Peak Force QNM Mode Retraction Hump

Chelsea posted on Tue, Feb 26 2013 8:24 PM

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Hi! We've been having an issue recently with Young's modulus calculation in standard amplitude QNM. The retract portion of the force curves get a sort of hump shape to them. They start out tracking alongside the relatively straight extend curve, but around about 90% of the maximum force, their slope shallows out and stays that way down to about 85% of the maximum force. Then they get steeper again and parallel the extend curve back down to baseline. The net result is the force curve ends up shaped like the tip of a violin bow. When this happens, the software returns modulus values ten times higher than we know our samples are (we expect about 500 kPa, and we see 5-15 MPa.) We've done high speed data capture and manual indentation analysis to confirm it's not just a software glitch; both the very top portion of the curve and the bottom do give slopes in that range. The shallow region gives a slope more like we thought we'd see, but we're not clear on the physics of what's causing the curve, so we don't feel comfortable using that value.

Does anyone know if external (non-sample-intrinsic) factors cause this force curve shape, how to fix it, or how to analyze these curves appropriately? We've seen it both on our tissue samples and on the soft PDMS calibration standard. We use borosilicate tips with spring constants around 0.06 N/m. Thanks much.