Urea's distinct role in agricultural spray deposition: modulating drying dynamics to enhance central deposition without wetting benefits

BACKGROUND: Urea is frequently tank-mixed with pesticides, yet whether it functions as a conventional spray adjuvant or acts at another stage of droplet evolution remains unclear. This study examined how 0-2.0 wt% urea affects spray liquid properties, atomization, impact, wetting, evaporation, and final deposition. RESULTS: Urea caused only minor changes in equilibrium surface tension and viscosity. Across nozzle and pressure conditions, changes in volume median diameter were generally within ±3%, while impact, wetting, and the time required for 90% volume loss (55-60 min) were not appreciably altered. In contrast, dried residue morphology changed markedly. In the sodium-fluorescein-containing 32% thifluzamide-tebuconazole suspension concentrate system on polytetrafluoroethylene (PTFE), which contained sodium fluorescein, the addition of 0.5 wt% urea increased the central integrated fluorescence fraction from 21.90% to 26.40% (+4.50 percentage points) and the center-to-periphery ratio from 2.24 to 2.87. Similar central enrichment occurred at 1.0-1.5 wt%, whereas 2.0 wt% reduced the fraction to 20.66%. More continuous residues were also observed qualitatively on maize and rice leaves. CONCLUSION: Urea primarily modifies deposition during late-stage droplet drying rather than through atomization or wetting. The observed redistribution is consistent with evaporation-driven solute enrichment, precipitation, and crystallization. These findings identify 0.5 wt% as the lowest tested concentration producing high central enrichment in the examined formulation and establish late-stage drying as a distinct target for improving pesticide deposit spatial distribution, although biological efficacy requires further verification. © 2026 Society of Chemical Industry.