separated slightly from the fang forming a clear venom tube (vertical arrow).

The efficacy of injecting venom depends on two main factors: the shape of the groove, and the properties of the venom. Both contribute to how well reptiles can envenomate prey.

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Biological organisms can sniff their way to the shortest path

Various organisms, like neutrophils–the white blood cells that attack invading bacteria, find their way around by following the “scent” of another organism. A researcher has shown that this process of following the chemical signals, or chemotaxis, put out by a bunch of invaders is enough for organisms to find good solutions to the traveling salesman problem, in which the shortest route between multiple targets is sought.

 

The chemotactic paths that microorganisms follow in simulations.

The chemotactic paths that microorganisms follow in simulations.

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Calculating the elements of life

Life depends on carbon and carbon was created in stars. But just how did that carbon come to be, given raw fuel of hydrogen and helium? A paper in Physical Review Letters describes some calculations from first principles that, for the first time, show how carbon can come into being in the heart of a star.

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Proton-electron mass ratio change limits grand unified theories

Some grand unified theories predict that the ratio of the proton to electron mass (about 1836) varies on cosmological timescales. Now new astronomical measurements have limited that variation to no more than eight parts per million over the past 10.6 billion years.

About a decade ago, some physicists detected what looks like a variation in the constant alpha, the strength of the electromagnetic force, over the age of the universe. (Yes, it seems odd that a constant could vary but that’s because we always just assumed that it is a constant–perhaps it isn’t.) It is a small change but a significant one as it tells us something fundamental is happening to the forces of nature.

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