Some movies to help conceptualize surface viscosity

Vikram Prasad and Eric Weeks (Please e-mail either of us if the movies below don't play for some reason - include your web browser and operating system)

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Air-water interface

In the movie shown below, I have suspended a paper clip on the surface of a petri dish filled with water. The paper clip doesn't fall because surface tension holds it in place. By waving a bar magnet in front of the paper clip, I am applying a magnetic force on it, which causes the paper clip to move. There is no significant drag on the paper clip, except that from the bulk water phase below (the air phase is assumed to have nearly zero viscosity). The velocity of the clip is quite high, as expected.

In the next movie, I have added a little bit of albumin (a protein) to the bulk water phase. The quantities of albumin are small enough that they do not affect the bulk viscosity of the water phase. However, albumin likes the air-water interface a lot, and therefore its surface concentration is quite high. What this means is that it adorns the interface and prevents the motion of objects there. In the movie below you will see that the movement of the paper clip is significantly slowed down, and since the bulk viscosity of albumin/water is not that much different from the water solution shown above, all the drag the clip is experiencing comes from surface effects, namely, surface viscosity. Note that both movies are playing at the same speed (video frame rates), so the two cases can be compared directly.

In fact, this paper-clip/bar magnet concept is actually used to measure surface viscosities by a device known as a magnetic needle viscometer. Instead of a paper clip, researchers use a small magnet placed in a hollow teflon cylinder, and large Helmholtz coils that apply a caliberated magnetic field. The dimensions of the small magnet are of order the size of the paper clip, especially the thickness, which is about 1 mm. Of course, the limitation of such a device is that the surface viscosity has to be reasonably large so that bulk effects do not dominate.

Soap films

A soap film is a special interfacial system consisting of a thin fluid (water/glycerol mixtures typically) that are separated from air phases above and below it by two interfaces that are stabilized by surfactant. The soap film in the movie below is one such example (50:50 w/w water glycerol mixture and 0.3 wt % DAWN dishwashing detergent). For some reason that is not entirely apparent (though I have some ideas) the soap film is quite rigid. The paper clip in this case feels a large drag too, and the hypothesis is that most of it is coming from surface viscosity effects. The bulk viscosity of the water/glycerol mixture is only about 6 times that of pure water, and cannot explain the low velocity of the clip (unless water has preferentially drained from the film, which seems unlikely).

For more information, please contact Eric Weeks: <weeks(at)> or Vikram Prasad: <vprasad(at)>