Question of the Month

Jul 132020
Spread the love

2 You might have encountered the phenomenon: You have a faucet running, for instance, while filling your bathtub, and quite a bit of water is flowing out. Then you want to stop the flow and quickly turn the valve to block the water. Suddenly, in the very moment you close the valve, you hear a banging sound in the walls around you. Sometimes it is rather quiet, sometimes it can be scarily loud. But what is actually causing the sounds and is it something you should worry about?

It turns out that the answer to this question is one of the most relevant design considerations for civil engineering.1,2 It all roots in a fundamental property of liquids: not being compressible. You may have noticed that a balloon filled with air can be compressed quite a bit while one filled with water only changes shape and cannot be made smaller. This very principle is the core of hydraulics, i.e. using the volume of a liquid like water or oil to transfer force in a system.

Maybe you are wondering now how this affects your pipes, considering that the volume of the pipe always stays the same. Nobody (hopefully) compresses the pipes in your walls. There is, however, also another important aspect to a liquid: its momentum. Once a liquid starts to move through a pipe it builds momentum just like a car or a train build momentum when they move (see Fig. 1). As long as the flow can continue undisturbed no ill effects occur and similarly, nothing happens if the flow is slowly brought to a halt by gradually closing the valve. To stay in our analogy, this would correspond to a train of cars slowing down when a road gradually narrows from two lanes to one and eventually to a road blockage.

Figure 1: Analogy of moving water and a train of cars. When the water is still, no momentum is stored in the system. Once the water moves, a significant amount of inertia can be present in the flow and if that flow is suddenly restricted by something (like a valve), the entire inertia must be transferred to the environment. That is the walls of the pipe and the valve itself. ©JUnQ.

The interesting effects happen when we force the flow of liquid to stop suddenly with a valve, essentially causing an accident on our road that causes all the cars to slam into the blockage. In this case, a rather large amount of inertia must be dissipated into the valve and the walls of the pipe causing a large pressure spike. If the pipe can move like a free garden hose (e.g. old pipes that are not properly fixed) this can lead to a sudden jerk and might loosen connections. The story is not so easy, though, for pipes that are fixed in your walls or buried beneath streets: those usually cannot move. As a result, the full force of the pressure acts on the valve and the walls of the pipe – causing the banging sound in your bathroom. The effect is officially called ‘water hammer’.3

But before you start wondering now if your pipes will one day burst and start flooding your apartment: civil engineering has developed a series of safety measured to prevent this from happening in our daily life. To understand how, it is important to know that the pressure spike caused by the sudden restriction in water flow is directly related to the flow rate. In our car analogy this would correspond to the relation between the number of lanes on the road and the number of cars that have to pass through a road segment at any given time. Together this dictates the speed the cars are traveling with and their cumulative inertia. In simple words: fast-moving cars on a narrow street do have a much harder time to slow down than a larger number of slower cars. Therefore, many supply pipes have large diameters, causing the water to flow slowly and thus avoiding the pressure spikes if the flow is restricted by one of the recipients. Additionally, air or spring-loaded pressure relief valves can engage to dissipate a dangerous spike without causing damage and water suppliers usually have build-in safety measures to make sure pumps are not starting up too quickly.

All in all, you usually do not have to worry about a slight banging sound caused by quickly closing a faucet, even though you should avoid it if possible. However, if it is loud or suddenly starts to appear you should be careful to reduce the wear and tear of your pipes or directly ask a pumper. A neat little starting point is Ref. [1]. Anyway, next time you use your sink or shower, think maybe of all the civil engineering that is necessary to let you shower or take a bath anytime at will.

Kai Litzius

Disclaimer: This article is meant to give an introduction into the physics behind the water hammer. In doubt always ask a specialist if your piping needs to be repaired.

