Why does water form a dome on a dirty surface?
Water does not particularly want to form a dome, or it would be
better to say, form a droplet, on a dirty surface.
See related questions below.
Water forms droplets when the cohesion of water with itself is
larger than the adhesion of water to the surface.
A dirt surface cold be anything; hydrophobic or hydrophilic,
soluble or insoluble, rough or smooth, hot or cold.
All these things matter.
See links below.