E-juices have varying quantities of 'dry deposits' - ingredients that will not vaporize and build up in the atomiser. Some of this will get burned on the coil and harden, which is one of the main routes to atomizer death.
Here are some pictures of dry deposits posted by Peter Knutsell at e-cigtest:
The pictures are not great but gives an indication of how much the residue can vary.
Not surprisingly, the unflavored juice leaves very little residue, as most is due to flavorings, flavorings by and large developed for food, where residue is not an issue.
More specifically, the dry residues are not the flavor chemical itself, because that is vaped, but other things that were dissolved during the extraction process, such as sugars, starches, colors, gums and resins.
Artificial flavors would be expected to leave very little dry residue, and probably none, because they are essentially one chemical; not all taste good though and a few might not be healthful (e.g. vanillin). Natural extracts will probably vary a lot.
In one or two cases it looks like the dry residue is not actually dry - that there might be some oil in the juice too, although it could be VG because that is very very slow to evaporate at room temperature.
Here are some pictures of dry deposits posted by Peter Knutsell at e-cigtest:

The pictures are not great but gives an indication of how much the residue can vary.
Not surprisingly, the unflavored juice leaves very little residue, as most is due to flavorings, flavorings by and large developed for food, where residue is not an issue.
More specifically, the dry residues are not the flavor chemical itself, because that is vaped, but other things that were dissolved during the extraction process, such as sugars, starches, colors, gums and resins.
Artificial flavors would be expected to leave very little dry residue, and probably none, because they are essentially one chemical; not all taste good though and a few might not be healthful (e.g. vanillin). Natural extracts will probably vary a lot.
In one or two cases it looks like the dry residue is not actually dry - that there might be some oil in the juice too, although it could be VG because that is very very slow to evaporate at room temperature.
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