Water Conference Handout

8/12/2019 Water Conference Handout

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Name: Water Is Life Conference 2014

Aquanamics!

In Physics, you dont have to go around making trouble for yourself - nature does it for you.

Frank Wilczek, theoretical physicist & Nobel laureate

1 Anti-Gravity Water

Have you ever observed this neat magic trick where someone fills a cup with water, covers the cupwith a water-permeable object, flips the cup upside down, and to your surprise, the water defiesgravity by not flowing out? The truth is, the water seems to defy gravity, but in fact, it does not!

Lets think about it. Gravity wants to pull the water out of the cup, but somehow the water doesnot want to come out. Hence, a logical deduction would be that there is a force that is actingagainst gravity and that is preventing the water from falling out. Indeed, physicists have discoveredsuch a force and they call it surface tension.

A Bite-Size Introduction

Surface tension can be thought of as a property that every kind of liquid has. It is formally definedas the force (per unit length) that pulls a liquid to a shape that reduces its surface area as much aspossible. On the microscopic scale, surface tension exists in a liquid because at the surface of a liquidthe liquids atoms/molecules are pulled to each other via inter-molecular forces of attraction. Thus,a liquids atoms/molecules are in the most favourable energy state when they are closest to each

other. This means that the surface area has to be minimised so that the liquids atoms/moleculescan be as close to each other as possible.

Figure 1: A water globule floats freely on the middeck ofspace shuttle Atlantis in front of NASA astronaut LelandMelvin. Note that the water globule forms a spherical

shape to minimise its surface area.

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A Little Clarification

It is important to realise, however, that sometimes when one discusses surface tension one mayreally be discussingadhesion, which usually comes hand-in-hand with surface tension. While surfacetension is about the cohesion between a liquids atoms/molecules at the liquids surface, adhesionis about the attractive forces between atoms/molecules of two different media. Thus, while the

scenario in Figure 1 is directly related to surface tension because the water globule is in vacuum,what we usually observe in the laboratory (e.g., hydrophobic surfaces, meniscus, etc.) and in nature(e.g., Lotus effect) are really the effects of adhesion. For example, the common phenomenon ofwater forming a concave surface in a narrow tube (i.e., meniscus) is a result of the stronger adhesionbetween water and the walls of the tube as compared to the surface tension and cohesion betweenwater molecules at and near the tubes walls.

Figure 2: A computer-generatedimage of the Lotus effect.

In this activity, surface tension will be the main actor since it explains the defiance of gravity bywater, while adhesion will be the side actor since it explains a few subtle but interesting phenomenathat accompany the main catch of the trick.

What Did You (Not) See?

After you have completed the experiment, you must have made some cool observations, so do penthem down and try to account for them! Do be specific so that what you write will make sense toyourself! Also, since the experimental procedures are not foolproof, there are no absolutely rightor wrong answers!

Q: What did you observe about the water? How did it defy gravity?

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Q: What did you observe about the tissue membrane? Did it curve upwards, downwards or remaintaut? Whichever the case, why?

Q: What happened when you shook or tilted the cup a little? Why did whatever you saw happen?

Q: What happened when you did not stretch the tissue membrane across the mouth of the cuptightly? Why did whatever you saw happen?

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Proposed Explanations

So, there are pores in the tissue membrane and yet the water does not seep through them just likehow water gushes through the opening of your water bottle into your mouth. The short answerwould be surface tension, but of course an elaborate explanation would be more satisfactory.

Figure 3: Illustration of water molecules andthe hydrogen bonds between them.

First, it may be good to examine what gives rise to waters surface tension. On the microscopiclevel, each water molecule is made up of two hydrogen atoms and one oxygen atom. The hydrogen

atoms link up with the oxygen atom by covalent bonding. Due to the strong electronegativity ofthe oxygen atom, however, the shared electrons in the bonds tend towards the oxygen atom, thuscreating a slight negative polarity on the oxygen atom and slight positive polarities on the hydrogenatoms. This phenomenon, termed hydrogen bonding, is the primary cause of water moleculespulling each other attractively, which in turn accounts for the surface tension of water.

Figure 4: Illustration of howwater defies gravity. Do

note that the size of the

water moleclues are grossly

exaggerated.

Now, let us zoom in on what happens at the interface of the tissue membrane and the water. Letus be clear about a few things. First, surface tension is not the only force that is supporting theentire weight of the water. Surface tension is relatively much weaker than the gravity acting on theentire water mass, which is why with a large hole like the opening of your water bottle water stillgushes out. Second, continuing from the first point, the tissue membrane supports the bulk of thewaters weight. Surface tension only becomes significant at the tiny pores on the tissue membrane,where it prevents water from trickling out due to gravity.

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Figure 5: An illustration which possi-bly explains why the tissue membrane

curved upwards.

