Sep 102019
 
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Curious things happen around us all the time – and sometimes we are so familiar with them that we do not even notice them anymore.

If you read the title you might now think that this article was about the Leidenfrost effect [1], that is, this little funny dance water droplets perform on a hot surface such as a frying pan. It is not, though. The Leidenfrost effect occurs when a material – usually a liquid – meets a surface far above its boiling temperature. A thin layer of the droplet’s surface will then evaporate rapidly, causing a protective gas coating to appear that effectively insulates the droplet and lets it last longer on the hot surface. Similar effects can also be seen with liquid nitrogen on a material at room temperature. These droplets appear to travel around due to ejected gasses. But does a similar phenomenon also occur without the necessity of a hot surface?

There is in fact a location where such an effect occurs regularly without us usually noticing: The bathroom. Under certain conditions water droplets can be seen moving on a surface of water as if they had hydrophobic properties. The easiest way to see them is in the shower, when the shower floor is already covered in a thin layer of water. If new water droplets now impact on this surface at certain angles and speeds, they can be seen rushing around for a while before disappearing. It turns out that in recent years a few scientific publications were dedicated to investigating this effect more closely. [2,3] With a high-speed camera, the bouncing effect can be visualized rather easily, as shown in Fig. 1: The droplet appears to cause a dent in the water surface and then bounce off without merging with the rest of the liquid. Of course, the first idea that comes into mind now is the Leidenfrost effect, where a similar behavior can be seen caused by a layer of vapor. However, here no high temperatures are involved and thus the generation of water vapor is negligible.

Figure 2: A schematic depiction of the resistance time phenomenon. On impact, a thin layer of gas (air) is compressed on the surface, causing a protection from immediate coalescence. However, eventually, the air escapes and the lower periphery of the droplet merges with the rest of the liquid. The surface tension can then rapidly squeeze the edges of the droplet together, causing the upper half of the droplet to be cut off from the rest. It can then repeat the bouncing process if the conditions are right. Reproduced from [4].

The first intuition of an air coating to protect the water droplet is still standing, though, and thus the scientists tried to model the behavior. It turns out that there is indeed a protective coating of air, which can get compressed when the droplet approaches the surface of the liquid underneath. The air simply cannot escape quickly enough and therefore protects the droplet on impact and pushes away from the water surface. This phenomenon causes what is called the residence time of a droplet, that is, the time a droplet can sit on top of a pool of the same liquid before coalescing (see Fig. 2). The theory was confirmed by lowering the ambient air pressure around the experiment, which caused the residence time to decrease. [4] However, one would expect that this thin layer of gas should not withstand a heavy impact of a droplet coming from e.g. the shower head with a lot of speed and thus kinetic energy.

An explanation can be found using a simple speaker membrane: When the droplets are put in contact with an oscillation surface, like water on an oscillating speaker, the bouncing is facilitated, and the droplets can remain intact for much longer. Moreover, the droplets now travel around just like they do in a shower! High-speed camera footage can show the reason for this change in behavior: The surface of the water pool, excited into periodic up- and down-movement patterns, gently catches the droplet if the surface is moving downwards in the moment of impact and therefore prevents the impact from destroying the protective gas layer. It is just like gently catching a water balloon with your hand by grabbing it in motion and then slowing it down. Additionally, the continuous movement of the surface seems to stabilize the gas layer and therefore massively increases the residence time, all while allowing the droplet to travel from minimum to minimum, thus creating the “walking water” effect. [6] In a shower, the impact of many, many droplets cause the surface of the water pool on the ground to oscillate in a similar manner, creating landing spots for some droplets that then move around the surface. The phenomenon can thus be explained by the residence time of a droplet together with an oscillating surface.

Finally, one can reproduce a similar behavior in space, where microgravity does not pull the droplets down. An air bubble inside of a water bubble can thus act like an isolated system where droplets can form and move… excited by the sound of a cello! If you got curious, please check out the beautiful footage in Ref. [6] where much of the inspiration of this article came from.

As stated initially, the most curious things happen around us and we simply have to notice them.

— Kai Litzius

References:

[1] https://www.engineersedge.com/physics/leidenfrost_effect_13089.htm

[2] Y. Couder et al., From Bouncing to Floating: Noncoalescence of Drops on a Fluid Bath, Phys. Rev. Lett. 94, 177801 (2005).

