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UnitGrades 10 - 12

Surface Tension

Four images (clockwise from top left): A water strider insect, a lotus flower with many white petals, apparatus to measure surface tension by how high a liquid climbs a capillary tube, and water droplets on a leaf.Surface tension can be seen in a variety of places

Lesson 1: The first lesson introduces surface tension and adhesive forces and explores the question of why liquid jets break into water droplets rather than remaining in continuous streams. In the associated activity, students use common household materials to explore surface tension phenomena and surfactants; they determine their own best soap solutions for making bubbles and experiment with additives. Calculations from a math homework explain why soap bubbles form into spheres.

Lesson 2: The second lesson builds on the first by introducing cohesive forces and demonstrating how the combination of adhesive forces and cohesive forces cause capillary motion. By studying the capillary motion of a liquid, its surface tension can be measured. Two activities are associated with this lesson: in the first, students study different effects of capillary motion; in the second, for more advanced students, they use capillary motion to measure the surface tension of water.

Lesson 3: The third lesson introduces the concepts of wetting and hydrophobic and hydrophilic surfaces. These phenomena also depend on the surface tension of the liquid and the strength of a liquid's attraction to the surface. In the associated activity, students explore how water's behavior changes when a surface is treated with different coatings.

Lesson 4: The fourth and final lesson in this unit introduces superhydrophobic surfaces with the lotus leaf being a natural example. Superhydrophobic surfaces are covered with tiny protrusions that trap a thin layer of air between a water droplet and the surface, and the air cushion gives rise to the water-repellent, self-cleaning property of the lotus leaf. In the associated activity, students examine the lotus effect on natural leaves and human-made superhydrophobic surfaces (an example of biomimicry), and explore how the lotus leaf repels dewy water through vibration.

Understanding the effects of surface tension has implications in a wide variety of engineering fields. For example, the functioning of the widely-used inkjet printer depends on the surface tension of the ink turning a jet of liquid into droplets that can be deployed into position. Environmental and chemical engineers study the interactions between surface tension and chemicals as they develop technologies to remove pollutants from our water and air resources. The surface tension of paints and other surface coatings is carefully designed to ensure that the coating spreads easily while maintaining a desired film thickness. Another consequence of surface tension, the wetting or beading of a liquid on a surface, is important in the design of coatings that to improve visibility through vehicle windows by helping water to shed quickly on the outside of windows while avoiding the formation of tiny droplets of condensation that create a "foggy" appearance on the inside of windows. Integrating the scientific understanding of capillarity and wetting, chemical engineers have created modern materials and coatings so that clothing and other surfaces repel water and are self-cleaning.

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