The Nanoscale World

Reverse path effect on force curves?

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nhung posted on Wed, Dec 14 2011 10:10 AM

I used ramp mode in Dimension Icon to obtain force curves. However, the inverse path effect appears and causes the retracting curve to pass over the approach curve. This is unphysical as at the same amount of indentation, the force exert on the sample during loading should be larger than those exerted when unloading. This effect occurs eventhough the voltage I used is not very big (500mv-6V)

What are the reasons of this artifacts? How can I eliminate the effect?


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Bruker Employee


I have seen this happen if you are ramping in open loop. Can you verify if you are ramping in closed loop (Z closed loop = ON) and monitoring Deflection/Force vs. Z sensor?




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nhung replied on Thu, Dec 15 2011 10:50 AM

Thank you for your reply.

Yes, I turned on Z closed loop, but the reverse path effect still happens in the deflection vs. Z sensor, especially when the trig threshold defleciton is high. is there any other factors?

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replied on Thu, Dec 15 2011 4:43 PM

What kind of sample and tip did you use?

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The artefact you describe can be caused by the fact that the cantilever is not parallel with the surface. This means that moving the cantilever vertically (with your Z ramp) while in contact applies a force along the length of the cantilever. This force is balanced by the restistance of the tip sample contact to shear. On the approach, the tip-sample contact applies a torque at the tip end which causes convex bending of the cantilever in addition to the bending caused by the normal force at the tip. As the beam-bounce system that is used in the vast majority of AFMs measures the angle of the upper surface of the cantilever, as opposed to its real vertical displacement, this means that you measure a smaller deflection than is actually applied. On the retract, the in-plane force is in the opposite direction, so the cantilever takes on a more concave shape, which is measured as more deflection than is really applied by the normal force.

This effect has actually been used as a way in which to measure friction between the tip and the sample. See Attard et al, Journal of Physics: Conference Series 61 (2007) 51–55, for example. I seem to remember that Bhushan has done something similar also.

I'd be surprised if you can eliminate this artefact, but you should be able to calculate how big it is and the effect that it has on your normal force measurement using the above paper.

Hope that helps,


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nhung replied on Fri, Dec 16 2011 8:59 AM

Thank you all for your replies. I am using Tap 150A in tapping mode. My sample is PLGA which has biconcave shape with diameter about 10-30 micron, height is about 3 micron. Anything else that I need to be aware of in order to minimize this artifact since if I can't not eliminate it, it causes trouble when trying to get the mechanical properties such as viscoelasticity.

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