  Liquid flow in sessile evaporating droplets of ultrapure water typically
results from two main contributions: a capillary flow pushing the liquid
towards the contact line from the bulk and a thermal Marangoni flow pulling the
drop free surface towards the summit. Current analytical and numerical models
are in good qualitative agreement with experimental observations however they
overestimate the interfacial velocity values by 2-3 orders of magnitude. This
discrepancy is generally ascribed to contamination of the water samples with
non-soluble surfactants, however an experimental confirmation of this
assumption has not yet been provided. In this work, we show that a small "ionic
contamination" can cause a significant effect in the flow pattern inside the
droplet. To provide the proof, we compare the flow in evaporating droplets of
ultrapure water with commercially available bottled water of different
mineralization levels. Mineral waters are bottled at natural springs, are
micro-biologically pure, and contain only traces of minerals (as well as traces
of other possible contaminants), therefore one would expect a slower
interfacial flow as the amount of "contaminants" increase. Surprisingly, our
results show that the magnitude of the interfacial flow is practically the same
for mineral waters with low content of minerals as that of ultrapure water.
However, for waters with larger content of minerals, the interfacial flow tends
to slow down due to the presence of ionic concentration gradients. Our results
show a much more complex scenario than it has been typically suspected, and
therefore a deeper and more comprehensive analysis of the huge differences
between numerical models and experiments is necessary.