[3] J. Molácek & J. W. M. Bush, Drops bouncing on a vibrating bath, J. Fluid Mech. 727, 582-611 (2013).

[4] I. Klyuzhin et al., Persisting Water Droplets on Water Surfaces, J. Phys. Chem. B 114, 14020-14027 (2010).

[5] https://upload.wikimedia.org/wikipedia/commons/1/1d/Bouncing_droplets.gif

[6] https://www.youtube.com/watch?v=KJDEsAy9RyM (Water bubble in space at time index 8:18).
 

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Sep 042019
 
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Superstitions are having hard times in our modern always progressing world. It is no longer easy to fool someone with a myth or a beautiful legend from childhood. But how about this one: have you ever heard that a thunderstorm could curdle milk

A correlation between thunderstorms and the souring or curdling of milk has been observed for centuries. As early as in 1685 the first clue was written down in the book “The Paradoxal Discourses of F. M. Van Helmont: Concerning the Macrocosm and Microcosm, Or the Greater and Lesser World, and Their Union” [1]:

“Now that the Thunder hath its peculiar working, may be partly perceived from hence, that at the time when it thunders, Beer, Milk, &c. turn sower in the Cellars … the Thunder doth everywhere introduce corruption and putrefaction”.

By the beginning of the 19th century there had been numerous attempts to find theories of a causal relationship. [2-7] They all were not plausible, many even contradicting. Later, after refrigeration and pasteurization became widespread, eliminating bacteria growth, interest in this phenomenon almost disappeared. While the most popular explanation remains that these occasions are only a correlation, we would like to draw the reader’s attention to some of the suggested theories. 

In order to understand what actually happens with milk during a thunderstorm we would need to know (i) what processes are behind the milk souring and (ii) what accompanies thunderstorm, e.g. lightning. While the latter is not yet entirely clear to scientists, [8] the simplified picture of the first point we will cover in the next few paragraphs.     


Figure1: Schematic image of casein micelles covering fat globules within milk as a colloid solution.

Fresh milk is a textbook example of colloid – a solution consisting of fat and protein molecules, mainly casein, floating in a water-based fluid. [9] The structure of milk is schematically illustrated in Fig. 1. Fat globules are coated with protein and charged phospholipids. Such a formation protects the fat from being quickly digested by bacteria, which also exist in milk. Casein proteins under normal conditions are negatively charged and repel each other so that these formations naturally distribute evenly through the liquid. Normally, milk is slightly acidic (pH ca. 6.4-6.8), [10] being sweet at the same time due to lactose, one of the other carbohydrates within the milk. When the acidity increases to pH lower than 4, proteins denature and are no longer charged. Thus, they bind to each other or coagulate into the clumps known as curds. The watery liquid that remains is called whey.

The acidity of milk is determined by the bacteria which produce lactic acid. The acids lower the pH of milk so the proteins can clump together. The bacteria living in milk naturally produce lactic acid as they digest lactose so they can grow and reproduce. This occurs for raw milk as well as for pasteurized milk. Refrigerating milk slows the growth of bacteria. Similarly, warm milk accommodates bacteria thrive and also increases the rate of the clumping reaction.

Now, we can think of a few things that may speed up the souring process. The first one could be ozone that is formed during a thunderstorm. In one of the works it was shown that a sufficient amount of ozone is generated at such times to coagulate milk by direct oxidation and a consequent production of lactic acids. [2] However, if this were the case, a similar effect would occur for sterilized milk. The corresponding studies were carried out by A. L. Treadwell, reporting that, indeed, the action of oxygen or oxygen and ozone coagulated milk faster Ref. [2]. But the effect was not observed if the milk had been sterilized. The conclusion drawn from this study was that the souring was produced by unusually rapid growth of bacteria in an oxygen rich environment.

In the meantime, a number of other investigations suggested that a rapid souring of milk was most likely due to the atmosphere that is well known to become sultry or hot just prior to a thunderstorm. This warm condition of the air is very favourable for the development of lactic acid in the milk. [3, 4] Thus, these studies were also in favour of thunderstorms affecting the bacteria.