Read more:

[3]A similar effect occurs with steam, the so-called ‘steam hammer’, which is an exceptionally dangerous phenomenon that can lead to steam pipes exploding once too much steam condenses and the condensate gets accelerated through the pipe. See also here for a great explanation:

Mar 102020
Spread the love

Traffic development

Absolute nothingness ( Śūnyata ) is one of the most exciting notions in Buddhism. Essentially, it cannot be interpreted anyhow but can be thought of as Ultimate Reality[1]. In Mediterranean tradition, ancient cosmologists introduced another term that sounds more familiar – The Chaos. It was associated with the infinite ocean and expressed an initial state of cosmos in potentia[2]. Not to get numb by the immensity of this semantic unit, we can consider chaos as noise having an infinite spectrum of all conceivable frequencies. And through interaction with external conditions, certain modes manage to become more pronounced as, for example, in the process of stimulated emission build-up in the laser[3] or during the process of natural selection in the theory of evolution.[4]

Traffic development

In the context of road traffic development, we can define the situation in ancient times as the initial chaotic state. As there were no roads as such, the traffic was chaotic. With the evolution of horse-drawn transport, the road map was developing. However, the roads were still only directions along which one could get from one place to another.

The situation changed when engine cars jolted the slow and stagnant horse traffic. Between the man and the road there was no middle link anymore that could choose a better way within the given direction on its own. Nonetheless, engine-drawn transport had an obvious advantage of higher achievable speed. In turn, the desire to move faster and faster required less scattering at the surface roughness, which inevitably resulted in roads getting smoother, i.e., less chaotic. In the meantime, the assembly line was progressing drastically and both factors lead to a dense cloud of potentially fast cars. But people were still scratching their heads why the average speed of the road traffic was not increasing. After a while, they figured out who is to blame in the residual scattering – the interaction of the drivers themselves with each other. With the absence of any predefined rules, everyone had to slow down and likely change the direction to avoid physical interaction with another participant of the traffic. Thus, the necessity of the traffic regulations was obvious.

The first “Convention with respect to the international circulation of motor vehicles” was signed in Paris in 1909[5]. Among others, it contained the sign depicted in Fig. 1, which indicated the road intersection. And naturally, originating from the ship traffic, the habitual priority-to-the-right rule was established to regulate the right-of-way for two vehicles with intersecting directions. Later a set of traffic regulations was complemented with priority signs and traffic lights.

Gödel’s theorems

In 1930 Kurt Gödel presented two theorems reflecting insuperable limitations of formal arithmetics. These theorems had a direct relation to the second problem from Hilbert’s list asking for the proof that arithmetics is consistent.[7] The first Gödel’s theorem (in Rosser form) states that within any consistent formal system S, one can come up with expression A that can be neither proved nor disproved[8]. In other words, the axiomatic system S is incomplete. Hao Wang published in his Logical Journey[9] the full text that Gödel had written about his discovery of the incompleteness theorems:

“In the summer of 1930 I began to study the consistency problem of classical analysis. It is mysterious why Hilbert wanted to prove directly the consistency of analysis by finitary methods. I saw two distinguishable problems: to prove the consistency of number theory by finitary number theory and to prove the consistency of analysis by number theory <…> Since the domain of finitary number theory was not well-defined, I began by tackling the second half <…> I represented real numbers by predicates in number theory <…> and found that I had to use the concept of truth (for number theory) to verify the axioms of analysis. By an enumeration of symbols, sentences and proofs within the given system, I quickly discovered that the concept of arithmetic truth cannot be defined in arithmetic. If it were possible to define truth in the system itself, we would have something like the liar paradox, showing the system to be inconsistent <…> Note that this argument can be formalized to show the existence of undecidable propositions without giving any individual instances. (If there were no undecidable propositions, all (and only) true propositions would be provable within the syosmos in potestem. But then we would have a contradiction.) <…> In contrast to truth, provability in a given formal system is an explicit combinatorial property of certain sentences of the system, which is formally specifiable by suitable elementary means…”

Traffic regulations in the context of the 1st Gödel’s theorem

We can consider any set of interrelated rules, including traffic regulations, as a formal axiomatic system where each axiom is not subject to prove and serves as a basis for further deriving the formulas and theorems (or behavior in a traffic situation). Clearly, the traffic regulations are consistent because otherwise, the number of car crashes would be much higher. Hence, according to the 1st Gödel’s theorem, the system is incomplete. This means that there would always exist a situation, which cannot be resolved regardless of the number of regulations (axioms) contained in the system.

