Time evolution of contact-angle dynamics for water and sodium chloride solution droplets during evaporation

Abstract

In this work, the time evolution of the contact-angle dynamics of distilled water droplets and 0.9% sodium chloride solution droplets placed on a solid glass surface was investigated experimentally during evaporation. The contact angle was determined using a simple geometric approach based on spherical-cap geometry from the droplet height and base diameter, without employing complex optical goniometric systems. The experiments were conducted under controlled environmental conditions, and the time-dependent variation of the contact angle was analyzed in detail. The results show that, for the 0.9% sodium chloride solution droplet, the contact angle decreases faster and in an almost monotonic manner compared with the distilled water droplet. In contrast, the water droplet exhibits stick–slip behavior associated with contact-line pinning and depinning processes. The obtained results indicate that electrolyte ions influence wetting properties and contactline dynamics. This study demonstrates the applicability of a simple geometric approach for contact-angle analysis without using complex goniometric systems. Furthermore, the obtained results for the 0.9% NaCl solution may serve as useful reference data for modeling the evaporation behavior of physiological saline droplets on solid substrates.