I'm trying to create a set of substrates that have a range of fixed values of surface-free-energy, in order to vary the contact-angle of samples of colloidal liquid placed on the substrates. The substrates have to be transparent (or at least translucent) to red light. So far, I've managed to do this by using a kind of pot-luck approach, in which I buy microscope slides from various different manufacturers (including silane-coated slides), which does result in the contact-angle varying (as measured from the geometry of a sessile droplet). However, the angles vary significantly, and are not very reproducible between experiments. I'd like to be able to produce a set of substrates with a predictable and reliable range of surface free-energies, that (for a test liquid like eq water) would yield contact angles of 10, 20, 40, 60, 80 degrees. What is the best means of producing these? If I were to ask eg a commercial organisation to quote for making them, how best to specify - by value of surface-free-energy (and if so, how best to calculate it - there seem to be a number of models), or by 'contact-angle of a sample of liquid X at temperature Y and humidity z"?
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