A smooth steel cylinder is rotated around its axis using a compressed-air-driven motor. It is coated by a film of silicone oil (polydimethysiloxane) provided by Dow Corning. The viscosity of the silicone oil is about 0.5 Poise, and its surface tension is about 25 dyne/cm. The cylinder is 28 cm long and has a radius of 3.4 cm, and can be rotated at up to 60 r.p.m.
With a coating of mean thickness around 0.3 mm, and a rotation rate around 40 r.p.m., interesting "rollers" form, wrapping around the cylinder. These are not axisymmetric, but rather thicker at the "front", where the cylinder moves upwards. This is due to the competition between
The shape of the film is smoothed by surface tension.
|
Rollers on a rotating cylinder |
|
Front view |
|
Top view |
|
View from the side and slightly below centre |
If the cylinder is slowed, to around 20 r.p.m., these "rollers" turn into "drippers", with structures reminiscent of the drop draining down a wall forming on the upward-moving side.
 |
Close-up of a dripper |
This behaviour is discussed further, and modelled, in a paper written with Len Schwartz and Valéry Roy: "Three-dimensional solutions for coating flow on a rotating horizontal cylinder: theory and experiment", Phys. Fluids, 17:072102, July 2005.
Photographs here are copyright © Peter Evans, 1998