A fundamentally different explanation was tested by e.g. A. Chizhevsky in Ref. [5]. It was suggested that the electric fields with particular characteristics produced during thunderstorms could stimulate a souring process. To check this hypothesis the coagulation of milk was studied under the influence of electric discharges of different strength. Importantly, in these experiments the electric pulses were kept short to eliminate any thermal phenomena. Eventually, the observed coagulation for certain parameter ranges was explained by breaking of protein-colloid system in milk due to the influence of the electric field.

Other experiments investigating the effect of electricity on the coagulation process in milk turned out to be astonishing. [6] When an electric current was passed directly through milk in a container, in all the test variations, the level of acidity rose less quickly in the ‘electrified’ milk samples compared with the ‘control’ sample. Which contradicted all the previous reports.

To conclude, while there is no established theory explaining why milk turns sour during thunderstorms, we cannot disregard numerous occasions of this curious phenomenon. [7] What scientists definitely know is that milk goes sour due to bacteria – bacilli acidi lactici – which produce lactic acid. These bacteria are known to be fairly inactive at low temperatures. Which is why having a fridge is very convenient for milk-lovers. However, when the temperature rises, the bacteria multiply with increasing rapidity until at ca. 50°C it becomes too hot for them to survive. Thus, in pre-refrigerator days, when this phenomenon was most popular, in thundery weather with its anomalous conditions the milk would often go off within a short time after being opened. Independently of the exact mechanism, i.e. increased bacteria activity or breaking of the protein-colloid system, the result is – curdled milk.

Should you ever witness this phenomenon yourself, do not be sad immediately. Try adding a bit brown sugar into your fresh milk curds…

— Mariia Filianina

Read more:

[1] F. M. van Helmont Franciscus “The Paradoxal Discourses of F. M. Van Helmont, Concerning the Macrocosm And Microcosm, Or The Greater and Lesser World, And their Union” set down in writing by J.B. and now published, London, 1685.

[2] A. L. Treadwell, “The Souring of Milk During Thunder-StormsScience Vol. XVIII, No. 425, 178 (1891).

[3] “Lightning and Milk”, Scientific American 13, 40, 315 (1858). doi:10.1038/scientificamerican06121858-315

[4] H. McClure, “Thunder and Sour Milk.” British Medical Journal vol. 2, 651 (1890).

[5]V. V. Fedynskii (Ed.), The earth in the universe” (orig. “Zemlya vo vselnnoi”), Moscow 1964, Translated from Russian by the Israel Program for Scientific Translations in 1968.

[6] W. G. Duffield and J. A. Murray, “Milk and Electrical Discharges”, Journal of the Röntgen Society 10(38), 9 (1914). doi:10.1459/jrs.194.0004

[7] “Influence of Thunderstorms on MilkThe Creamery and Milk Plant Monthly 11, 40 (1922).

[8] K. Litzius, “How does a lightning bolt find its target?” Journal of Unsolved Questions 9(2) (2019).

[9] R. Jost (Ed.), “Milk and Dairy Products.” In Ullmann’s Encyclopedia of Industrial Chemistry (2007). doi: 10.1002/14356007.a16_589.pub3

[10] https://en.wikipedia.org/wiki/Milk

Feb 052019
 
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Imagine you are on an airplane, ten thousand meters up in the sky. Now, if you close your eyes you know exactly which way the airplane has started moving, whether it has begun to manoeuvre to the right or to descend. This ability we owe to our inner ear as a part the humans’ vestibular system.

The vestibular system is designed to send information about the position of the head to the brain’s movement control centre, that is the cerebellum. It is made up of three semi-circular canals and two pockets called the otolith organs (Fig. 1), which together provide constant feedback to the cerebellum about head movement. Each of the semi-circular canals is orthogonal to the two others so that they detect the variety of movements in three independent directions: rotation around the neck (horizontal canal), nodding (superior canal) and tilting to the sides (posterior canal). Movement of fluid inside these canals due to the head movement stimulates tiny hairs that send signals via the vestibular nerve to the cerebellum. The two otolith organs (called the saccule and utricle) signal to the brain about linear movements (backwards/forwards or upwards/downwards) and also about where the head is in relation to gravity. These organs contain small crystals that are displaced during linear movements and stimulate tiny hairs communicating via the vestibular, or balance nerve to the cerebellum.