The example of such a situation can be observed on the road intersection regulated by priority-to-the-right rule depicted in Fig. 2. Here four vehicles coming from every direction want to pass this intersection each going straight. There is no way to resolve this situation (to derive the formula) within the traffic regulations system and the drivers in every certain situation are supposed to make the decision: who has the priority.

We can incrementally enhance our axiomatic system by introducing another rule to resolve such a dead-end situation. A rule that gives priority to go first, say, to a red car. Again, four red cars on the same road crossing end up with the same confusion. As long as we add the rules (axioms) into the system enumerably, which is the case for the traffic regulations, such situations will always appear. Introducing the priority signs, constant or variable in time, like traffic lights, or topological road junctions (see Fig. 3) can only decrease the probability of this situation emerging.

Nowadays, most of the intersections are controlled (or topologically resolved). And let’s assume that the preposterous situation with four red cars trying to figure out the right-of-way on the uncontrolled intersection hasn’t happened up to the moment in our complex but finite system of road traffic. Hence, the drivers’ behavior seems to be fully governed with the traffic regulations. However, there still is a possibility of an unresolvable situation, namely, if one comes up with an expression: “I’m not going to obey the rules. For the axiomatic system of traffic regulations, this expression serves as a “liar paradox” and cannot be resolved. Thus people had to come up with the penalty system for acceptable performance of the traffic regulations. But again, it is impossible to nullify the probability of such a situation emerging.

Instead of conclusion

The aim of this text was not to establish a solid theory in either mathematics or law, and the presented examples may not be in strict compliance with the described statements. However, the author finds entertaining the fact that there are bridges between different islands of knowledge accumulated by mankind over the infinite ocean of the unknown.

— Sergei Sobolev

Read more:
[1] D. Mathers, M. Miller, O. Ando. Self and No-Self: Continuing the Dialogue Between Buddhism and Psychotherapy. 2013 Routledge
[3] W. Koechner. Solid-State Laser Engineering, 2006 Springer
[4] C. Darwin. The origin of species by means of natural selection; or, the preservation of favoured races in the struggle for life. 1859 London
[5] Convention with Respect to the International Circulation of Motor Vehicles. The American Journal of International Law Vol. 4, No. 4, Supplement: Official Documents (Oct., 1910), pp. 316-328
[7] D. Hilbert. “Mathematical Problems”. Bulletin of the American Mathematical Society. 8 (10): 437–479, 1902.
[8] Introduction to metamathematics. S. Kleene, 1952 D. Van Nostrand Company, Inc.
[9] H. Wang. A Logical Journey. From Gödel to Philosophy. 1996 The MIT Press.

Jan 122020
Spread the love

Just the thought of getting in touch with or even ingesting urine repels many people. But medical treatment with urine – also called urotherapy – has been a valuable approach in the traditional medicine of many cultures over the last centuries. Usually, endogenous urine is used but animals are also popular sources. The utilization of urine in conventional medicine is not uncommon too. Urokinase, for example, can be isolated from (human) urine and is an important thrombolytic agent.[1] The drug Premarin®, which is used for hormone treatment, contains estrogens that are extracted from the urine of pregnant mares.[2]

Besides milk, camel (i. e. camelus dromedarius) urine plays a special role for desert dwelling people like the Bedouin. Its use was advised by Prophet Mohammed, thus it has found its way into the Islamic prophetic medicine.[3] Apparently, this body liquid cures diseases like tuberculosis, hepatitis, digestion problems, impotence, hemorrhoids, and flatulence, just to name a few. In 2013, one liter of urine from a virgin camel was worth about 15 € (ca 20 USD) in Yemen, where it is not only used for universal medical treatment but also as a cosmetic product for skin and hair care.[4]

src="Dromadaire4478.jpg" alt="Journal of Unsolved Questions (JUnQ): A Moroccan dromedary camel ‒ a favored livestock of Bedouin people. It's urine is said to be medicative."
A Moroccan dromedary camel ‒ a favored livestock of Bedouin people.[5] (public domain – wikimedia commons)

