Electrowetting: Thermodynamic Foundation and Application to Microdevices

Surface tension is a line force, directly proportional to the length. However, surface forces (e.g., pressure) are proportional to the square of the length, and body forces (e.g., inertia or gravity) are proportional to the cubic of the length. Therefore, surface tension becomes dominant over pressure or gravity as the dimension of interest gets smaller – in practice, below millimeter range. Many examples are found in our daily lives. An ant can carry a load that is several times heavier than its own weight, demonstrating its relative strength, but it also can be trapped in a water droplet showing its relative weakness. Small creatures climb up walls. Water striders hop on water surfaces. Note these examples are in millimeter scale where inertia forces are still comparable to surface tension. In micrometer scale, surface tension starts to dominate over not only the inertia forces but most other physical forces.