Surface Tension in Fluid Mechanics

The “Surface Tension in Fluid Mechanics” video exhibited the many ways in which surface tension can act as a powerful force on fluids. Through a large variety of experiments and demonstrations, the relevance of surface tension and the science behind how it works was effectively communicated. I thought that this video more than others did a great job of explaining concepts through demos rather than simply writing down an equation. Conceptually, this made surface tension a very interesting aspect in fluids.

I thought the capillary action demonstration was the most relevant experiment shown in the film. Using two thin pieces of plastic attached at one edge and slightly separated at the other (by a paperclip), the scientist showed how the 2-D meniscus forms when water is added to the system. I’ve seen this before on YouTube videos published by astronauts aboard the ISS. In a similarly designed vessel, astronauts sip water and coffee from these plastic containers in zero gravity without any spilling. I think it’s a very creative design since most wouldn’t expect open-containers of liquid to be a good idea in zero gravity surrounded by expensive equipment.

Another very interesting demo was the iron nail and mercury droplet creating an electrochemical cell. Touching the nail to the mercury short-circuited the battery causing charges to move. When this happened, the mercury began to oscillate. The video didn’t comment much on the geometry of the mercury drop, but personally, I found that to be the most fascinating part of the demonstration. The mercury drop inverts between two symmetric, curved right-triangle geometries. The aesthetics of the motion exhibited by the mercury drop really interested me. The addition of nitric acid to the mercury drop was very exciting too. When the nitric acid was added, the mercury drop swam towards it, until, suddenly, it touched and the mercury started shape-shifting all over the petri dish, usually breaking itself up into smaller droplets.

Similarly, the hot wire running through the acetone/water mixture was fascinating. The bubbles forming on nucleation sites on the wire ran back and forth towards hotter temperature regions. Though not intuitive to me at first, the explanation was effective: the colder side of the bubble has a higher surface density causing warmer fluid around it to flow towards the cold side and therefore propelling the bubble itself into the hot temperature fluid. This example effectively showed how these small bubbles acted as miniature heat engines. At one point, the camera focused on a single nucleation site. Bubbles scurried out of frame, nearly always alternating sides.