Second-Harmonic Generation Imaging of Humidity-Induced Salt Crystallization in Spider Aggregate Glue

Spider aggregate glue droplets exhibit complex physicochemical behaviors that influence web adhesion. Here, we investigate structural and compositional changes in aggregate glue droplets before and after surface attachment under controlled humidity conditions using second-harmonic generation (SHG) microscopy, high-speed optical imaging, and scanning electron microscopy with energy-dispersive X-ray spectrometry. Spatially localized SHG signals are observed and correlated with potassium dihydrogen phosphate crystallization, which emerges upon droplet attachment or under abrupt reductions in relative humidity, while suspended droplets remain largely amorphous under stable conditions. The onset and morphology of crystallization are strongly dependent on humidity history and dehydration rate, with rapid water loss promoting dendritic crystal formation. These results demonstrate that SHG selectively probes ordered, noncentrosymmetric inorganic domains formed under kinetic perturbations, providing insight into humidity-driven phase behavior in spider web adhesives.