I'm attempting to create a thin (microns-thick), uniform film of ~1-2% agar on a cover slip using a spin coater. So far, all I've managed to do is create blotches near the center of the cover slip. Does anyone have any insight into how fast to spin, how long to spin, what temperature is ideal to spread the agar mixture uniformly, and how best to accelerate the spinner?
Hello,
I've never had to spin agar itself, but if what you've achieved so far is only blotches near the center of the slip (I assume the blotches are where you dropped the agar), perhaps you need to dilute the agar to reduce it's viscosity. I'm not sure what you dilute it with - maybe water?
Typical spin speeds are:
4000rpm, 30sec at speed, accel/decel = 1000rpm/s
I'd try altering the dilution until you get within a factor of 5-10 of your target thickness, and then try varying the spin speed to more closely dial-in your thickness.
Check the viscosities of typical photoresists:
http://www.microchem.com/pdf/SU-82000DataSheet2000_5thru2015Ver4.pdf
and see the spin curves (Fig. 3) here:
http://www.microchem.com/PDFs_Dow/SPR%20220%20DATA%20SHEET%20R%26H.pdf
To get an idea of typical thicknesses for a given viscosity & spin speed.
I think what you need is a solvent that evaporates more slowly than the one you are using and also in which the agar is more soluble. Also I agree that you may have to reduce the concentration by one order of magnitude.
Besides the viscosity as mentioned in the previous answer, the adhesion of agar on substrate is also very important. You can try making your substrate (glass) hydrophilc. I am not sure how much hydrophilicity is required for getting a good uniform spread of agar on the glass, however I would have tried the followings;
Clean glass with some surfactant (e.g Triton x100 or any detergenet ) followed by washing thoroughly in water and finally isopropyl alcohol. This practice will remove oily/greasy layer on the surface. If you want to make it more hydrophilic, you can clean glasses in piranha solution (1:3 H2O2 (30%): H2SO4 (concentrated)). This treatment can reduce the contact angle of water on glass upto 1-2 degree deepending on how long you treat your substrates.
good luck
Regards,
Rekha
Is agar hydrophilic? If yes, then piranha solution is a very good suggestion. If hydrophobic, then either very mild HF etching or an OTS coating will be good.
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