  Diffusion of colored dye on water saturated paper substrates has been
traditionally exploited with great skill by renowned water color artists. The
same physics finds more recent practical applications in paper based diagnostic
devices deploying chemicals that react with a bodily fluid yielding
colorimetric signals for disease detection. During spontaneous imbibition
through the tortuous pathways of a porous electrolyte saturated paper matrix, a
dye molecule undergoes diffusion in a complex network of pores. The advancing
front forms a strongly correlated interface that propagates diffusively but
with an enhanced effective diffusivity. We measure this effective diffusivity
and show that it is several orders of magnitude greater than the free solution
diffusivity and has a significant dependence on the solution pH and salt
concentration in the background electrolyte. We attribute this to electrically
mediated interfacial interactions between the ionic species in the liquid dye
and spontaneous surface charges developed at porous interfaces, and introduce a
simple theory to explain this phenomenon.
