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

This study investigates how substrate surface energy (<b>γ</b>_{<i><b>S</b></i>}) and thickness (<i><b>h</b></i>_{<i><b>f</b></i>}) 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 <b>γ</b>_{<i><b>S</b></i>}, where weak pinning (<i><b>F</b></i>_<b>Pin</b>) and high nucleation barriers yield condensed central crystals, and (ii) high <b>γ</b>_{<i><b>S</b></i>}, where strong pinning and early crystallization produce coffee-ring deposits, with needle-like morphologies at higher <b>γ</b>_{<i><b>S</b></i>}. Thickness effects emerged under prolonged UVO: thin films (<i><b>h</b></i>_{<i><b>f</b></i>} ≈ 617 nm) exhibited uniform crystal deposits upon dewetting, while thicker films (<i><b>h</b></i>_{<i><b>f</b></i>} ≈ 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 (<i><b>S</b></i>) and <i><b>F</b></i>_<b>Pin</b>, revealing coffee-ring formation above ∼0.1 mN and dewetting-driven patterns at <i><b>F</b></i>_<b>Pin</b> ≈ 0.0005 mN. Results demonstrate how coupled substrate properties dictate evaporative deposition, offering pathways for controlled coating and surface patterning.