Key concepts
Physics
Mixture
Materials science
Water droplets
Introduction
Have you ever watched raindrops on your window as they move and run down the glass? It is fascinating to observe how some of them 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 liquids spreading on surfaces, known as "wetting," is actually important for many things—beyond simply watching the rain. For some purposes, you might want a surface to repel a liquid; just think of your water-repellent jacket as one example. Other times you need the liquid 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!
Background
What determines how a liquid behaves on a surface? The answer seems easy: it depends on the liquid and surface. 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, such as Teflon or plant leaves, exhibit the opposite behavior and are water-fearing or hydrophobic, meaning 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 it is on.
In liquids surface tension is the property responsible for the shape of liquid droplets. This is how it works: Liquid molecules inside a droplet want to cling to their surrounding neighbors. This makes the molecules inside the liquid stick together. On the surface, however, 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 has rained—or water-strider bugs 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!
Materials
- Adult helper
- Water
- Paper towel
- White paper
- Microscope glass slide and pliers (optional)
- Heat-tolerant, flat glass surface, such as a Pyrex glass tray or baking dish (in good condition with no cracks)
- Liquid food coloring (that contains propylene glycol as the main ingredient)
- Medicine dropper
- Oven mitts
- Gas stove or a camp stove or Sterno cooking fuel with matches or a lighter (Be sure to use your heat source in an appropriate area with appropriate precautions; always follow all manufacturer instructions.)
- Yogurt container lid or wax paper
- Safety glasses (to protect experimenters’ and observers’ eyes)
- Paper towels
- One sheet of white paper
- Permanent marker (optional)
Preparation
- Place paper towels or other protective layers that can get dirty on your work area.
- Take the medicine dropper and put a line of five 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 coloring 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 five water droplets. At the end, you should have a line with six droplets of food coloring in different concentrations.
- Rinse your medicine dropper with fresh water.
- Repeat the whole mixing procedure with a new line of five water droplets and another selection 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-coloring droplets later in the activity.
- Have your adult helper light the camping or gas stove. If you do not have either one available, light the Sterno with a match or lighter and use this flame. Use extreme caution when handling open flames. Check your environment for safety and tie up loose hanging hair.
- Have everyone put on eye protection.
- Have your adult helper hold the heat-resistant glass dish with the oven mitts. Pass part of the bottom flat area (an area of three by six centimeters is enough) slowly through the gas or camping stove flame for about 30 to 45 seconds. The flame should touch the glass. If you use Sterno, pass the flat glass surface through the flame for about one to two minutes. Make sure the glass touches the flame. Remember where the spot on your dish is that touches the flame—you will use this area for your activity. (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 all bystanders should be outfitted with eye protection, such as safety glasses.)
- Alternatively, if you have microscope slides, hold them with pliers and pass them in the flame for 30 seconds (gas or camping stove) or one minute (Sterno).
- Use caution, the glass will be very hot. Carefully place the glass dish or microscope slide on your work area. (The side that you passed through the flame should face upward.) Let the glass cool for approximately 10 minutes.
Procedure
- Once your glass has cooled a bit, place a white sheet of paper on your work area. Next, place your still-warm glass dish or microscope slide on the paper. Remember that the area that faced the flame should face upward.
- 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, now 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 three to six millimeters 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 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 there. 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—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.
- Make sure to rinse your medicine dropper again. Then, 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 so neither drop touches. 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: 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? (Food coloring and oil-based substances can stain, so be sure to get permission before trying these liquids on different surfaces.)
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. The concentrated food coloring, however, 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 drop 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? (Click here to read more and for a video explanation by the scientists who figured out how dancing droplets work.)
Cleanup
Rinse all your glassware or microscope slides with water to get rid of the food coloring. Wash all the other materials you used (yogurt lid, medicine dropper) and recycle the yogurt lid if you are not planning to reuse it. Clean your work area with water and soap and do not forget to wash your hands.
More to explore
Stanford Researchers Solve the Mystery of the Dancing Droplets, from Stanford News
2 Common Liquids Spontaneously Form Dancing Droplets, from Scientific American
Vapor-Mediated Sensing and Motility in Two-Component Droplets (pdf), from Nature (Scientific American is part of Nature Publishing Group.)
Droplet Dancing, from Physics Central
Science Activity for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