Conventional medicine offers plenty of pharmaceutical cancer treatments which are a blessing and a curse for the patients at the same time. Besides the tedious and exhaustive treatment, patients are confronted with severe side-effects including nausea, fatigue, hair loss, inflammation, and temporary immunodeficiency. The demand for alternatives that are at the same time highly effective, easy to use, mild, and in the best case based on renewable resources is therefore very high.

Camel urine has long been claimed to be an efficient cancer treatment but detailed research on its actual potency and effect on human health is scarce. The soothing effect of pure camel urine on digestive problems can sufficiently be explained through its relatively high content of electrolytes like sodium and zinc as found by Al-Attas, in 2009 – a result that certainly might be achieved just as well by drinking a bouillon.[6] Kohrshid et al. were the first to show an inhibiting effect of lyophilized camel urine on carcinoma cells in animals.[7] In 2011, Alhaider et al. found that treatment of murine hepatoma cells (Hepa 1c1c7, i. e. liver cells) with camel urine inhibited the induction of Cyp 1a1 (a well-known cancer-activator) gene expression by TCDD, a potent Cyp 1a1 inducer and a known carcinogen. Among virgin, pregnant, and lactating camels, the virgin’s urine was found to be most potent while the urine of pregnant camels showed the least potency.[8] One year later, Khorsihd et al. showed that the potency of camel urine to reduce a specific type of lung cancer cells (A549) is somewhat dependent on the breed (Majaheem urine was found to be more effective than Magateer urine) and the age of the camels. The depletion of the cancer cells ranged between 85‒93% of the starting cell number.[9,10] The bioactive subfraction PMF which is believed to be responsible for these effects is obtained from lyophilized camel urine (in literature frequently called PM701).[10] Clinical trials on the oral uptake of PM701 fractions showed no negative effects on human health so far.[11] Apparently, the urine contains a high amount of antibodies of such a small size, that they can be easily absorbed by the patient’s digestive system.[12] Other experiments also show antimicrobial effects of camel urine on bacteria and fungi.[13] Aiming at the environmentally friendly substitution of synthetic agents which are usually obtained from complex multistep reactions this approach is most honorable. It is exciting to see that a waste product has the potential to cure severe diseases although much more research must be done on this subject to clearly verify the efficacy. After all, urine is an excretion that contains various less beneficial digestive metabolites, and even toxins that the body wants to get rid of and indisputable evidence for the efficacy and safety of the PM701 fractions are vital.

For those people who are curious enough to try camel urine for whatever reason but are too disgusted by the idea to drink it pure, a solution might be on the way: there are capsules of PM701, or PMF respectively, but they are not yet available on the market.[10] Another alternative might be camel milk which sounds much more enjoyable and is supposed to be a medicine just as magical as camel urine. It is said to “reduce blood sugar […] solve the problems of autism in children, enhance the immunity of the body…” and many more.[14] Alas, some bad news comes from the World Health Organization (WHO) concerning the use of camel milk and urine: shortly after the Middle East respiratory syndrome coronavirus (MERS-CoV) outbreak in Saudi Arabia in the year 2012 dromedary camels were found to be zoonotic transmitters, meaning that the virus is rapidly transferred from animals to humans – just as we experience right now with the latest outbreak of a coronavirus (COVID-19).[15] As a consequence the WHO advises to avoid contact with camels or consuming raw camel milk and urine.[16] This surely dampens the enthusiasm to utilize camel urine and we might have to wait a few years more for some groundbreaking results in cancer research.

