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Influence of Substrate Surface Energy and Thickness on the Evaporation Dynamics of Sessile Saline Droplets
Soumya Mattupalli1, Nandini Bhandaru1
1Department of Chemical Engineering, Birla Institute of Technology and Science, Pilani, Hyderabad Campus, Jawahar Nagar, Kapra Mandal, Medchal District, Telangana 500078, India.
Abstract:
This study investigates how substrate surface energy (γS) and thickness (hf) 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 γS, where weak pinning (FPin) and high nucleation barriers yield condensed central crystals, and (ii) high γS, where strong pinning and early crystallization produce coffee-ring deposits, with needle-like morphologies at higher γS. Thickness effects emerged under prolonged UVO: thin films (hf ≈ 617 nm) exhibited uniform crystal deposits upon dewetting, while thicker films (hf ≈ 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 FPin, revealing coffee-ring formation above ∼0.1 mN and dewetting-driven patterns at FPin ≈ 0.0005 mN. Results demonstrate how coupled substrate properties dictate evaporative deposition, offering pathways for controlled coating and surface patterning.
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