Dancing Water Droplets
Do you like watching raindrops on your window as they move and run down the glass? It is fascinating to observe how some of them just sit there by themselves, whereas others combine to build a larger drop. Have you ever wondered what makes them move and behave in different ways? Controlling and influencing wetting, the spreading of liquids on surfaces, is actually important for many industries. For some purposes, you want a surface to repel a liquid; just think of your water-repellent jacket as one example. Other times you need the fluid to spread evenly across a surface, such as paint on a wall. But can you predict what a drop of water will do on a specific surface? Will it spread or form a droplet? It is even more complex if the liquid is not only water, but a mixture of different components. Sometimes the outcome can be very unexpected, and even fascinating to watch. Try this activity and see for yourself!
What determines how a liquid behaves on a surface? The answer seems easy: it depends on the liquid and surface properties. Different surfaces have different molecular structures, which result in different affinities for water or other liquids. Materials that do attract water, such as clean glass, are called hydrophilic, meaning water-loving, and water spreads easily on them. Other surfaces, like Teflon or plant leaves, exhibit the opposite behavior and are water-fearing or hydrophobic, and water will form a round droplet on their surfaces. The shapes of a droplet on a specific surface can actually tell you a lot about the characteristics of the material.
In liquids, surface tension is the property responsible for the shape of liquid droplets. This is how it works: The liquid molecules inside a droplet want to cling to their surrounding molecules. This makes the molecules inside the liquid stick together. However, on the surface, there are fewer liquid molecules around, which leads to the formation of stronger bonding between the surface molecules. As they get pulled mostly inward by the molecules below, all the surface molecules help “hold together” to form a droplet. You have probably seen examples of surface tension in action without realizing it: Tiny droplets of water on the window or plant leaves after it's rained or water striders walking on water are just a few of them. In this activity, you will explore another example of surface tension in action and you will discover that other factors influence droplet behavior, too. Be prepared for some colorful droplet action!
- Adult helper
- Paper towel
- White paper
- Optional: microscope glass slide and pliers
- Heat-tolerant, flat glass surface, such as a borosilicate Pyrex glass tray or baking dish (in good condition with no cracks)
- Liquid food coloring (needs to contain propylene glycol as main ingredient)
- Medicine dropper
- Oven mitts
- Permanent marker
- Gas stove or a camp stove or Sterno
- Lighter or match
- Yogurt lid or wax paper
- Safety glasses
- Paper towels
- One sheet of white paper
Warning This project places a heat resistant glass in a flame. For safety reasons, it is essential to use a heat resistant glass in good condition and to let all bystanders wear eye protection.
- Place paper towels or other protective layers that can get dirty on your work area.
- Take the medicine dropper and put a line of 5 water droplets on a plastic yogurt lid or wax paper. They should all be approximately the same size.
- Choose one color of food coloring (blue, green, red or yellow) and place one drop of concentrated food color from the bottle next to the line of water drops.
- With the medicine dropper, take a small amount of the concentrated food coloring from the drop next to your line, and put it into the first water droplet of the line. Mix it by sucking the droplet in and out the dropper.
- Now suck up a little amount of this first droplet with the medicine dropper and mix it into the next fresh water droplet in the line.
- Continue doing this for all 5 water droplets. At the end, you should have a line with 6 droplets of food coloring in different concentrations.
- Rinse your medicinal dropper with fresh water.
- Repeat the whole mixing procedure with a new line of 5 water droplets and another of food coloring (blue, green, red or yellow). Make sure to rinse your medicine dropper with fresh water in between colors. Set the yogurt lid aside for now; you will need the food color droplets later in the experiment.
- Ask an adult to perform the next steps for you.
- Adult: Light up the camping stove or the gas stove. If you do not have either one available, light up the Sterno with a match or lighter and use this flame. Note: it will work better with the stove. Caution: When handling open flames, always be very careful! Check your environment for safety and tie up loose hanging hair to not set them on fire. Be extra careful to not to burn yourself.
- Adult: Put on your safety glasses and hold the heat-resistant glass dish with the oven mitts. Pass the bottom flat area (an area of 3 cm x 6 cm is enough) slowly through the gas stove flame or the camping stove flame for about 30-45 seconds. The flame should touch the glass. If you use the Sterno, pass the flat glass surface through the flame for about 1-2 minutes. Make sure the glass actually touches the flame. Remember where the spot on your dish is that touches the flame, as you will use this area for your experiment. Caution: Be careful when passing the glass through the flame. Small microscopic cracks inside the glass can make it shatter when exposed to heat. Make sure to only use a heat-resistant or heat-tolerant glass dish, and protect the eyes of all bystanders with eye protection (safety glasses).
- Alternatively, if you have microscope slides, hold them with pliers and pass them in the flame for 30 seconds (gas stove or camping stove) or 1 minute (Sterno).
- Caution: The glass will be very hot. Carefully put the glass dish or microscope slide on your work area (the site that you passed through the flame needs to face upwards) and let the glass cool for approximately 10 minutes.
- Once your glass has cooled a bit, put a white sheet of paper on your work area and place your glass dish (it actually works better if it is still warm), or microscope slide on the paper. Remember that the area that faced the flame has to face upwards.
- Using your rinsed medicine dropper, put a small drop of pure water onto the glass surface. What happens to the drop? Does it spread on the surface or does it keep its drop shape?
