Near-wall fluid flow near the pinned contact line during droplet evaporation

• The flow pattern near the pinned contact line during droplet evaporation is observed.
• The thin film thickness in ethanol solution decreases earlier than in deionized water.
• The thin film thickness varies linearly just before the droplet shrinks.
• The transition time is earlier for high-concentration solutions due to higher evaporation flux.
• The transition thickness is larger for high-concentration solutions due to variation of surface tension.

Drop evaporation is a fundamental and complex phenomenon in nature and industrial processes. For the case of pinned contact line mode, however, we still lack a comprehensive understanding of the near-wall fluid flow during drop evaporation. In this paper, we measure the in-plane average velocities near the pinned contact line during evaporation of deionized water and aqueous ethanol solutions by the multilayer nano-particle image velocimetry (MnPIV) technique. The results show that the Marangoni convection near the triple-phase contact line is negligible for the flow pattern during the natural evaporation process without heat source being applied. The near-wall velocity profiles experience the decrease of thin film thickness just before the droplet shrinking process. The thin film thickness in the ethanol solution appears to decrease earlier than that in deionized water and the total shrinking time is much shorter than water. The thin film thickness decreases linearly before decreasing exponentially in about the first 100 nm. The transition time also becomes earlier for the high-concentration solutions due to higher evaporation flux, while the transition thickness is larger for ethanol solutions with higher concentrations due to the inverse relation between the transition thickness and the surface tension.