The photograph also appeared in the book by
M. Samimy, P. Steen, K. Breuer, G. Leal
"Gallery of Fluid Motion",  2003, published by Cambridge University Press,
and will appear in the book by S.P. Lin
"Breakup of Liquid Sheets and Jets",  2003, by Cambridge University Press.

A thick viscous film of castor oil on a vertical wire is unstable and forms beads.
The Reynolds number of the flow is very small, typically 0.01. The small marks on
ruler are 1 mm apart. As a fiber, nylon fishing line of radius 0.25 mm was used.

In the second case, the drop train is highly organized;
the shape, speed and distance between the drops are permanent in time.
This regime was observed in a relatively small range of flow rates.

For very small flow rates, the drops become larger, they are substantially more
separated in space, and the film in the space between the large drops is not uniform,
but shows a periodic growth of disturbances. Large drops collide with growing lobes
ahead of them, consume them, and continue to move. During the collision, the large
drops move faster. As a result, the instantaneous velocity of large drops changes
periodically in time.

The fuller set of photographs will appear in the book by R.P. Chhabra
"Bubbles, Drops and Particles in Non-Newtonian Fluids",  2nd Edition,
published by CRC Press.

Everyday experience shows that a stream of air in a  Newtonian liquid, such as  water,
breaks down into bubbles through the Rayleigh­Taylor instability. In other words,
aquarium pump in aquarium full of water will produce usual bubbles. Now, what will
happen if aquarium is full of liquid hand soap ? 
Remarkably enough, the result might be something like shown on the picture above.
The bubbles may form a  very stable, continuous, slowly rising, connected long chain
similar to beads, or bubble  " sausage", or necklace. 

It turns out that liquid hand soap contain polymers; this is important.
In concentrated polymer solutions  such as hand soap  the elastic properties
of the polymers can alter the development of the Rayleigh­Taylor instability. 
In particular, the breakdown of the air stream can be arrested and, as a result,
instead of individual bubbles, a long chain of bubbles forms. The bubbles in the
chain are connected by thin necks. There is relatively fast motion of the 
polymer liquid near the necks of rising bubbles. The presence of polymers 
prevents the necks from collapsing. The complete structure is very stable and 
can be observed in a broad variety of polymer solutions, both water-based and 
organic. The phenomenon does not appear in diluted polymer solutions. 
There is a minimal concentration threshold for the chain formation. 
For aqueous methocel solutions, the minimal concentration is 2%. 
Also, there is a minimal air flow rate necessary to support the chain; for a
very small flow rate, the air stream breaks down even in concentrated 
polymer solutions. For 2% aqueous methocel solution, the minimal flow 
rate is about 3.7 cm3/s.