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Surface tension, obsessive-compulsively

Ian Hopkinson talks about the need to attend obsessively, compulsively to detail when measuring surface tension:

Langmuir trough experiments are ideal for obsessive-compulsives: before you start your actual experiment you have to get the surface of the liquid you’re using absolutely clean. To do this you clean your trough, add in the ultrapure water, compress the surface, hoover (with a glass pipette connected to a vacuum pump) contaminants off the surface if there was an upturn in the surface tension, then go back to compressing the surface, hoovering the surface etc. Some times it just doesn’t work and you spend a morning trying to get your trough clean. Doing this for an oil/water interface is difficult, much more difficult actually I never succeeded. The core of the problem is that you don’t need much material to make a surface dirty, imagine painting a ball – the amount of paint required to cover the surface is much smaller than the volume of the ball.

(Thanks to investigator Alice Bell for bringing this to our attention.)

Some prefer to call the device a Langmuir-Blodgett Trough (some prefer not too, fearing that a sloppy pronunciation would make it sound like the name of a spy in a cheap intrigue novel: “Langmuir Blodgetroff”). This low-key video shows one person’s experiment using a Langmuir or Langmuir-Blodgett trough. Note the slight but dramatic pause before the narrator speaks the phrase “pure water”—that moment occurs near the 1:05 point of the video:

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