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Video: Demonstration of Solvent Drying During Spin Coating by Interference Effects

This video demonstrates the interference effects of silicon dioxide as a solvent slowly dries while spin coating at 2000 rpm. The solvent in this video is trichlorobenzene (without any additives or solute), which has a very high boiling point.

As a film dries it generally reduces in thickness from a wet layer of tens-hundreds of microns to a dry layer of a tens-hundreds of nanometres. During this time the film thickness is of the order of a few wavelengths of visible light which means that the reflectance properties will be interference based and highly non-linear. As such, even for a nominally transparent material in a transparent solvent the apparent colour will change as the film decreases in thickness and once the film stops changing colour it is predominantly dry (note that residual solvent monolayers is another issue and can be very hard to remove).

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