Patrick Wessels
Jean Hertzberg 
Department of Mechanical Engineering
University of Colorado, Boulder

Air (yellow region) fingers into a mixture of vegetable oil, water and dye in a diverging Hele Shaw cell.

A famous fingering instability is found when air is injected into a thin layer of water (or any more viscous or stiffer fluid), sandwiched between two flat sheets. Here, we are looking down on a sandwich of two sheets of Plexiglas, with a mixture of water and vegetable oil dyed green as the filling of the sandwich. When the top sheet is lifted a little bit, air is pulled into the middle of the sandwich. It doesn't enter uniformly; instead it forms fingers pushing into the mixture. You may have noticed a similar pattern when peeling apart any kind of gooey sandwich. In a laboratory, the sandwich is called a Hele Shaw cell, and the fingering instability is the Saffman Taylor (Tabeling et al. 1987).  

References

Tabeling, P., Zocchi, G., Libchaber, A., (1987). An Experimental Study of the Saffman-­‐Taylor Instability. Journal of Fluid Mechanics Digital Archive 177: 67-­‐82.  

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