I am doing DNA solution rheology, and the DNA is dissolved in a buffer solution. I put this solution is a parallel plate of ARG2 and did some frequency sweep and creep tests, all in the linear viscoelastic regime. A silicone oil of viscosity of 0.05 Pa.s was wrapped around the edge to prevent water evaporation. But after 3 days of tests, the sample shrunk by about dV/V of 10%. I wonder what is the reason for this sample loss? Is it possible the water can dissolve a little bit in silicone oil, since the volume of the silicone oil is much large than the water solution (4 ml >> 0.02 ml). Is there a conventional way to measure the liquid-liquid solubility?
I think the problem is that the silicone oil is of such low viscosity. Yet, I can't imagine that using a higher viscosity oil would be helpful at all as that would greatly impact the results. (You are putting a viscous material at the outer edge of the plate, the very region that has the greatest influence over the measured stress.)
A larger diameter plate would reduce the relative surface area available for evaporation.
I think that it is possible for the h2o to diffuse theough the oil. In microfluidics aqeuous bubbles in silicone oil will slowly lose volume through this mechanism...perhaps another oil? Or is there anyway to do this test without waiting 3days?
I tried another oil (DuPont Krytox GPL 107) and no water is coming into this oil. But I am not sure why this made a difference, probably because this oil is fluorinated and more hydrophobic compared with silicone oil. But I am not sure, really. Or is it because it has a higher viscosity (1.6 Pa.s compared with 0.05 Pa.s of silicone oil)? Or is it because it has a higher density (~ 2 g/ml)?
If the mechanism is based on hydrophobicity, we do want to use a less viscous oil (because more viscous oil is applying an extra torque on the transducer). And DuPont has the same oil with lower viscosity. But at this point, I am really not sure what is making this difference in water solubility?
Thank you!
I think its a solubility not viscosity issue.....i would try the lower viscosity higher density dupont stuff.
Consider buying or building a "solvent trap" to contain the evaporation.
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