User:Brian P. Josey/Notebook/2010/06/08

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I am experimenting and getting a feel for making and using emulsions in preparation for the salad dressing idea. What I do today and tomorrow is going to be very rough, basic, and fundamental. Ultimately, I would like to have a good idea of what I'm doing in a general sense before I go on vacation on Thursday to let it gel while I'm out of town.

An emulsion is a type of mixture that results from mixing two chemically different liquids together. The liquids are immiscible, and typically the mixture is of one hydrophilic and one hydrophobic chemicals. Today, I am using water with dissolved quantum dots in mineral oil. Because the chemicals are immiscible, they naturally want to separate from each other, so a surfactant is needed to suspend droplets of one liquid in the other. The surfactant needs to be chosen so that it has both a polar and non-polar end and will surround one in the other. In my case, I need an emulsifying agent that is more soluble in oil than in water because oil is the in the greater quantity. Because of this, I am using Tween 20, or Polysorbate 20.

Procedure and Notes

Generally, the outline of today's project is to suspend quantum dots in solution, suspend that solution in mineral oil using Tween, and look at it under the microscope.

Quantum Dots

I am using the green quantum dots today. The procedure for preparing them is:

  • Spin the stock solution of quantum dots at 5,000 g for 3 mins in the centrifuge in the refrigerator. Use a counterbalance.
  • Put 1 μL of the dots in a tube, 2 μL if you want a higher concentration, and dissolve it in 99 μL (or 98 μL) of the large stock solution from the storage cylinder.
  • Store at 4°C

This dilution creates a clear solution without any distinguishing features.

Emulsion 1:5 Water in Oil

In a new centrifuge tube, I mixed the following chemicals in this order:

  • 20 uL of Qdots
  • 2 uL of Tween 20
  • 78 uL of Mineral Oil

This created a liquid with two phases separated by their densities, both clear and with a thin layer in between it. The lower and more dense layer was water, while the upper layer was oil. Due to either the surface tension of the Tween or greater adhesion of the oil to side of the tube, the water had a rounded surface with a curvature facing downward. As expected, shaking the tube did not mix the two. I put the liquid in the sonicator, resulting in a milky-white clouding of the liquid, confirming I had created an emulsion.

Flow Cell

I put some of the emulsion into a flow cell, and I noticed that the emulsion was not completely consistent in how opaque it was. Maybe it was from the denser water settling into larger droplets, but the bottom of the tube was more opaque than the top. When I collected for the flow cell, I drew from the center of the tube, to get as close to average density of water droplets in oil as possible. In the flow cell, the emulsion had a texture, and it hinted at the uneven distribution of water droplets in the oil.


When I put the flow cell onto the microscope, it became clear that I had too much water in the emulsion. For one, the solution was far too opaque, and everything was fluorescing as green. It appeared that there was a thick texture in the flow cell, and what I assume was the water-oil interface traced a heavy thick pattern around the field of view. With the exception of a few spots, it was impossible to see through this texture. I am going to take this as one extreme in density of water in oil, and I am going to repeat it at least one more time with a much lower concentration of water.

Emulsion 1:100 Water in Oil

This new emulsion is made with the same procedure, but substituting in the following volumes for the three chemicals:

  • 1 μL quantum dots solution
  • 0.1 μL Tween 20
  • 98.9 μL mineral oil

I also am putting this one into the sonicator for a longer period of time, to get a hopefully more even mixture.


A rough mixing of the chemicals makes it impossible to see any difference between the emulsion and the raw mineral oil, the volume of the quantum dots solution is so small in comparison. However, after being mixed in the sonicator, the liquid turns a very slight milky-white, confirming that there is an emulsion in the tube.

Looking into the microscope, it is clear that I don't need the quantum dots to see the bubbles, as I have a fairly consistent spreading of droplets of water that are mixed in with much larger drops of water.


I am repeating the procedure from the last one, this time I am replacing the 1μL quantum dot solution with 1 μL stock ferritin solution.

Again, the ferritin solution is denser than the oil, and is easier to see as it is significantly darker. When I first put it into the sonicator, the ferritin as a whole jumped up in the tube, but after a while it finally broke down, creating a milky-white color. After taking this out of the sonicator, it had a yellowish-orange color to it. I don't know if this is from the red color of the ferritin or from diffusion of light around the droplets. Either way, ferritin appears at this level to have gone into a successful emulsion.

In the flow cell, it looks like a more opaque version of my 1:10 dilution of ferritin.

Through the microscope, the droplets were much, much smaller, but more evenly spread out than in the quantum dots case. I placed a magnet on the slide to see what, if anything, effect there would be, and I wasn't able to get any movement. Some of the smaller droplets were moving a little. But I could not distinquish the movement as being from Brownian motion or from the magnetic force. This is something I will have to repeat in the morning.