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!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
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!
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.
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!
More to Explore
Svenja Lohner, PhD, Science Buddies
Science Buddies |
Water droplets, two-component mixture, materials science
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