Flows of one fluid into another are common in both technology and nature. These may be entirely different fluids or the same type of fluid but with different properties (such as warm water flowing into cold). Viscous fingering occurs when a fluid of lower viscosity flows into a fluid of higher viscosity resulting in a fingering pattern. The behavior of these flows is significant in petroleum recovery, climatology, carbon sequestration, and pollutant dispersal. Here we study the flow of one transparent fluid into another using the optical technique called Schlieren. Use of this sensitive technique has made it possible for us to image the internal structure within the flows. We see that even for very thin flows with thicknesses of about one third of a millimeter, the flows must still be treated as three-dimenstional, in contrast to the two-dimensional approach more widely employed.
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