Influence of Substrate Surface Energy and Thickness on the Evaporation Dynamics of Sessile Saline Droplets

This study investigates how substrate surface energy ( γ S ) and thickness ( h f ) govern the evaporation and crystallization of aqueous NaCl droplets on polydimethylsiloxane (PDMS). Surface energy was tuned via UV–ozone (UVO) treatment on spin-coated thin films and self-standing blocks. Two deposition regimes were identified: (i) low γ S, where weak pinning ( F Pin ) and high nucleation barriers yield condensed central crystals, and (ii) high γ S, where strong pinning and early crystallization produce coffee-ring deposits, with needle-like morphologies at higher γ S . Thickness effects emerged under prolonged UVO: thin films ( h f ≈ 617 nm) exhibited uniform crystal deposits upon dewetting, while thicker films ( h f ≈ 1.3–3.6 μm) promoted needle-shaped growth. The Péclet number (Pe) delineated deposition modes, with Pe > 45 favoring rings and Pe < 45 condensed deposits. A morphology phase diagram maps the interplay of supersaturation ( S ) and F Pin, revealing coffee-ring formation above ∼0.1 mN and dewetting-driven patterns at F Pin ≈ 0.0005 mN. Results demonstrate how coupled substrate properties dictate evaporative deposition, offering pathways for controlled coating and surface patterning.