This is one of the more fun science experiments I’ve done in a while.
I wasted half the day watching these drops of water dance around.
I even forgot to pick up the kids from school (almost – I did get there late).
With a hurricane getting ready to hit town this may be just what you need to keep busy when you’re stuck inside.
This spring a group of scientists at Stanford University published an article describing the motion of droplets on a clean glass surface that was just plain mesmerizing. Colored dots chase each other around and seem to dance together.
Take a moment to check out the video. You’ll be glad you did.
The basic science is this:
Take any two liquids that mix but do not react together. In the case of food coloring this is water and propylene glycol. If you place a drop on a smooth, clean surface it will form a round droplet.
However, the mixed liquids will have different evaporation rates so that as one of the liquids evaporates the concentration of the other will become higher. This evaporation also changes the surface tension of the droplet so that the outside of the drop, where evaporation takes place, changes its surface tension and causes the liquid inside the drop to sink and rise – which starts the drop moving.
If you add another drop with a different concentration of liquids, the evaporating vapor from each drop act as a signal to the other causing them to move towards each other and dance around on the slide.
This phenomenon is a possible explanation of the physics behind the biological process of chemotaxis, in which cells move in response to a chemical stimulus. For example, white blood cells move towards bacteria in a cut or scratch.
This particular instance of chemotaxis in liquid molecules has potential applications for cleaning surfaces. For example such a process could lead to self-cleaning solar panels
If you want to learn more, check out this article from Stanford.
The best part about this discovery is that it uses materials that you can find in most kitchens – or at least can purchase cheaply, if you don’t already have them in your basic chemistry set. The scientists even provided a video showing us all how to observe the effect ourselves. However, they obviously use a pipette and other materials you can only find in a lab.
I wanted to see if I could do it in my kitchen. Turns out I could!
Here are the materials you will need:
- Microscope slides (I found some good ones for $5 on Amazon)
- Food coloring
- Droppers (the smaller the better! Mine were pretty big and it still worked)
- Tweezers, forceps or some other means for holding the slide over a flame.
- Gas stove top (or a Bunsen burner if you are at school – candles don’t get hot enough)
- 3 Cups
- Measuring spoons
- Sharpie (optional)
- Ruler (optional)
- A clean dry surface (I used a piece of paper)
Prepare your liquids
- Pour a small amount of straight up food coloring in one cup.
- Make a 50/50 solution of water and food coloring in the second cup. I used 1/8 teaspoon of each.
- Make a 75/25 solution of water and food coloring in the last cup. I used 1/8 teaspoon of food coloring and 3/8 teaspoon of water.
Prepare your glass slide
- Use your forceps or tweezers to tightly hold the short end of your slide. The metal will get very hot. I used a small cloth to protect my fingers but make sure you keep the cloth clear of the flame.
- Turn the stove burner on high.
- Gently and slowly move the class around in the flame so that the bottom side of the slide is exposed for at least 20 seconds.
- Turn off the burner but continue to hold the slide in the air for another 20 seconds before gently putting it down UPSIDE DOWN on the paper. You want the side that was exposed to the flame to be facing up. The hot flame effectively cleans the slide by burning off any dirt on the surface.
*NOTE – if you put the slide down too soon it will burn the paper and likely crack. I learned this the hard way.
You are now ready to play with your droplets!
Place a few drops of different concentrations of food coloring on the slide. In this case, smaller drops are preferred.
Try drawing a track with a permanent marker to race your droplets. What other shapes can you try?
Below is a video of a couple experiments in my kitchen. The speed is 4x which causes my one-handed cell phone filming to look a little jumpy, but it is still mesmerizing to watch!
If this sounds like so much fun you want to use it for your science fair project, here are some questions you could explore:
- Do different colors of food coloring behave differently?
- How close to the drops need to be to “sense” each other?
- What is the smallest concentration of food coloring that causes this type of motion?
- What happens if you don’t clean the glass with a hot flame first?