‒ Tatjana Dänzer


[1] “Abstracts of Papers Read”. American Journal of Physiology. Legacy Content., 1952, 171, 704–781.
[2] D. Brügger, „Hormone aus Stutenharn“, pharma-kritik, 2019, Nr. 5/6/1997.
[3] Alhaidar, A., Gader, A. G. M. A., Mousa, S. A., The Journal of Alternative and Complementary Medicine, 2011, 17, 803‒808.
[6] Al-Attas, A. S., Arab J. Nucl. Sci. Appl., 2009, 42, 59–67.
[7] Khorshid F., International Journal of Pharmacology, 2008, 4, 443‒451
[8] Alhaidar, A. A.; El Gendy, M. A. M.; Korashy, H. M.; El-Kadi, A. O. S., Journal of Ethnopharmacology, 2011, 133, 184–190.
[9] Alghamdi, Z.; Khorshid, F., Journal of Natural Sciences Research, 2012, 2, 9‒16.
[10] Khorshid, F. A., 2009, US 20090297622.
[11] Khorshid, F. A., Alshazly, H., Al Jefery, A., Osman, M. A.-M., Journal of Pharmacology and Toxicology 2010, 5, 91‒97.
[12] Hamers-Casterman, C.; Atarhouch, T.; Muyldermans, S.; Robinson, G., Hammers, C.; Songa, E. B.; Bendahman, N. and Hammers, R., Nature, 1993, 363, 446‒448.
[13] Mostafa, M. S.; Dwedar, R. A., British Journal of Pharmaceutical Research, 2016, 13, 1‒6.
[14] Hammam, A. R. A., Emirates Journal of Food and Agriculture, 2019, 31, 148‒152.

Dec 172019
Spread the love

Due to technical improvements during the last years, machines outcompete humans in a couple of specialized tasks: Whereas it can take a human person very long to calculate the square root of a (non-square) number, a computer can finish this calculation at high precision within a fraction of a second. However, there are some areas in which machines still cannot compete with nature (yet). One of them is olfaction: Currently, no device is available that could replace police dogs with the ability to detect trace amounts of molecules. Similarly, farmers sometimes even train pigs to search for truffles hidden in the soil. Of course, the ability to detect relevant molecules in low amounts offers an enormous advantage and is thus subject to extensive optimization by evolution.
L’odorat, Honoré Daumier (circa 1839, public domain – wikimedia)

How exactly olfaction works in higher organisms has not been known for a long time. Nonetheless, it had been intuitively clear that there must be specific receptors interacting with the corresponding odours. This simple assumption has a remarkable consequence: Since mammals can distinguish a high number of odours, there also must be a high number of different receptors encoded in the genome. Indeed, the two scientists Linda Buck and Richard Axel discovered a comparatively large family of genes encoding for odorant receptors [1]. For this discovery, they were awarded the Nobel Prize in Physiology or Medicine in 2004. The activation of these receptors on the cell surface always results in similar intracellular reactions. If a cell had receptors for different odour molecules on its surface, it could therefore not distinguish these odours. In accordance to this consideration, it turned out that each olfactory cell only carries one type of all the different odorant receptors encoded in its genome. Why exactly this is the case is still not known in detail to date. Even more surprisingly, it even turned out that the axons of cells, which carry the same type of odorant receptor on their surface, end on the same set of cells.

An odour can of course consist of several kinds of molecules. The activation of different combinations of olfactory sensory neurons further increases the number of differentiable odours. A phenomenon seemingly similar to the exclusive expression of a single odorant receptor by an olfactory sensory neuron is the generation of only one type of antigen receptor by immune cells. They achieve this by a complicated recombination of genes, which is clearly not observed in olfactory neurons.

Investigating how a biological structure develops is often very helpful: In a later work, Linda Buck could show that in contrast to mature olfactory neurons, there are multiple mRNAs for different odorant receptors in immature neurons [2]. Why cells of our body can have entirely different morphologies and properties even though they all carry a copy of the same genome is a fundamental question which keeps many biologists busy. It is the differential expression of the genes in a cell, which causes these differences. This gives muscle cells the ability to contract and enables neurons to generate action potentials.

