FIG. 5.

(a) Plot showing measured intensity of dye in a paper channel [black circles, measured at the same location as Fig. 2(a)] while adjusting the concentration of dye in the solution reservoir via step changes (as shown by the blue plot). (b) Intensity plotted vs dye concentration demonstrates the expected linear correlation. (c) “Tree ring” pattern formation on evaporation pad due to the accumulation of multiple dyes and salts for more than 5 h of operation.