Droplet evaporation and wetting regime transitions on nature-inspired superhydrophobic soft surfaces

Patterned superhydrophobic surfaces in the Cassie-Baxter (CB) wetting state enhance droplet mobility, self-cleaning, anti-fouling, and reduce pressure requirements in microfluidics. The lotus leaf exemplifies this through its hierarchical micro/nanoscale structure and stable CB state. Developing biomimetic analogues requires fundamental understanding of wetting, evaporation dynamics, and fabrication. We investigated evaporation of sessile water and saline (0.5 M NaCl) droplets on natural lotus leaf (LL) and PDMS-based biomimetic positive replicas (PRs) fabricated via replica molding at prepolymer-to-curing-agent ratios of 10:1, 20:1, and 30:1 to examine the role of substrate stiffness. LL and PRs exhibited apparent contact angles (ӨApp) of ~162° to ~139° and contact angle hysteresis (CAH) of ~6° to ~59°. Despite similar static wetting, significant differences emerged during evaporation. LL maintained CB throughout evaporation lifetime (tE), while PRs underwent CB-to-Wenzel (W) transitions at ~251 s, 224 s, and 129 s for PR 10:1, 20:1, and 30:1, respectively. Time-resolved optical microscopy confirmed persistent air pockets on LL versus rapid disappearance on PRs, triggering liquid penetration. LL showed an initial constant contact angle (CCA) mode transitioning to constant contact radius (CCR), while all PRs displayed a CCR-dominated regime with stick-slip motion and intermittent TPCL pinning/depinning. Analysis of ӨApp vs. penetration depth showed LL had the highest wetting resistance; stiffer PR 10:1 most closely mimicked LL, while softer PR 30:1 showed deepest penetration. Thermodynamic analysis confirmed energetically favorable W states for all PRs, with negative Gibbs free energy differences (ΔG) ranging from −54.9 to −57.7 mJ/m² with increasing compliance. For saline evaporation, crystal deposition morphology depended strongly on compliance: PR 10:1 showed continuous TPCL recession yielding a central deposit, while PR 30:1 displayed strong TPCL pinning producing a coffee-ring pattern. PR 10:1 had the highest initial saline contact angle (ӨNaCl,App) (~163°) and longest tE (~2000 s); PR 30:1 had the lowest (~140°) and shortest (~900 s). These findings identify substrate compliance as a critical design parameter governing evaporation-induced wetting transitions, contact-line dynamics, and crystallization pathways on biomimetic superhydrophobic surfaces, providing new strategies for engineering anti-fouling, self-cleaning, and microfluidic materials.

Sai Soumya Sri Mattupalli is a PhD candidate in the Department of Chemical Engineering at BITS Pilani, Hyderabad Campus, India, working under the supervision of Prof. Nandini Bhandaru. Her research focuses on sessile droplet evaporation dynamics, contact line pinning, wetting transitions, and evaporation-induced deposition morphology on micro/nanostructured and bioinspired PDMS surfaces. She combines experimental characterization with mechanistic analysis to understand anisotropic pinning behavior on patterned substrates. She has authored a publication in Langmuir on flat PDMS substrates and continues to investigate structure–property relationships governing crystallization and deposition patterns in evaporating saline droplets on engineered soft surfaces.