- With the medicine dropper, take a small droplet of concentrated food coloring from your yogurt lid or wax paper (pick any color) and carefully place it on the glass. How does this droplet behave? Does it look different than the pure water droplet? Do you notice any other difference between the two drops?
- Now put a droplet of the same concentrated food coloring next to the other concentrated food coloring droplet on the glass. They should not touch each other, but should be in close proximity (about 3-6 mm apart). How do the two droplets behave? Are they moving or are they still? If they are moving, in which direction do they move and what happens to them? Tip: If you do not see anything happening, you probably haven’t heated up the glass surface enough. Wipe off the glass and try to pass the glass surface through the flame again, making sure to concentrate the flame to one area and give it enough time in the flame. It works best with microscope slides, but flat surfaces of a heat-resistant glass dish are also fine to use. You can also try to put the droplets on a different spot of the heated glass surface, as some areas might work better than others.
- After rinsing your medicine dropper, take another droplet of concentrated food coloring (pick any color) and place it on the warm glass surface.
- From your line of food coloring droplets on the yogurt lid or wax paper, pick the least concentrated one of a different color and put a small droplet next to the concentrated food coloring droplet on the glass. Make sure to rinse your medicine dropper first and place the second droplet so it does not touch the other one. Do these two droplets show the same behavior as the two concentrated food coloring droplets? Do you see any differences? How do they behave differently?
- Now pick several colors and concentrations of food coloring from your yogurt lid or wax paper and place many different small droplets randomly on your warm glass surface. Make sure to rinse your medicine dropper before you switch colors or concentrations. Try to also make different droplet sizes. What happens to the different droplets? Does the color or concentration change their behavior? Can you make out any behavioral patterns?
Extra: Use a permanent marker and once your glass surface becomes lukewarm, draw two parallel lines, about 3-5 mm apart from each other, on the surface (don't worry, the marker will wipe off). Put one small concentrated droplet of food coloring in between the two lines. Next, pick a low-concentration food coloring droplet of another color and place it in front of the concentrated droplet in between the lines. The droplets should not touch, but should be close to each other. How will the concentrated droplet interact with the low concentrated droplet? What does the permanent marker line do? How do the droplets interact with the marker line?
Extra: What other droplet moves can you create? Get creative with the permanent marker to design different tasks for the droplets. Can you make two droplets jump on top of each other? What about a droplet chase in a circle? Can you find a way to sort droplets depending on food coloring and concentration?
Extra: Can you think of other ways to influence the behavior of your droplets on the glass or any other surface? They will not dance on every surface, but you can try to play around with droplet formation and spreading. Think of surfaces such as the fabric of your rain jacket, plant leafs, plastic or a wooden table. Will treating the surfaces with soap, olive oil or alcohol change how droplets form? What happens if you change the surface roughness?
Observations and Results
You should have seen all kinds of droplet behaviors on the glass surface. The droplet of pure water probably spread out pretty fast. Treating the glass dish or microscope slide with the flame made the surface very unfavorable for droplet formation and more susceptible for water spreading. However, the concentrated food coloring contains not only water, but also a chemical called propylene glycol. The mixture of both components allows the formation of a more sphere shaped droplet, which you should have noticed when putting the concentrated food coloring on your glass surface. The water on the droplet surface evaporates faster than propylene glycol, and at the same time, has a higher surface tension. These differences result in a symmetric internal water flow inside the droplet that stabilizes it and stops spreading.
You might have seen that the food coloring droplet was floating or moving on the glass a little bit. When putting a second food coloring droplet next to the first one, they glide over the glass and move toward each other. This attractive motion is vapor-mediated, meaning that one droplet influences the water evaporation of the second droplet, leading to an imbalance of the stabilizing internal water flow. The resulting asymmetry starts the movement of the droplets toward each other. Once the two droplets meet, their water/propylene glycol mixture determines their behavior. If the mixture is the same, the two droplets join to become a bigger droplet, as you might have seen. If you choose a very different food coloring concentration for your second droplet, you can observe one drop chasing the other. The reason for this chasing behavior is the difference in surface tension of the two droplets. Lower-surface-tension droplets (droplets with higher concentration of food color) will chase the droplets with higher surface tension (droplets that have lower food coloring concentration). This makes for some fun observations: Did you see your droplets dancing once you put many different droplets with lots of different colors and food coloring concentrations on your warm glass surface?
If you start introducing permanent marker lines to the warm glass surface, you can guide the movement of the droplets, as the marker lines are very water repellent and the droplets cannot cross them. Knowing this, you can create lots of fun exercises for the droplets, making them run in circles or race each other. Just try it!
Ask an Expert
- Rinse all your glassware or microscope slides with water to get rid of the food coloring.
- If you used a glass dish, you can rub away the permanent marker lines with a paper towel.
- Wash all the other materials you used (yogurt lid, medicine dropper) and throw used paper towels or papers in the trash.
- Clean your work area with water and soap and do not forget to wash your hands.
- Stanford researchers solve the mystery of the dancing droplets, from Stanford News
- 2 common liquids spontaneously form dancing droplets, from Scientific American
- How to easily observe dancing droplets using everyday materials, at Vimeo
- Dancing droplets, at YouTube
- Science Activity for All Ages!, from Science Buddies