Glowing Floating Spheres in Sugar Water

Tuesday, October 21st, 2014

Here are the instructions as a PDF. (Making Glow in the Dark Floating Balls)

Description:  A while ago, I found these index matching hydrophilic spheres.  They seemed pretty cool and worth the $3 investment.  They look like little balls, and when placed in water, they slowly swell, and they disappear.  They don’t really disappear, they get the same index of refraction as water, so light doesn’t bend when it goes through water and the balls, so they appear to be invisible.

Not long after buying them I started to wonder if they could absorb other compounds.  I found out that they absorb sugar quite nicely, and some dyes like rhodamine B.  That led me to the image you see below.

Glowing floating index matching balls.

Glowing floating index matching balls.

 

Materials

  • 4 beakers
  • Regular Growing Spheres (hydrophilic water gel)
  • Sugar
  • Buret
  • Ring stand and buret clamp
  • Short piece of rubber tubing
  • Short piece of glass tubing
  • Rhodamine B dye (optional)
  • Food Coloring (optional)

Instructions:

  1. Make about 100mL of 4 different sugar solutions with warm water.
    • 0g of sugar in 100mL of water – 0%
    • 5g of sugar in 100mL of water – 5%
    • 10g of sugar in 100mL of water – 10%
    • 15g of sugar in 100mL of water – 15%
  2. Drop a 5-6 hydrospheres each of the solutions above.  It will take several hours for the balls to swell and disappear, but you should be able to see that they have changed pretty quickly.  I usually let this sit overnight.
  3. If you want to dye the balls or the water, add the dye now.  Food coloring can work well, but it they only faintly absorb the color.  It absorbs rhodamine B very well, and you only need a few crystals to dye the liquid.

Making a Sugar Density Gradient

  1. In a 100mL graduated cylinder, place a ball from the 15% solution, followed by the 10% solution, followed by the 5% solution, and then the 0% solution.  There should be 4 balls in total.
  2. Attach a piece of glass tubing as long as the cylinder to a buret with a piece of rubber tubing.  Make sure the stopcock is closed.  (Use set up shown below)

    A buret, piece of rubber tubing, and piece of glass tubing are used to pour liquids to the bottom each time.

    A buret, piece of rubber tubing, and piece of glass tubing are used to pour liquids to the bottom each time.

  3. Pour in about 15mL of the 0% solution and open the stopcock to a slow flow.  When the solution in the buret has almost run down, pour in about 15mL of the 5% solution.  Repeat with the 10% solution, and the 15% solution.  Try not to let there be a gap between solutions, which could let in air.
  4. When all of the solutions have been added, quickly remove the tube.
  5. To make the fluorescence happen, use a UV source like this UV flashlight.
Balls floating in a sugar density gradient.

Balls floating in a sugar density gradient.

Glowing floating index matching balls.

Turn on the UV light for extra impressiveness.

 

What happened?

Normally, different concentrations of sugar solutions, when mixed, would make a new sugar solution with a new concentration and density.  When you mix them slowly and carefully, they still mix, but it takes a long time.  In the meantime, you have a heterogeneous mixture of sugar and water that is separated by density.

The balls, when placed in the solutions will absorb the sugar and the water, and get the same density of the solution, so you when you stack them in the graduated cylinder and pour in the liquids, they will be pushed up by the more dense solutions.  When you are done, you have balls that are layered by density.

To improve upon this, you can dye the solutions different colors or you can dye the balls different colors, or both.  I was lucky to have some fluorescent dye and a UV flashlight which leads to the ultra cool image you see here.

Have fun!


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