Simulation of water sensitive papers for spray analysis

Aim of this study was to simulate the water sensitive paper (WSP) behaviour for estimate the unitary spray deposition on the target at varying spray features and target coverage. WSP images were produced assuming some simplifying hypotheses: spherical drops and circular stains randomly placed on the images. Sprays were described in terms of probability distribution function (PDF) of drop size (log-normal and Rosin-Rammler), coefficient of variation of diameters (CV), arithmetic mean diameter (AMD), and volume median diameter (VMD). The results from the simulations showed that the overlap between stains was independent of spray features: when the percentage of covered surface of WSP images ranged from 4.7 up to 61.6%, the overlap between stains ranged from 0.3 up to 33.4%. On the contrary, unitary deposit (volume per square centimetre, L cm-2) and particle density (number of distinct particles per square centimetre, cm-2) could be obtained from the percentage of covered surface if CV, AMD, VMD and PDF were known. However, assuming as spray parameter the VMD, the drop size distribution had almost no effect on the unitary deposit: with the same percentage of covered surface, the Rosin-Rammler PDF provided an estimate of the unitary deposit higher than log-normal distribution between 5 and 7% only.