Marangoni Effects

04.30.19

Experiment leaders: Jane, Rocky, and Charlie

UMN Science for all volunteers visited Venture Academy on April 30th to do interactive experiments on Marangoni Flow with 7th grade students.

Three total experiments were done. The first experiment is the liquid droplets on pennies experiment. This experiment aims to let the students visualize and conceptually learn about surface tension of different liquids. During the experiments, students used a medicine dropper to drop water, isopropyl alcohol (IPA) and soapy water onto pennies and counted the number of drops a penny can hold before overflowing. They observed that a penny can hold the most amount of water and the least amount of soapy water because water, IPA and soapy water have decreasing surface tensions.

The second experiment is the visualization of Marangoni Effects. In this experiment, each student had a petri dish filled with water with some black pepper ground evenly sprinkled on the water surface. They observed that when IPA or soapy water is dropped into the center of the petri dish, the pepper particles quickly flowed away from the center. They learned that it is due to the soap reducing the surface tension of the water in the center of the petri dish and therefore inducing Marangoni flow towards the edge of the dish.

In the third experiment the students made use of the Marangoni effect to propel a cardboard boat. Each student used a piece of cardboard made from milk cartons to cut out and decorate their own boat. Then they picked the “fuel” that propels their boats (IPA or soap). They lined up their boats at one end of a tray that is filled with water and placed one or more drops of “fuel” to the back of the boat and observed the boat moving forward in water under the force of Marangoni effect. In the end, each table elected a representative to race in a large, 36 inches tray.

One cautionary note is that for the boat racing experiment, the water in the tray can only sustain Marangoni effect once. During the experiment preparation, students are having trouble keeping soap laden droppers out of the water and tend to drop IPA or soap into the water prematurely. This drastically decreased the strength of the intended Marangoni effect. For future experiments, figuring out how to protect the water body from contamination is an area that can be improved.

The students had lots of fun doing the experiments and learning about surface tension and Marangoni Effects. The three experiments get increasingly engaging and exciting. The school visit ended with a high note where students were cheering as they gather to watch the boat races.