However, all olfactory neurons express a very similar pattern of genes except for their odorant receptor. One of the reasons for the transcription of different amounts of RNAs from different genes is the spatial arrangement of the DNA in the nucleus. Had it not been tightly packed into the nucleus, the DNA in each cell would have a total length of 1.8 m and highly condensed sections of DNA are usually not accessible for transcription into RNA. Stavros Lomvardas, a former member of the group of Richard Axel, could show that DNA segments encoding for odorant receptors on different chromosomes get pulled close to each other in a small spatial region in the nucleus. Interactions between the different DNA segments encoding for odorant receptors could contribute to the exclusive transcription of one specific odorant receptor gene [3,4].

The relevance of the spatial arrangement of the DNA within the nucleus for gene expression is an open question of major interest beyond olfaction. To which degree there is a specific nuclear arrangement of DNA and how this is established after cell division would then be further important for other unsolved questions in biology.

— Tobias Ruff


  • [1] Buck, L. and Axel, R. , A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 1991, 65-1 PP175-187 DOI:10.1016/0092-8674(91)90418-x
  • [2] Hanchate, N. K. and Kondoh, K. and Lu, Z. and Kuang, D. and Ye, X. and Qiu, X. and Pachter, L. and Trapnell, C. and Buck, L. B. , Science 2015, 350-6265 PP1251–1255
  • [3] Clowney, E. J. and LeGros, M. A. and Mosley, C. P. and Clowney, F. G. and Markenskoff-Papadimitriou, E. C. and Myllys, M. and Barnea, G. and Larabell, C. A. and Lomvardas, S., Cell 2012, 151-4 PP724–737
  • [4] Markenscoff-Papadimitriou, E. and Allen, W. E. and Colquitt, B. M. and Goh, T. and Murphy, K. K. and Monahan, K. and Mosley, C. P. and Ahituv, N. and Lomvardas, S., Cell 2014, 159-3 PP543–557

Sep 102019
Spread the love

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



[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).


[6] (Water bubble in space at time index 8:18).

 Tagged with:
Sep 042019
Spread the love

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


May 222019
Spread the love

Once, thunderstorms with thunder and lightning were interpreted as signs of the god’s wrath; nowadays, we are taught the mechanics behind a thunderstorm in school. You are probably already thinking about ice crystals that are smashed together by strong winds inside clouds, creating static charges in the process. How does a lightning bolt, though, find its way from the cloud to the ground? This question still keeps scientists awake at night – and there is still not a clear answer to how exactly the formation and movement of a lightning bolt work. This Question of the Month will give a brief summary on how a lightning bolt selects its target.

Lightning [1,2] occurs always when a large thunderstorm cloud with strong winds generates sufficient electrostatic charge that it must discharge towards the ground. The discharge itself occurs (simplified) in a twostep process, consisting of a main lightning bold and a preflash: The preflash travels as comparably weak (but still dangerous!) current downwards from the cloud. This usually happens in little jumps, which have been investigated with high-speed cameras. They show that the current path is apparently selected randomly by slowing down at a given position and then randomly selecting the next to jump to. This random selection appears to happen within a sphere of a few tens of meters in diameter around the tip of the growing lightning bolt. The process also involves growing many tendrils with individual tips and thus covers a large area (see also Fig. 1). With this procedure, the lightning bold eventually “feels” its way to the ground until it reaches it either directly or via a structure connected to it.

Figure 1: Lightning bolts are branching off into many tendrils. [3]

Therefore, if a conductive object reaches into such a sphere, the bolt will immediately jump to it and use it as a low-resistance shortcut to the ground – as a result, if possible, shortening the path for the discharge. This behavior leads to the curious effect of exclusion areas around structures that are protected with lightning rods, in which practically no ground strike will occur, and a person will not be hit directly. Unfortunately, this will not completely protect the person, as the electricity can still be dangerous within the ground.

Now that the preflash has found a path to the ground, the second phase starts, and the majority of the charge starts to flow with up to 20 000 A along the path found by the preflash. This is also the portion of the discharge that is visible by bare eye. It can consist of several distinct discharges that all follow the path of ionized air of the previous one, creating the characteristic flickering of a lightning bolt.

