Evaporation Dynamics of Completely Pinned and Partially Pinned Sessile Droplets in Multi-Droplet Configurations

TL;DR

This work investigates evaporation dynamics of sessile droplets in multi-droplet configurations under ambient conditions, focusing on the interplay between neighbor-induced vapor shielding and contact-line mobility (completely pinned vs partially pinned). It experimentally compares isolated, two-, three-, and five-droplet arrays using nanoparticle pinning to enforce complete pinning, with high-speed shadowgraphy and IR thermography to track height, diameter, angle, volume, and surface temperature, and it develops a diffusion-based model that incorporates evaporative cooling via a suppression matrix $[\psi]$. Key findings show that lifetime increases with the number of droplets due to shielding, with five-droplet lifetimes up to $1.89$× (partially pinned) and $2.24$× (completely pinned) the isolated case, and that completely pinned droplets exhibit stronger relative shielding due to constant contact radius; a universal, lifetime-normalized behavior emerges for droplet parameters across configurations. The theoretical framework (Method 1 and refined Method 2) agrees with central-droplet lifetimes within about 7–12%, validating the coupled physics of diffusion, pinning, and evaporative cooling, and highlighting practical implications for inkjet printing, spray cooling, biosensor fabrication, and coating technologies where precise control of evaporation is critical.

Abstract

The evaporation of sessile droplets placed in close proximity is influenced by complex vapor-vapor interactions, producing a shielding effect that can significantly extend droplet lifetimes. This study presents a systematic experimental investigation of evaporation dynamics in multi-droplet configurations under ambient conditions, comparing completely pinned and partially pinned contact line modes. Completely pinned droplets are generated by introducing alumina nanoparticles, while partially pinned droplets consist of pure water. An isolated droplet is compared with arrays of two, three, and five droplets, each arranged at a fixed spacing ratio. High-speed shadowgraphy is used to measure droplet height, contact angle, volume, and lifetime. Results show that, although completely pinned droplets evaporate faster in absolute terms due to a constant contact radius, they experience a more pronounced relative lifetime increase from vapor shielding than partially pinned droplets. In five-droplet configurations, lifetimes increase by up to 89% and 124% compared to isolated droplets for partially pinned and completely pinned modes, respectively. A theoretical model incorporating evaporative cooling predicts central droplet lifetimes with good agreement. These findings underscore the coupled influence of contact line mobility and droplet proximity on evaporation rates.

Figures (11)

• Figure 1: Schematic diagram of the experimental setup. The side view of the droplets is captured using a CMOS camera in a shadowgraphy arrangement, while the top view is recorded by an infrared (IR) camera. Droplets are deposited on an aluminum substrate coated with a 100 $\mu$m thick polytetrafluoroethylene (PTFE) tape.
• Figure 2: Various droplet configurations considered in the present study. Here, $N$ denotes the number of droplets arranged in each configuration, $d$ is the equatorial diameter of a droplet, and $L$ represents the center-to-center distance between the central and side droplets.
• Figure 3: Variation of (a) droplet lifetimes (in seconds), (b) normalized droplet lifetime ($\tau$), and (c) relative deviation in lifetime ($t_{dev}$ in %) between partially pinned and completely pinned droplets with $N$.
• Figure 4: Temporal evolution of droplet contours for the (a) partially pinned and (b) completely pinned configurations.
• Figure 5: Temporal evolution of top-view droplet profiles, with the first and third rows corresponding to partially pinned droplets, and the second and fourth rows depicting completely pinned droplets. A scale bar is provided in the bottom-right panel.