A general word that decribes "the residue" left behind when a fluid runs over a surface

synonyms

I'm looking for a word that describes the film left behind when a fluid touches a surface.

For example:

A blood drop runs down a wall -> it leaves a ...
A puddle drying-out leaves a ...
Oil that leaks from a barrel and sinks in the ground leaves a ...

The words I've came up with are: film, layer, residue (describes it best, but sound wrong), smear, trace, ...

Constraints:

The word should be neutral/scientific
Be applicable for all kinds of fluids and invariant to size
Ideally, it should be specific to fluids ("layer" is too general - "layer of a cake")

Solution 1:

Stain is the first thing that comes to my mind.

Solution 2:

The word deposit, in its sense "Anything left behind on a surface", is used in some "coffee ring effect" articles, as for example near the middle of an mpg.de webpage called "Dynamics of drying colloidal dispersions". That webpage also uses previously-mentioned terms residue and stain.

All of residue, stain, and deposit work for your first two examples (blood or puddles) but for the third (oil in ground) stain and deposit don't work. To your sample words (film, layer, residue, smear, trace) you might add contaminant, concentration, vestige, remnant, and DNAPL.