So why is that, even equipped with such a tool, sometimes we get a feeling sitting on an airplane that it is falling down when in fact it is not? Why is that some people, particularly underwater divers, may lose direction and no longer know which way is up?[1] Surely, typical divers should still have the inner ear, unless a shark has bitten their heads off. Is it all caused by stress? Actually, there is much more to it!

Humans have evolved to maintain spatial orientation on the ground, whereas the three-dimensional environment of flight or underwater is unfamiliar to the human body, creating sensory conflicts and illusions that make spatial orientation difficult. Normally, changes in linear and angular accelerations and gravity, detected by the vestibular system, and the relative position of parts of our own bodies, provided by muscles and joints to the proprioceptive system, are compared in the brain with visual information. In unusual conditions, these sensory stimuli vary in magnitude, direction, and frequency. Any differences or discrepancies between visual, vestibular, and proprioceptive sensory inputs result in a sensory mismatch that can produce illusions. Often the result of these various visual and nonvisual illusions is spatial disorientation.

For example, fighter pilots who turn and climb at the same time (they call it “bank and yank”), feel a strong sensation of heaviness. That feeling, caused by their acceleration, surpasses the pull of gravity. Now, if you asked them while blindfolded to tell which way was down using only their vestibular organ, they would point to the cues provided by the turn, not to the cues provided by the earth’s gravity. [2]

Furthermore, the vestibular system detects only changes in acceleration, thus a prolonged rotation of 15-20 seconds [3] results in a cessation of semi-circular output. As a result, the brain adjusts and does not feel the acceleration anymore which can even result in the perception of motion in the opposite direction. In other words, it is possible to gradually climb or descend without a noticeable change in pressure against the seat. Moreover, in some airplanes, it is even possible to execute a loop without exerting negative G-forces so that, without visual reference, the pilot could be upside down without being aware of it.

Another interesting example is the phenomenon of loopy walking. When lost in a desert or a thick forest terrain without landmarks people tend to walk in circles. Recent studies performed by researchers of Max Planck Institute for Biological Cybernetics, Germany, revealed that blindfolded people show the same tendency. Lacking external reference points, they curve around in loops as tight as 20 meters in diameter while believing they are walking in straight lines. [4]

Seemingly the vestibular system is quite easy to trick by eliminating other sensory inputs. However, even when visual information is accessible, e.g. underwater, spatial disorientation can still occur [any scuba diving forum – for the reference]. The obvious fact that water changes visual and proprioceptive perception is crucial here: humans move slower, see differently and let’s not forget the Archimedes’ principle. It happened a lot, that a confused diver thought that the surface was down, especially when the bottom seemed brighter because of reflections. This can be a dangerous mirage in such an unusual gravity. On top of it, water can affect the vestibular system directly through the outer ear. When the cold water penetrates and reaches the vestibular system, it can cause thermal effects on the walls of the semi-circular canals, leading to slight movements of the fluid inside, which are enough to be detected by the brain.[5] Just like in the situations described before this causes the symptoms of spatial disorientation and dizziness.



Fig. 1. Schematic structure of a humans’ inner ear [6].

The vestibular system is indeed frightfully complicated. We can trick it for fun riding roller coasters in an adventure park, but when incorrect interpretation of the signals coming from the vestibular system occurs at the wrong moment this can lead to serious consequences. Luckily, nowadays the airplanes and even divers are equipped with precise instruments used to complement the awareness of the situation and thus avert dangerous situations.

P.S. If you are interested, try riding an elevator while seated on a bike.

— Mariia Filianina

References:

  1. The Editors of Encyclopaedia Britannica, (2012). Spatial disorientation, Encyclopædia Britannica, inc.,
  2. L. King, (2017). The science of psychology: An appreciative view. (4th. ed.) McGraw-Hill, New York.
  3. Previc, F. H., & Ercoline, W. R. (2004). Spatial disorientation in aviation. Reston, VA: American Institute of Astronautics and Aeronautics.
  4. J. L. Souman, I. Frissen, M. N. Sreenivasa and M. O. Ernst,Walking straight into circles, Current Biology 19, 1538 (2009).
  5. http://www.videodive.ru/diving/vizov5.shtml
  6. http://www.nidcd.nih.gov/health/balance/balance_disorders.asp