Figure 5. Evolution in time of the mean c/c0 in half of the Hele-Shaw cell. The cross marks the time at which the fingers reached the cell boundary in each experiment. Fingers did not reach either horizontal boundary in the R r 5 2.8 experiment. Also plotted are square root of time curves, showing that R r 5 2.4 and2.8approximatediffusivebehavior;thecurveforR r 5 2.4 exactly overlies the data.