Friday, May 23, 2008

Oil Migration - Moebius vs. Mobil

Oiling a watch movement is serious business. If you use the wrong oil, there's plenty of people out there to wag their finger at you and say "I told you so!". On the other hand, Swiss watch oil is pretty expensive and some might be tempted to seek alternatives.

On a whim, I decided to compare Swiss watch oil with automotive synthetic oil. After all, motor oil is designed for a much harsher environment than the inside of a watch. I took two flat glass 30mm watch crystals, cleaned them with alcohol and held them together with a small plastic clothes pin. Using a watch oiler, I put 1 'measure' of the following oils at the edge where the crystals were touching - a slight chamfer on the edges formed a convenient 'V' to touch the oiler to.

Mobil 1 5W/30 synthetic motor oil (upper)
Moebius 9010/2 sythetic light train oil (middle)
Moebius 941 synthetic pallet jewel/escape teeth oil (lower)

I was completely unprepared for the result!

As I touched the oiler to the edge of the crystals, I noticed that, not unexpectedly, each oil blob moved inward a little, leaving no oil at the edge. I noticed also that an optical fringe effect was actually indicating the interfacial stress caused by the clothes pin pressure. "Must take pictures!", I thought. While I was setting up the camera, I noticed that the motor oil seemed to have changed shape and moved a bit!. As you can see from the above shots - it eventually moved a lot, disappearing under the clothes pin in less than an hour!! The first shots were a couple minutes apart; the last four were taken at 5, 10, 15 and 60 minutes elapsed time.

The motor oil moved in the direction it did because the attractive molecular forces were greater on the side nearer to the clothes pin. Attractive forces are higher when two surfaces are closer together as shown by lower spatial frequency of the fringe pattern toward the pin. However, the blob stayed essentially whole because of it's own surface tension except where some some surface irregularity caused it to shed a small piece (conflicting attractive forces exceeded the shear strength of the oil).

I conclude that Swiss oil does tend to stay in place as advertised, and that oil can migrate away from where you put it - leaving dry, un-lubricated areas if the forces of capillary action so dictate, or if you just don't put enough.

I have to say that this is an astounding sequence of photos, made alive by the wonders of today's Internet technology. Do forgive me if I sound a little immodest!

Best regards,



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Eugene said...

Did you try to replicate your experiment with alternating the positions of the oil on the crystal?

Ted Cossins said...

No, Eugene, I did not.

Is there a good reason why I should have?