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I am trying to image oil deposition on surface using our AFM (an Icon system with Peak Force QNM). I am expecting thin film or small droplets formed on the surface. The oil is a hydrophobic small molecular weight ester (very soft). The substrate is fairly hard (modulus close to polystyrene)
Does any of you have any suggestion about particular AFM mode and tip for this task?
Thanks.
Hi,
I would think of starting with scanasyst in fluid using normal SCANASYST-FLUID tips. Prob you will need more efforts on sample preparation then imaging part should be straightforward. Please update your progress here if anything unexpected come out so we can help you more specifically.
LA
Thanks!
AFM in-air would be more idle for our purpose. If in fluid, oil might be dispersed into the fuild, or at least change its deposition morphology as to in air.
I am guessing patches of soft & hydrophoic deposition formed on the hard substrate. I tried both scanayst using soft tip and tapping mode, but could not find out any consistent indication of deposition. I also tried Peak Force QNM, hoping to track modulus or adhesive force differences. However, Peak Force QNM seems to unconsistent so far.
Liz,
You will not be able to measure modulus on both, your liquid and your hard substrate with the same tip at the same time. For modulus measurments the tip has to deform the sample a few nm and the cantilever that is capable of doing that on your hard surface may not be sensitive enough to image your liquid at the same time.
I think what Ang suggested is using the workspace for PeakForce Tapping in fluids not covering your sample physically in fluids (but this is just a guess of course).
Not sure what you mean when you say that you can't find "any cosistent indication of deposition". Do you get reproducible AFM images or now stable images at all?
Stefan
Hi, Liz,
I was thinking of your sample as imaging lipid like molecules so suggested in a liquid environment. Is your surface hydrophilic or hydrophobic and how is your molecules deposited, nonspecifically? and what would happen during drying process? Do you have other means (like optical imaging) to check your surface first to see the spreading of your molecules (do they form patches or just spread all over the surface, thus you can't see contrast by any imaging modes)? I still suspect the problem is more on your sample prep step instead of imaging step.
Hi Ang,
Thank you for your answer. I hope I can explain better this time about what I am looking for.
In my system, ~ 10 nm fluidic oil (olive oil alike) film is deposited on a hetrogeneniuos hydrophobic surface. I want to use AFM to map the oil film thickness using PeakForce QNM. I am uisng a soft tip (k ~0.4) and hoping the deformation value obtained correlates the thickness of the oil film.
Any comment or suggestion.
Thanks a lot
Zhi Li
Hi Stefan,
Thanks for your message. I gave up using modulus to map the oil on my hard surface. However, I did find decent maps in both dissipation and deformation channels. Can I find any correlation between deformation and thickness of liquid film (< 10 nm)?
Note that the fluidic non-evaporating oil does not (or at least does not appear to be in my images) form contineous film on the hard heterogeneous hydrophobic surface. Rather, it forms isolated patches. I want to figure out a way to measure the thickness (particular, if in the range of 1 - 10 nm).
Hi, if correctly calibrated, contrasst in deformation channel should also appear in modulus channel, usless your deflection sensitivity was not calibrated accurately. I guess you can roughly correlate the measured deformation to the thickness of your oil patches if you have very high indentation force to fully penetrate the oil layer. But a quantitative calculation would depend on your calibration method that you have to figure out in some way.
Hi Angus,
Oil droplets are not easy to image. Sample prep wise, I can recommend squizing it between a glass slide and a cover slip and spread it so that you have a VERY thin layer. Parameter wise, I noted that this type of sample is so viscous that you really have to used the special version of QNM having a 1 micrometer drive amplitude. Otherwise the tip gets stuck and you can't pull off.
Good luck!
Alex.
Hi Li,
I don't think that is a viable way to measure thickness. I suggest to find a patch where you can get to the surface and top of the film within the scan range of your AFM and then measure the step height which will be the thickness of your layer. This is analogous to solid thin films where you basically have to make a scratch, e.g. with a razorblade, to generate a step that you can subequently image.