How the entire process from preflash to main discharge works is still not completely understood today and much of the presented insights were simply gathered phenomenologically by camera imaging. Additionally, there are many more types of and effects related to lightning bolts, which are relevant for our understanding of a variety of weather phenomena. All in all, thunderstorms are still something magical today, even if only figuratively.

— Kai Litzius

Further reading:




[4] Chem. Unserer Zeit, 2019, 53. DOI: 10.1002/ciuz.201980045

Mar 012019
Spread the love

Just a few years before Dolly was born as the first surviving clone of a sheep in 1996, the movie Jurassic Park was launched, based on the same-named novel by Michael Crichton.[1,2] In this story scientists insert genetic material derived from fossils into amphibious eggs to bring all sorts of dinosaurs back to life. The actual cloning of animals follows a quite similar approach called somatic cell nuclear transfer or SCNT (fig 1): a nucleus with the desired DNA is isolated from a somatic (body) cell and introduced into an emptied ovum of the same species. Several electrical impulses excite the cell and stimulate proliferation in a nutritional medium. The most stable cell clusters, called blastomeres, can then be transferred to a host mother and grow into an embryo.[1] Dolly managed to fully develop into a lamb and lived 13 years until she died of an infection. She even gave birth to a lamb, proving the viability of cloned creatures.[3] Blastomeres that are dissected instead of implanted can be used to treat diseases or might enable the growth of tissue. Maybe in the future we will be even able to grow a whole surrogate organ ‒ an approach that is highly controversial since human somatic cells are mostly derived from embryotic tissue.[4]

Fig 1: Schematic depiction of the SCNT process: The nucleus with the desired genetic material is inserted into an empty egg cell which is growing into a blastomere.[5]

According to a report from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) about one million species of an estimated number of around 8 million species (only counting eukaryotes) on earth are currently endangered or threatened with loss of habitat.[6,7] In the history of Earth extinction has mostly been a consequence of natural disasters like climate change, volcanic eruptions, or meteorite impacts until human population started to expand.[8,9] The IPBES report demonstrates the present impact of human behaviour on biodiversity and it seems that we are facing many more extinctions caused by anthropogenic reasons in the next decades. It has become a growing interest to not only preserve existing species but also to revive those that have already died out.

One attempt is currently being made to revive Quaggas, a subspecies of the living plain zebra that has died out in the 1880s (fig 2), by selective breeding. Due to their close genetic relation some plain zebras that resemble the characteristic pattern of the quaggas have been selected in the hope to one day give birth to a zebra that looks just like them and shows similar genetic information.[10,11,12]

Fig 2: Taxidermied Quagga foal in the Museum of Natural History in Mainz, Germany. (© Tatjana Dänzer)

More demanding is the CRISPR Cas9 method: the DNA that can be extracted from most fossils like the woolly mammoth could be much too old to produce a healthy individuum. But their DNA might be partially recovered by replacing some sequences in the DNA of their closest living relative, the elephant, with extracted mammoth DNA. The genome will not be the same as it was millions of years ago and no one really knows how this will influence the livability of the animals.[13]

But most of the extinct species do not have such close relatives anymore. Interspecies nuclear transfer like in Jurassic Park can be another possibility for de-extinction, that means to revive species that have gone extinct or are on the verge of extinction. The San Diego Zoo Institute for Conservation Research maintains a large collection of cells and embryos called Frozen Zoo®.[14] By using reproductive technologies they develop methods to prevent endangered species like the northern white rhino or the Przewalski horse from extinction or inbreeding.[ 15] The first animal of an endangered species that was successfully cloned was a gaur (bos gaurus), an Asian ox, in 2001 by Advanced Cell Technology using genetic material from the San Diego Zoo. DNA from the skin cells of a male gaur were implanted into empty cow egg cells, grown into blastomeres that were then transferred into the wombs of domestic cows. One of eight embryos developed to a full-grown calf. Unfortunately, after being born, the gaur did not live for more than two days. However, the cause of death is considered to be an infection and not the fact that it is a trans-species clone.[16] The second clone that was created with the very same method had a higher life expectance. It was a banteng (bos javanicus), another endangered Asian cattle. Also remarkable is, that the used fibroblasts were taken and frozen 25 years before, in 1978.[17] An attempt to clone a species that has already gone extinct, the Pyrenean ibex (capra pyrenaica pyrenaica) failed since the kid was born with a deformed lung.[18]

