Surface Tension Basics
Surface tension in soap film causes the characteristic spherical shape of soap bubbles.Copyright 2004 Microsoft Corporation, One Microsoft Way, Redmond, WA 98052-6399 USA. All rights reserved.
Students are presented with the question: "Why does a liquid jet break up into droplets?" and introduced to its importance in inkjet printers. A discussion of cohesive forces and surface tension is included, as well as surface acting agents (surfactants) and their ability to weaken the surface tension of water. Students observe the effects of surface tension using common household materials. Finally, students return to the original question through a homework assignment that helps them relate surface tension and surface area to the creation of water droplets from a liquid jet.
The inkjet printer has become one of the most widely used printer types. The fundamental principle that enables the operation of inkjet printers is the tendency of a continuous stream of water to break apart and form droplets. Droplets form due to the surface tension of the liquid—a larger surface area requires more energy to maintain due to the molecular forces associated with surface tension. When water transitions from a column into droplets, the same volume of water has a smaller surface area and therefore requires less energy. Chemical engineers must finely adjust the ink's surface tension so it forms droplets of the desired size and so it adheres to the paper surface without smearing or bleeding. Aside from desktop printing on paper, inkjet printers are today being used for many industrial applications such as automotive coatings, decoration of curved and irregularly-shaped surfaces, printing conductive patterns with metallic particles, replacing screen printing on everything from ceramics to textiles, and creating rapid 3D prototypes.
After this lesson, students should be able to:
- Explain certain properties of water using the concepts of cohesive forces and surface tension.
- Describe how the properties of water change when surfactants are added.
- Describe how surface tension encourages liquid droplets and soap films to minimize their surface areas.
- Discuss why columns of liquids form droplets, as happens in inkjet printers.