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Schematic of vortex ring formation

Vortex rings – suggestions please

My project over the last 3 years could be summed up as “getting stuff to stick to fibre optic sensors”. The sensors were kindly prepared by various other members of the group (best ones were by Rebecca) and I was then responsible for finding the right material to coat on to the fibre to make it sensitive to X. Exciting stuff, I know. Most of my coating work was done using a technique called Langmuir Blodgett, the first stage of which is preparing monolayers of material spread on a large trough of water. These monolayers are explained in slightly more detail in my previous post about our Lego Brewster Angle microscope. Now, for reasons I can’t quite remember, I looked at a monolayer one day and thought to myself “What if I add food dye?”. The video below shows the result.

This video was shot using a small USB camera overlooking the surface of the water. The food dye is a 1:100 dilution in some water and as shown, the drop was simply touched to the surface of the water which then caused the formation of the ring. If the drop fell naturally from the tip of the syringe, it will still created a ring but these have a much greater variation in shape (more information on that effect is detailed here). The rings in the video were created using a monolayer of stearic acid but further work showed that these rings form in clean water as well. Although, without a monolayer, they were slower and more prone to simply dissipating as a blob of colour. The video below shows the comparison of rings formed with and without a monolayer.

Update: Video now available for download at FigShare

 The formation of these rings has been the subject of considerable interest over the last 100 years. This interest has mostly been on the formation of vortex rings in fluids, the study of air current vortex rings or by teenagers trying to blow smoke rings. However, the formation vortex ring through monolayer materials has garnered little recent attention aside from a handful of articles by a group headed by John Saylor at Clemson University. It is from this prior data that we know that the rings are formed by the ‘spinning’ of the edges of the drop as it merges with the water.
Schematic of vortex ring formation

Vortex ring formation, arrows show the direction of flow

Although this model only explains why they form in water and not why the existence of a monolayer material accelerates them so significantly. This acceleration is explained by a more complex series of forces and is still not quite fully understood.

Now where this work gets even more interesting is that while making these rings, I found that if I used a multilayer (2-3 layers) insted of a monolayer, I occasionally got quite different rings that defy my knowledge of the vortex ring effect.

Update: Video now available for download at FigShare

The rings created here were made though a layer of calix[4]resorcinarene C11 but I have also made them using a stearic acid monolayer. For an as of yet unexplained reason these rings split into two (never more) at the point of formation. This split is not always equal and the later rings have only very small off shoots. They don’t always occur with a multilayer, a work experience student (Lucy Ackroyd) very kindly spent an entire morning making multi-layers trying to re-create these and managed to produce precisely zero (got loads of other good data though!). Honestly I can’t even being to wrap my head around what is causing these split vortex rings. I don’t know enough about fluid dynamics to tackle this problem. So this blog post is my way of reaching out to anyone who thinks they have a theory of what is causing this effect. Please post any suggestions either as to the cause of these split drops or for any further experiments that you think might help solve this problem. As it happens, it is relatively simple to make these rings yourself (described below) so I would be very interest to see if anyone can produce any other weird effects!

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By Matthew (@MCeeP), ago