How do we deal with the slope in the approach curve?

[paulmueller]

Sometimes the approach curve (before indentation) exhibits a non-zero slope. While many might be of the opinion that such curves should be discarded altogether, sometimes there is just not enough data of better quality to analyze.

I believe the question is not whether we should avoid such an analysis (because the need is there). But how a solution could look like. There are two approaches with benefits and drawbacks:

1. Measure the slope and subtract it from the entire approach part (including the indentation part). Here we are effectively manipulating measurement data, which is not so good. In addition we would have to make sure that approach and retract curve "match up" after subtraction of the approach curve. This results in an offset of the approach curve and might also affect hysteresis.

2. Fit the slope and subtract it only up until the fittted contact point. In contrast to point 1, this is not anymore a preprocessing step, but part of the fit model. Here we are not touching the indentation data, which might be better.

Personally, I would prefer approach 2. Open for discussion. Please attach your data as .zip files to this issue if you would like to contribute to the discussion.

[felixrico]

The tilt in the force curve is inherent to many AFM systems. In particular, those moving the cantilever and not the sample. Thus, it is a known artifact that affects both contact and non-contact regions.
I would say that the way to correct it is fit a first order polynomial to the force versus indentation (not z movement). Using the indentation would minimize the correction applied to the contact part.
Regarding the approach and retract correction, I would say that this depends on the later analysis but if viscoelasticity is to be considered, the correction should be the same. That is, use the exact same parameters for approach and retract to correct for the tilt.

[paulmueller]

@felixrico That would imply the approach 1 with the extension that it should be applied to the retract part as well, right?.

What physical process leads to this tilt? Is this artifact well-understood in the AFM community?

[felixrico]

From my understanding, the attached picture may reflect what happens for cantilever scanning configuration. Of course, it will depend on each AFM configuration.

[felixrico]

I would allow choosing which trace (approach or retract) use as a reference. For indentation measurements, the approach might be the most useful, but for single molecule force spectroscopy, it might be the retract.

[paulmueller]

Ah ok. That means it is an inherent, systematic "error" of the system.

I was actually talking about this artifact as something that appears only sometimes and is thus sample-specific. But I don't have sufficient experience experiment-wise.

[paulmueller]

see #22