Archive for February, 2013

In our last post we discussed the goal and methods around capturing the birefringence found in frozen soap bubbles.   The good news is that we established a solid method for capturing birefringence, but so far have yet to see any real noticeable birefringence on soap bubbles.

Along the way we did capture some amazing photos of soap bubbles in several different states of frozen, and finally just shot some images of just ice crystals forming.  We have some videos of the growth of crystals which we might post later.

In order to capture images such as these, you need to polarizing sheets.   One covers your light source (flash, led, etc), the other over your camera. It is important that the polarizing sheet is the last optical element before the subject you wish to photograph.

First, more photos of "ice crystals we really wished showed birefringence ".  We have a few theories about why this doesn't work (including: thin film from glycerine causing scatter, too many layers of ice, etc).  At this point it's still unknown, any comments welcomed!  The good news is that the soap is showing it's lovely colors!

Well, that sure was neat, but we didn't really get the birefringence in water crystals we wanted. So it was time to just try old fashioned ice crystals and liquid nitrogen. The results where spectacular:

Here's where it got interesting. By rotating the polarizing film different parts of the crystals would show birefringence based on the changing angles interacting with each other.



Frozen bubbles!

Posted by 3ricj on 3 February 2013

Transparent solids can show birefringence when they are under mechanical stress. This stress can be present in a part after it's manufactured (in the case of plastic) or present due to thermal expansion. You can view these birefringence patterns if you view it between two crossed polarizers.

These patterns can also be found in ice. I decided, on a whim, that would attempt to photograph birefringent (cross polarized) crystals in frozen soap bubbles. This is what is hopefully going to be short set of posts with attempts to do so.

For starters, making frozen bubbles has it's own challenges. When air cools, it compresses. This would likely lead to a fracture of the bubble. The first attempt to make frozen bubbles confirmed this - if you inflate a bubble using (warm) air from your lungs, it pops the moment it gets close to something cold. In this case, we tried this with a pool of liquid nitrogen - -  it fractured well before hitting the liquid. We did manage to make some 'broken half bubbles', which floated around on the gaseous nitrogen. I don't have any photos of this, but let's just say it didn't work so well. After some trials and tribulations we developed the following method to make frozen bubbles:

  • Take a short (12") copper pipe
  • dip one end into a "bubble solution", adding additional glycerin may help.
  • Make sure that there is bubble solution coating the outside of the pipe; a thin film will work fine.
  • Submerge the other end into a cup of liquid nitrogen.
  • The warm copper will cause a phase change in the nitrogen, which will inflate the bubble with chilled nitrogen.
  • Before it pops, gently shake the pipe such that bubble 'slides' down to the pipe.
  • Take the pipe and hold it carefully over a pool of liquid nitrogen. There will be a thermal gradient there which enables the bottom of the bubble to freeze.
  • With some luck and skill, you can "thaw" and "refreeze" your bubble many times before it bursts.

Here are some photos of our first round of testing.  At the time we didn't have a good setup for capturing the birefringence in the crystals then we ran out of liquid nitrogen!. We will have to try again. On the next post I'll provide more information about how to capture birefringence using a camera.

A nitrogen filled bubble:

Frozen bubbles!

A shot of an old fashioned ice-cube under cross polarization (you can see birefringence!!):

According to the USB Battery Charging Specification, a device plugging into a USB port to charge may find itself connected to a source that is capable of data transfer as well as power, or it may be connected to a source that provides power only. If the source supports data, the device is expected to do a trickle charge only, but if the source does not support data, the device may draw more current because the source is likely to be a wall socket. (More detail on Wikipedia.)

So those of us who use USB car chargers with our Android phones really want the phones to charge as fast as possible. Unfortunately, most car chargers do not short the data pins together, which is the spec-compliant way to indicate that the power source does not support data. It would seem that this gets past manufacturers' QA because the iFail devices apparently ignore the spec and draw as much current as they want, regardless of the state of the data pins. This leaves Android users stuck with trickle charge off their car chargers, unless they go out and buy a specialized charge-only USB cable which shorts the data pins.

For those of us who want a car charger that supplies 1+ amps without needing a special cable, the Mediabridge dual port high output charger is easy to take apart and add solder to short the pins, and this post shows how to do it. I wrote this up because I've done it at least 3 times so far and I always forget the fastest way to put it back together. I am indebted to this review of the charger model in question.

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