The Hourglass and some science - Contents of life

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The Hourglass and some science

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I posted some still shots of  the hourglass in motion, which usually is lying on my office table. Mostly unnoticed, except for that moment of urge to flip it. One fine day, I felt, it looks interesting and worth sharing. So, I tool some snaps and posted them here. It caught attention of a lot many people and some interesting queries also came up.
So, then I decided to post its complete video. Here it is:

Now, a normal or an original hourglass has sand as the real constituent for the measurement of time. To begin with, there are two connected glass bulbs carrying sand, and small hole through which the sand can swiftly pass from one glass bulb to another. The amount of time required for complete transfer of sand from one bulb to another, will be a measure of time. Usually, it comprise of sand, which counts upto an hour for its complete transfer. There is a big history related to the hourglass and can be found here.
Now, the hourglass, lying on my table is not that ancient one. Infact, recently bought from a streetshop located on an island hill of Elephanta caves. (More on that later.) It is a constructed showpiece, the size of which vary with the amount you are ready to shell out. In my case, more than the size, I got fascinated with the color, and so this hourglass is not actually a hourglass, but, just one minute and  ~ 30 seconds glass.
There is no sand inside the glass. Instead, a mixture of two immiscible liquids. Immiscible means, the two liquids which will not at all, mix with each other. Example, Oil and water. Depending on the density, one of the liquid will always tend to be at the bottom. And this tendency of the heavy liquid will initiate the process into the motion. So, as the hourglass is flipped, the heavy liquid goes at the top. Because of its heavy nature, it will tend to move downwards. Since liquids have high degree of freedom, they can replace other liquid component from its place. So, the heavy liquid, moves through the small towards the bottom. And at the same time, replaces the light liquid from its position, compelling it move into the upper glass. The small hole assures that a very small volume is transferred at a time. And as the liquids are immiscible, these small volumes end up getting separated in form small liquid drops. This makes the view even more fascinating.
There are some nice observations to make here. The downward flow of drops is accompanied by an upward flow of drops too. This shows an equal exchange of liquids occupying each others space.
Second and more interesting thing is, check video around 1.10 minutes. You can see a bubble form appearing in the upper glass. From where is this bubble appearing? After all, it is composed of only two liquids and no air!
The thing is, as the transfer of liquid is getting over, there is very small amount of dense liquid remaining in upper glass. As more and more of light liquid rising up, at some point, the dense liquid assumes a layer form, pushed up by the upcoming light liquid. The dense liquid remain in layer form, failing to escape the upper glass. Slowly, this layer surrounds the upcoming light liquid, giving a feel of a bubble. This layer can resist the upthrust of light liquid to an extent and then it brakes  away into tiny liquid drops which can now escape the upper glass. This point, where the liquid brakes away from its thin layer form into tiny liquid  drops can give a measure of interfacial tension which exist between the two liquids.
Just related to this, do you know why the liquid always take form of spherical drops? Why don't we ever see squarish or pyramidal kind of water drops. The reason is again the interfacial tension. Lesser is the surface area, lesser is the tension. And we know, sphere has the minimum surface area.

One more query, out of the two liquids seen in this hourglass, which one do you think is water?
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