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We are interested in measuring the rigidity of silicon surfaces that should range between 1 -3 MPa. The tip we are using is a 0.95N/m cantilever with a 4.5um polystyrene spherical tip. We previously used a 0.35N/m cantilever also with a 4.5um PS tip, but that was deemed too soft. In any case, the indentation of the material produced unusual curves.
At a ramp size of 8.6um:
With increasing ramp size, at 8.7um:
At 8.9um:
At 9.1um:
And at 9.3um:
Questions: 1.) This final curve is reproducible between experiments and reveals rigidities in an appropriate range. Is it acceptable to analyze curves with so small a baseline?
2.) In the past, using the NanoScope Analysis Software I have ignored adhesion forces and corrected for baseline. In these instances I notice an improved fit by not correcting for baseline (perhaps because there is no appreciable one) and also including the adhesion force. Is this appropriate?
Thank you!
Dear Sedeleo,
Do you measure silicon or silicone? Your modulus sounds to me as to be too low for silicon.
Your curves look awful. When I get curves like those, I readjust the laser point or even change the tip. Did you obtain better curves on a standard calibration sample, like saphire or the polystyrene calibration sample? The curves should be constant for a few hundred nanometers to get reasonable curves. It seems to me as if there is some reason for the machine not to be able to measure deflections higher than 220 nm. Maybe because the maximum deflection was reached? The constant part you can see in your last plot (9.3 µm) is the same as the flat part at -220 nm in the plots above. It is not a real material response, but some kind of measurement error.
To get to better curves you should use a fresh AFM-tip and try to get to clean "heartbeat" curves on standard samples (saphire, PDMS). If it doesn't work immidiately, try to readjust the laser point. Sometimes that helps already.
Regards, Dietmar
Hi Dietmar,
Thank you for your quick reply. I misspoke, the measurements were taken on 1:10 PDMS. I was able to get pretty clean curves on polystyrene, as you mentioned. I can try to adjust the laser point next time. Would it suggest that the 0.95N/m tip is not appropriate for this sample?
Hi, Sedeleo,
They recommend to use "ScanAsyst" probes for a stiffness like that, which has a spring constant of 0.5 N/m. So your probe should be fine. Is the PDMS sample you mention a reference sample? If not, try to image the PDMS reference sample next time and try to create the very same force-distance curves from. They should look like that:
It looks to me you are using trigger off mode and it's recommended to use trigger relative mode for force ramp. You should have a normal force curve as Polis showed (although the wavy baseline is not very normal if it's not peakforce curve) for an accurate analysis.
Ang Li
Hi Sara,
I believe I just switched to a new tip. It's unfortunate that PDMS is so sticky (sometimes we incubate in a small percentage BSA solution to reduce this), but it means that the tips must be switched out more often. The post above may also help, but I haven't seen it until now: indeed I was using trigger off mode when perhaps I should use trigger relative mode.
Good luck!