Let us shift our focus to the other subtle phenomena now. The first phenomenon that we (theworkshop conductors) observed was that after the cup was flipped upside down, the tissue membraneactually curved upwards a little!

We believe that this occurred due to two reasons. First, the adhesion between the cups wall andthe water was stronger than the cohesion and surface tension between the water molecules nearthe cups wall, hence a concave meniscus was formed. Second, the adhesion between the tissuemembrane and the water at the centre of the water-tissue interface was so strong that the waterpulled the tissue membrane upwards by a little.

Another thing that we observed was that the water spilled out of the cup both when the cup wasshaken or tilted and when the tissue was not stretched across the cups mouth tightly. We believethat the water spilled out in both cases because of imbalances in the waters pressure on the tissuemembrane (which resulted from tilting, shaking, and a loose tissue membrane).

Conclusion

Figure 6: A sequence of camerashots showing a water droplet

bouncing off a water surface.

While you might not have observed the subtle phenomena, hopefully you managed to observe waterdefy gravity! While surface tension and adhesion are extremely weak when compared to the forceswe usually observe in daily life (e.g., pushing someone, watching rain fall, etc.), those of us who areattentive would know that they are really ubiquitous. In fact, they play an important role in manyareas of science such as materials science!

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2 Soap-Powered Boat

Figure 7: A common designof the boat you can use. The

green-coloured part is where

the soap should be applied.

Heres another awesome trick create a light boat using paper or cardboard, place it gently onwater, apply a drop of soap directly behind the rear of the boat, and the boat will zoom off!

As with all magic tricks, once you understand the science behind that small but essential drop ofsoap, you will be more satisfied than surprised!

What Did You (Not) See?

We are jumping straight into the recording of your observations because the science behind thistrick is actually just an extension of the concept of surface tension and we hope you will be able tocome up with the explanation on your own!

Q: Did the boat move forward, backward or did it remain static? Why? Do draw diagrams if thathelps you understand and explain the phenomenon better!

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What Really Happened

If you were successful in your experiment, your boat should have moved forward quite quickly. Somequestions would then pop up why did the boat move forward, and how did it move so fast?

Physicists have discovered the mechanism behind the boats motion and they call it the Marangoni

effect. It is the phenomenon where mass is transferred along an interface between two liquids withdifferent surface tensions. It occurs because the liquid with higher surface tension will pull on itssurrounding more strongly than the liquid with lower surface tension, thus causing mass to flowfrom the liquid with lower surface tension to the liquid with higher surface tension.

Figure 8: Flow visualisation of surfactant re-leased by a water strider at its rear in water.

This provides a good analogy of how the soap-

powered boat works.

With this knowledge, we can now answer the three questions that were asked earlier. The boatmoved forward because the drop of soap caused the soap-water liquid mass to spread outwardradially, and the part of the liquid mass that moved forward pushed the boat forward.

Figure 9: 2D illustration of how the soap in-teracts with the water to create a force that

pushes the boat forward.

Then, why did the boat move quickly? First, realise that fast is a relative concept, so let uscompare the scenario where the Marangoni effect is cause of the boat moving forward with thescenario where the slight ripple caused by the drop of soap is the cause of the boat moving forward.It is immediately intuitive that in the latter scenario, the ripple would be too weak to acceleratethe boat to such a high speed. Try dripping a drop of water at the rear of the boat and you wouldmost certainly either not see the boat nudge or see it move really slightly. Hence, we can inferthat the boat moved quickly because the difference in surface tensions of the soap and the waterresulted in a net force of significant magnitude that pushed the liquid mass and the boat forward.Of course, another factor would be the sheer lightness of the boat but even if the boat were light,a slight ripple would still not be able to accelerate the boat to such a high speed.

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Conclusion

With just a drop of soap we managed to demonstrate the sheer power of the Marangoni effect!The effect has been made used of both in nature and by Mankind some water insects use it as ameans of locomotion and semiconductor wafers can be dried because of the effect.

Figure 10: Tears of wine onthe upper walls of a wine-filled

glass.

One famous example which demonstrates the effect is the tears of wine phenomenon, where some

wine droplets are seen flowing down the upper walls of a wine-filled glass. What happens is thatthe wine near the walls of the glass climbs up the walls slightly due to the strong adhesion betweenthe glass and the wine. Then, since alcohol is highly volatile, the alcohol insde the wine that hasclimbed up the walls evaporates, leaving water behind. This creates a surface tension gradientacross the wine that is near the walls and the water that is already on the walls, thus allowing theMarangoni effect to occur (i.e., the water gets pushed upwards). This process repeats itself, thuscreating the beautiful tears we often see when we drink wine.

So the moral of the story is: the next time you see the tears, if you do not like them do not blamethe waiter or waitress for not pouring the wine into your glass properly!

3 Magnet-Propelled Water Boat

We have yet another jaw-dropping trick to show you, using just water and magnets. In case youare wondering how these two entities are even related, you will see from this trick that a magnetcan repel water!

