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Schlagwörter:
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Zusammenfassung:
Quantitative experiments on the evaporation from sessile droplets of aqueous saline (NaCl) solutions
show a strong dependence on salt concentration and droplet shape. The experiments were performed
with seven decades of initial NaCl concentrations, with various droplet sizes and with different contact
angles. The evaporation rate is significantly lower for high salt concentrations and small contact angles
than what is expected from the well-accepted diffusion-controlled evaporation scenario for sessile
droplets, even if the change of the vapor pressure due to the salt is taken into account. Particle tracking
velocimetry reveals that this modification of the evaporation behavior is caused by marangoni flows that
are induced by surface tension gradients originating from the local evaporative peripheral salt
enrichment. In addition it is found that already very low salt concentrations lead to a pinning of the
three phase contact line. Whereas droplets with concentration Z106 M NaCl are pinned as soon as
evaporation starts, droplets with lower salt concentration do evaporate in a constant contact angle
mode. Aside from new, fundamental insights the findings are also relevant for a better understanding of
the widespread phenomenon of corrosion initiated by sessile droplets.
