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.