Tuesday, 26 April 2011

Ants Create a Lifeboat

So imagine this.  You and 1000 other people are stranded on one side of a river.  You need to get downriver for more food and safety.  To float you have to resist surface tension of water.  You could build a boat to try to resist the surface tension of water.  If you are a fire ant you would have solved this problem.  I learned that not only can these ants leave a nasty bite they are also pretty smart.  By linking legs the worker ants create a living raft supported by the surface tension of the water they can now float downstream to greener pastures in the Amazon.  The Queen sits nicely on top of the raft with the rest of the colony resisting surface tension.

Fire Ants Surf Floods on Rafts of Their Own Bodies - ScienceNOW

Fire Ants Surf Floods on Rafts of Their Own Bodies - ScienceNOW

Thursday, 14 April 2011

Surface Tension of Acid Rain

So in the previous post I talked about how my first grade teacher distorted physical principles of the raindrop.  When I was going home I thought about acid rain.  What is the effect of acid rain on surface tension?
Acid rain occurs when sulfur dioxide and nitrogen oxide react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds in the atmosphere.  If you have ever been to the Acropolis in Greece you can see the result of the rain as much of the building gets damaged by the rain resulting from the pollution around Athens.  Aquatic life is also harmed by the acid rain.   So how does the acid in the water affect surface tension?

Yes adding an acid would change the hydrogen bonding within the water and at its surface.  So it would lower the surface tension of the water.  This would obviously have an affect on aquatic life that lives on the surface of the water as well as it would change the temperature of the water.  I asked this question from Kibron, the company we bought the AquaPi.  They provided me with some of this answer.  They also mentioned that oil companies use the AquaPi to measure acidic water at the refineries.    They actually go with this instrument into the field and scoop up water then measure it.  Cool!

Wednesday, 13 April 2011

How my first grade teacher distorted me

Not physically correct

A proper raindrop

I still hate how my first grade teacher have distorted my view of the world.  It still amazes me that they teach creationism in schools in the US (so I am glad I was not educated there).  Like most children I spent some of the time drawing in first grade.  We drew things like a heart, a star and a raindrop.  They say these are supposed to look a certain shape that is in no way resembling of that shape.  I drew it how I thought it might look and the teacher said I have to try again.  After many tries I drew the heart, the star and the raindrop that she wanted.  Fifteen years later I find myself disappointed when I am dissecting a cat in a vertebrate biology class to see nothing like the heart in my grade one class.  
Also I learn that raindrops are nothing like as I drew them in grade one.  Nearly every drawing of raindrops I have seen depicts them as having a sharp upper point. This like many other things in life the first grade teachers are completely wrong. If they had studied a little more physics and a little less (what do first grade teachers study?).  I would not be as confused as I am now.  The surface tension of water acts like a stretched "bag" around the water. Unless some other force is acting, it pulls the water into a spherical shape. When we see droplets of water our eyes see them the tiny droplets as a blur.  With slow motion photography shows that small raindrops are nearly spherical. The larger ones are distorted by the pressure of moving air, but this doesn't make points!  It makes them somewhat flattened. Think of it this way: underwater bubbles are not pointed as they rise, just as falling water drops are not pointed as they fall. 
One might point out that when water drips from a faucet it has a point.  However, the water droplet never actually has a point. Instead it has a narrow neck.  It holds on to the faucet due to its surface tension.  After the neck has snapped, it is yanked back into the falling ball of water. 

If you will excuse me I have to take that failed drawing I got of improperly shaped droplet back to my first grade teacher and complain...

Friday, 8 April 2011

Cap'n' Crunch Does it again....

Look into your bowl of cereal when the last remaining chunks of cereal remain.  Do it!  Those last bit of Rice Krispies, Fruit Loops or Cap'n Crunch are floating to the top doing  waiting for you to put that bit of sugary goodness into your mouth.  Before your shove that food into your face did you stop to think?  Why is my Cap'n Crunch floating together like those garbage piles floating in the Pacific?  You have a great big bowl.  Why is it not randomly dispersed around the bowl to increase its entropy?

The last pieces of cereal always clump together in milk due to surface tension interactions of the milk hitting the side of the bowl.  The side of your IKEA bowl (I just know you shop there) is made of ceramic.  Milk adjacent to the side of a container curves either upward or downward, depending on whether the liquid is attracted to or repulsed by the material of the wall.  Since milk in this case is attracted to ceramic, the water surface in this container will curve upwards near the container walls, as this shape increases the contact area between the water and the ceramic. Dense objects like Cap'n Crunch or garbage can rest on liquid surfaces due to surface tension. These objects deform the liquid surface downward. Other dense objects, seeking to move downward (to the middle of your bowl) but constrained to the surface by surface tension, will be attracted to the first.  Objects with an irregular meniscus (since the Cap'n Crunch is rough) also deform the water surface, forming "capillary multipoles". When such objects come close to each other, they first rotate in the plane of the water surface until they find an optimum relative orientation. Subsequently they are attracted to each other by surface tension.

This is explained by  Dominic Vella and L. Mahadevan of Harvard University (scientists actually study this but they used Cheerios instead.

where γ is the surface tension, K1 is a modified Bessel function of the first kind, B = ρgR2 / γ is the Bond number, and

is a nondimensional factor in terms of the contact angle θ. This a convenient meniscus length scale:

Useful references taken from Wiki:

  1.  "Scientists explain the 'Cheerio Effect'". MSNBC. http://www.msnbc.msn.com/id/9425907/. Retrieved 2006-08-28. 
  2.  Chan, D.Y.C.; Henry, J.D.; White, L.R. (1979). "The interaction of colloidal particles collected at the fluid interface". Journal of Colloid and Interface Science 79 (9): 410–418. 
  3.  Stamou, D.; Duschl, C.; Johannsmann, D. (2000). "Long-range attraction between colloidal spheres at the air–water interface: The consequence of an irregular meniscus". Physical Review E 62 (4): 5263–5272. doi:10.1103/PhysRevE.62.5263. 
  4.  Vella, D.; Mahadevan, L. (September 2005). "The Cheerios effect". American Journal of Physics 73 (9): 817–825. doi:10.1119/1.1898523.

Disclaimer:  I hate Cap'n Crunch because it is not crunchy enough and is not really so healthy for any meal of the day.  So Captain Horatio Crunch can suck it.  Rice Krispies so much better.  The Rice Krispies treats very delicious.  Snap, Krackle and Pop should sink Cap'n Crunch's fictitious boat.  Seriously ironically!!