A Bite-Size Introduction

Yes, you did not read wrongly. Magnets can repel water! This is due to something called diamag-netism, which is an intrinsic property of all materials. In the presence of an external magnetic field,

diamagnetism would cause the creation of a magnetic field that opposes the external magneticfield, thus resulting in repulsion. To truly understand this phenomenon, we would need to delveinto the crazy world of quantum mechanics. However, there is, fortunately, a rougher yet simplermodel that can explain the phenomenon adequately.

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Figure 11: Illustration of Lenzs law.

This model involves a law in electromagnetism called Lenzs law, which in our context can simplybe understood as that an external magnetic field passing through a conductive ring will inducean electric current in the ring which will in turn create a magnetic field that opposes the externalmagnetic field. Now, let us recall that all materials are made up of atoms, and every atom has anucleus that is surrounded by revolving electrons. Thus, if we apply an external magnetic field toany material and treat the revolving electrons as conductive rings, we are effectively allowing Lenzslaw to take effect!

Of course, your next question would then be: why is it that I dont see every material being

repelled by a magnet? There are two reasons for this. First, the electrons are constrained bythe pull of the positive nucleus and the Pauli Exclusion Principle. This is very complicated stuff,but the basic idea is that in reality the external magnetic field does not induce a steady andsignificant electric current (i.e., a strong and consistent circular flow of electrons)that in turncreates a significant opposing magnetic field. Second, in some materials, diamagnetism can beovershadowed by paramagnetism and ferromagnetism, which are other forms of magnetism where anexternal magnetic field causes a material to produce a complementary magnetic field, thus resultingin the material being attracted to the source of the external magnetic field. For example, an ironbar is both a diamagnet and a ferromagnet, but its ferromagnetism far outweighs its diamagnetism.You may then ask other questions: what gives rise to ferromagnetism and paramagnetism, and whyis water purely diamagnetic? The answers to these are beyond the scope of this workshop, but ifyou are interested you may explore the concepts magnetic domains andspin magnetic moment.

Conclusion

Figure 12: Real image of a froghovering over a magnet. The

water content of the frog is

what enables this remarkable

feat to be achieved.

If your boat did not move much, do not be surprised. Diamagnetism is really weak, so it is normalthat you do not see the boat get repelled strongly by the magnet. If you felt the slight repulsion ofyour hand while holding the magnet near the water-filled test tube, consider that an achievement!Anyway, because of its sheer weakness, diamagnetism has very few applications. Nevertheless, itis a really interesting phenomenon, and who would know if in the future scientists would discovernew ways to utilise this marvelous property?

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4 Jumping On Water

Figure 13: Image of two people walking on

water. An event was organised in Malaysia,where people were asked to travel across the

pool by walking, running, and even cycling!

Finally, its time for the last magic trick of the day! Before you jump to conclusions, let us clarifyone thing of course you cannot jump on pure water! Nevertheless, you can still jump on water,that is, water with cornstarch added to it. This still does sound pretty impressive, isnt it?

Truth Of The Miracle

So how did you manage to jump on something which you would expect to sink in? The answer isthat adding cornstarch to water creates a new fluid mixture (people call it oobleck) which can beclassified as a non-newtonian fluid.

For our purposes, we do not need to know what this term means, but it would be useful to know thatthere are many different types of non-newtonian fluids and that oobleck can be further classifiedas a shear-thickening fluid. A shear-thickening fluid behaves such that when the rate at which thefluid is being deformed increases, its viscosity(i.e., internal resistance to flow) increases too.

Figure 14: Shear-thickening fluids have recentlysparked interest among military scientists, for rea-

sons which are stated in the annotated graphic.

Now, let us analyse the situation. Perhaps it is simply because of the stronger cohesion (i.e., surfacetension) between the oobleck particles that has allowed you to jump on the oobleck. Sure, it doesplay a significant role in supporting your weight, but if you try to stand on a particular spot on theoobleck you would realise that you would start sinking!

This is where the concept of a shear-thickening fluid comes in handy. Since at each instant whenyour feet press against the oobleck you are effectively applying a non-linear increasing amount offorce, the amount of deformation that results increases at an increasing rate. Then, due to thePoisson effect, which is the phenomenon where when a material gets stretched or compressed inone direction, it gets compressed and stretched in the other two perpendicular directions, the fluidgets shear-strained at an increasing rate. This, by definition of a shear-thickening fluid, causes theviscosity of the oobleck to increase. In other words, the oobleck hardens and behaves more like asolid. Now you know why you could walk on water!

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5 Some Last Words...

At last, we have come to the end of the workshop! We hope that you have had fun with the magictricks and that you have learnt some Physics about water! Something that occupies more than70% of the Earths surface is certainly worth studying about! Anyway, heres a joke to end the

stressful day!

The End

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Water Conference Handout