The fact that cloned cells do in principle develop to embryos and even prolific adult animals (like Dolly) gives hope that one day species that have recently been wiped out could come back to life. But besides the challenging and time-consuming scientific research these plans also evoke a lot of critical questions in the society:

How is decided which species will be revived and which stays extinct?

It is clearly difficult to revive every species that we know has ever lived on this planet. There would just not be enough space and food and we might soon experience another wave of mass extinction. Since DNA from fossils might be too old, mammoths and dinosaurs are still out of question. This is shifting the focus on species of the recent past. But how can we select which species can live again and which won’t? We surely must consider the preservation of still existing species as a priority.

Where should they live?

If it is possible to clone many animals of one kind that can even mate, there must be a safe and nourishing environment, most likely captivity. Who knows how an entire species that has been created in captivity will develop? And the knowledge about the behaviour and needs of most of those animals is very little.[13]

Who is going to pay?

The scientist’s motivation might surely be an idealistic one but somehow all the research and maintenance must be financed. Innovations will always attract temporizers that try to exploit it financially. Zoos and wildlife parks that exhibit animals are the lesser problem. Some worry that wealthy poachers and “gourmets” who don’t withhold from hunting and eating endangered species now will just as much be attracted by the thought of getting hold of a cloned specimen. Paying to hunt an endangered species to support the protection financially is already practised in southern Africa and raises a lot of ethical issues.[19,20]

To see living “fossils” like dinosaurs, mammoths, dodos and all the others is surely an exciting thought. But if mankind proceeds like this, in just a few decades there might be much less animals on earth than there are now. Let’s hope that combined common sense, technical progress, and less vanity will lead to a preserved and healthy nature in our future.

‒Tatjana Dänzer

Read more:

[1] I. Wilmut, A. E. Schnieke, J. McWhir, A. J. Kind, K. H. S. Campbell, Nature 1997, 385, 810–813.

[2] M. Crichton, Jurassic Park, Alfred A. Knopf, Inc., 1990.


[4] S. Lü, Y. Li, S. Gao, S. Liu, H. Wang, W. He, J. Zhou, Z. Liu, Y. Zhang, Q. Lin, C. Duan, X. Yang, C. Wang, J. Cell. Mol. Med. 2010, 14, 2771‒2779.

[5] By en: converted to SVG by Belkorin, modified and translated by Wikibob – derived from image drawn by / de: Quelle: Zeichner: Schorschski / Dr. Jürgen Groth, with text translated, CC BY-SA 3.0,


[7] C. Mora, D. P. Tittensor, S. Adl, A. g. B. Simpson, B. Worm, PLoS Biology, 2011, 9, 1‒8.

[8] D. B. Weishampel, P. Dodson, H. Osmólksa, The Dinosauria, 2nd ed., University of California, 2004.

[9] D. P. G. Bond, P. B. Wignall, Geological Society of America Special Papers, 2014, 505, 29–55.



[12] J. A. Leonard, N. Rohland, S. Glaberman, R. C. Fleischer, A. Caccone, M. Hofreiter, Biol. Lett., 2005, 1, 291‒295.

[13] B. Shapiro, Genome Biology, 2015, 16, 1‒3.

[14], [15]


[17] D. L. Janssen, A. L. Edwards, J. A. Koster, R. P. Lanza, O. A. Ryder, Reproduction, Fertility and Development, 2004, 16, 224‒224.




Feb 052019
Spread the love

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


  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).