Correlation between electrostatic response localization and droplet impact behavior on femtosecond laser–structured stainless steel

Abstract

Femtosecond laser structuring enables the formation of hierarchical metallic surfaces with tailored interfacial properties; however, the role of morphology-induced electrostatic-response localization in liquid–surface interactions remains insufficiently understood. Here, we demonstrate that ultrafast laser processing of AISI 321 stainless steel produces distinct electrostatic response regimes that correlate with droplet impact behavior. Two surface classes—two-scale periodic TS–LIPSS and deep hierarchical QP–LIDSS morphologies—were generated and systematically characterized using correlated SEM/EDS, AFM, and electrostatic force microscopy (EFM). While TS–LIPSS surfaces exhibit moderately enhanced but spatially continuous electrostatic responses, QP–LIDSS structures display strongly localized electrostatic hot-spots and heavy-tailed EFM amplitude distributions, indicative of outlier-dominated force-gradient regimes. These surfaces also show pronounced oxygen enrichment and increased topographic curvature, indicating concurrent morphological and chemical modification that may contribute to the observed electrostatic-response heterogeneity. Static contact angle measurements reveal a transition toward hydrophobic behavior; however, pre-contact lateral deflection was observed only for droplets approaching the QP–LIDSS surfaces under the investigated experimental conditions. Simplified numerical modeling suggests that, under assumed droplet-charge and field-strength conditions, a localized and asymmetric electrostatic interaction could generate forces approaching the order of magnitude of gravitational loading under assumed charge conditions, providing a plausible physical explanation for the observed trajectory perturbation. These results reveal a strong nanoscale-to-macroscale correlation between laser-induced morphology–chemistry coupling, electrostatic-response localization, and droplet dynamics. The findings identify electrostatic interaction regimes as an additional functional design parameter for structured metallic interfaces, suggesting the possibility of contactless manipulation of liquid behavior via engineered surface electrostatic responses.

The authors' abstract, as published at the source. Applied Surface Science, 2026 · DOI ↗