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Dear all,
For my experiments on a Catalyst I've been using the "point and shoot" window. In this window, I import a canvas image, on which I can then select where I want the system to record force curves. To calculate a reliable result I always collect 3 ramps on each point. I often notice that the first out of 3 ramps on each point is often an error. When the system moves over the the next point after performing 3 ramps on this point, the first ramp on the next point is then often again an error. I wonder what could be causing this?
I uploaded an example picture below. On the left you can see a bad ramp that is recorded first, on the right is a curve that was ramped second, but on exactly the same location.
The measurements are performed in contact mode in fluid, using 10um polystyrene bead-functionalized cantilevers with spring constants around 0.06N/m. Ramp speeds are in the range of 1-3um/s
Hi Sjoerd,
Can you share the key settings for your force measurements, please? It looks like the first curve 'stopped' at a quite low maximum force, while the second one went to a higher force level. Maybe there is some optimization on the force triggering to be done?
Peter
Hi Peter,
Do you mean the triggering threshold? For all curves I use a threshold value of 2nN. To me it looks like the extend curves in both pictures have made the ramp over the the full 2nN, although in the left curve it seems to have started at a lower force. Indeed maybe something is different in the initial approach, although I don't think I have control over that. All 3 ramps should be performed executed the same.
Best,
Sjoerd
The first curve never seem to reach the 2nN trigger value, maybe because there is a huge adhesion peak (5nN), whcih I do not see in the 2nd cruev. Could there be any specific reason for this big adhesion upon the first curve? do you see the same behavior if you colelct individual curves 1-by-1 in a manual way?
I don't understand. The blue curve in the left plot travels from approx -4.3 to -2.3nN. So the system has somehow registered a 2nN force. The huge adhesion peak I can't explain. The substrate is indeed sticky, because I always get quite some adhesion as also seen in the plot on the right, which is what I usually see. The plot as seen on the left I never see when collecting individual curves. To me it looks very artificial, I don't see why the approach curve starts at -4.3nN whereas some seconds later on the same spot it starts at -1.8nN (where it was aligned at that moment). I fear that it has something to do with that the system starts ramping immediately after changing xy position and that something is not right there.
As far as I can see it, it has something to do with your sample, is it "very" rough? I think the first curve is not good because the start of the curve is with the tip on the surface and not away from the surface, it still gets the 2nN reletive trigger, then withdraws.
Check it on a glas sample withe a normal tip if you get the same, don't expect it, if you still get it there there is a setting wrong.
Thank you for your comment.
I briefly checked it and I didn't see these bad curves on glass. For some reason, in my last 2-3 experiments I didn't experience so many bad curves in my own samples either. Indeed maybe it has to do with roughness of the sample; these collagen gels are not flat.
I know this is more than a year old. Yet, if anyone is still interested try this experiment.
On one of the polymer samples with a fairly stiff cantilever, approach the surface and image it at around 2 µm scan size as softly as possible for the point and shoot. Save the images. Be sure to perform an up and a down scan. If the cantilever is stiff enough maybe a TESPA cantilever then you will see a plastic deformation in the center of the image from the approach at 0.1500 V. Then in point and shoot mode draw a vertical line with about 5 spots to perform indentations perform 4 indentations at each point at a fairly high force setpoint at least 0.15 V maybe 0.5 V would be even better. Perform the indentations and then reimage the surface at a light setpoint. What you will likely see is 5 rows of indentations with 2 holes in each row or effectively a 5 row 2 column image of the plastic deformations. Now one of the holes is going to be not very deep while the other is quite deep . The shallow hole that may look stretched is from the 1st ramp, while the 2nd hole is much deeper where the tip indented 3 times. You may also see that the picked indentation locations are also compressed closer together relative to where they were selected. i.e. something like half the distance that they should be from the point and shoot selected area. I once saw this while working on some standards and never had the chance to upload any images. This may have been fixed in more recent updates of the software. If anyone wants to test this please do and upload your findings.
Jeremy R. Eskelsen, Ph.D.