A note on the visualization of wetting film structures and a nonwetting immiscible fluid in a pore network micromodel using a solvatochromic dye

Micromodel technologies are a useful and important method to study pore-scale fluidic processes, using two-dimensional formats that enable direct visualization of processes within patterned microstructures. In this technical note, Nile red, 9-diethylamino-5H-benzo [alpha]phenoxazine-5-one, is demonstrated as a single dye whose solvatochromism enables selective visualization of two immiscible liquid fluids in a pore network micromodel containing a homogeneous array of pillars. Nile red dissolves in, and partitions between, hexadecane as a nonwetting fluid and polyethylene glycol 200 (PEG200) as a hydrophilic wetting fluid in a micromodel with silicon oxide surfaces. Both the absorption spectra and fluorescence emission spectra are sensitive to the solvent environment, such that the two phases can be distinguished by the observed color or the fluorescence emission band. Bright field, epifluorescence, confocal fluorescence, and hyperspectral microscopy methods were used to image the micromodel after displacing PEG200 in the model with hexadecane. Using a single solvatochromic dye facilitates direct visualization and identification of both phases anywhere in the micromodel on the basis of color and also enables collection of complementary fluorescent images for each phase. The use of Nile red with these imaging methods facilitates selective visualization of phase identity at specific locations; the interfaces between the two immiscible liquid phases; wetting behavior of the wetting phase within the pore network; and retention of the wetting phase as thin films around pillars and as bridges across the pore throats. The pillars and wetting phase bridges create a network of obstacles defining a tortuous flow path for the displacing nonwetting phase.