Evaporation and Deposition Behavior of Surrogate Respiratory Sessile Droplets: Influence of Substrate Wettability and Solute Effect

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The evaporation and deposition of respiratory sessile droplets play a critical role in the transmission of infectious pathogens. This study investigated the drying behavior of surrogate respiratory fluids containing salts, polymers, and surfactants, with a focus on the impact of substrate wettability and solute effects on evaporation kinetics and deposition patterns. First-order statistics (FOS) and gray level co-occurrence matrices (GLCM) were employed to quantify the texture features of the dried deposits. The results demonstrated that droplets evaporated in constant contact radius (CCR) mode on ordinary glass, polydimethylsiloxane (PDMS), and nanocoated surfaces, yet crystalline nucleation and final deposit morphology varied markedly. Hydrophilic substrates induced intracrystal ring nucleation, forming a composite morphology comprising a ring-like deposition band (edge), a gel network, and dendrite clusters (center). In contrast, hydrophobic substrates promoted crystal nucleation near the triple-phase contact line (TPCL). Higher hydrophobicity widened the ring-like deposition band from 0.1 (glass) to 0.22 (nanocoated surface) and increased deposit complexity, evidenced by a drop in mean gray value (143 to 108) and a rise in entropy (6.9 to 8.1). Surfactants enhanced spreading and stabilized contact line pinning by reducing surface tension, thereby driving polymer migration toward the TPCL, where they self-assembled into ring-like gel networks. Salt ions induced polymer aggregation and phase separation via electrostatic interactions and cooperated with surfactants to suppress the coffee-ring effect. Moreover, increasing salt concentrations shifted crystal morphology from needlelike to dendritic and eventually petal-like, producing a "dense edge - sparse center" distribution with greater textural complexity. Conversely, higher polymer concentrations led to reversed amorphization of deposit morphology and nonmonotonic textural evolution.