With the increasing use of pesticides comes the tangible risk of growing resistance in weeds, which calls for using more and more pesticides. This untenable cycle calls for solutions that will help put the agricultural industry on a more sustainable path. In particular, bioactivators (i.e., inerts that can be built in or added in the tank to increase the efficacy of pesticides) remain in high demand. Their design is usually quite long and consuming from a research standpoint because it involves numerous trials and errors and back and forth between formulation, in vitro and in vivo testing in greenhouses and, eventually, fields. From a formulator standpoint, quick selection methods that correlate with efficacy would be of value early in the design phase. We present the design and use of an in vitro penetration technique based on confocal microscopy conducted under controlled temperature and humidity that can be used as a sorting test before an adjuvant of design is moved to in vivo testing. The method helps quantify the extent of penetration of aqueous agricultural formulations inside parafilm, which is used as a model hydrophobic substrate. A fluorescent dye with physicochemical parameters (log P; electric charge) close to those of a pesticide of interest (in the present case, glufosinate) is used as a probe under the assumption that its penetration is representative of that of the entire formulation. Parafilm then acts similarly to a solid-state chromatographic substrate, spreading the penetration of the fluorescent dye over several tens of microns. Penetration depths are averaged over several confocal microscopy experiments and automatized image analysis. We show how penetration depth in this in vitro test shows correlation with the agronomic efficacy of Liberty® 280 SL formulations added or not with new adjuvants of our design.