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Hello,
I'm running some force curve tests in liquid medium(via fluid meniscus) with the di3100 afm. My problem is that I can't seem to engage properly on my sample.After aligning my laser onto my cantilever tip (sum~2.5-3.5) and focusing on my surface(in fluid), I hit the engage button. What i notice is that most of the time after doing the pre-engage check, the afm scanner makes beeping/squeaky noises while the "approaching surface" message is present in my screen. What happens next is that I see the cantilever being moved in the "fast approach" mode but once it transitions to the "incremental/safety approach" mode, it abruptly halts and beeps to say that it has engaged.I know for a fact that my tip is not properly engaged because of the large gap in focus between the sample and cantilever tip.This happens almost all the time for the past few days. To be specific, i have a glass coverslip onto which biological samples are adhered.I place about 3 drops of fluid on my sample and then one drop of fluid on my cantilever prior to engage as my setup(this constitutes the fluid meniscus).I also adjust my laser's vertical deflection point to -2.0 and horizontal to 0.0. My deflection setpoint for engage is always 0.0. My cantilever is very soft,actually with a k=0.02 N/m, however, i've seen people use cantilevers similar those in K before.I also did take into account the change in index of refraction due to fluid by focusing about 300um below my actual surface, but whether i do this or not, i still often experience the same engage issue, just at different heights,respectively.I'm not sure what I'm doing wrong.Other users with the same system work in air and they say that the system works just fine,without any such issues and noises by the scanner.Any ideas?
I think i found a solution to the issue:
Due to the effects of fluid, it is necessary to decrease the "minimize engage gain" value found in the scan parameters list window. On average, the value is set at about 3.It is advised to minimize this value to approximately 0.4.One should also try to decrease the integral and proportional gains(0.2 and 0.3 respectively). The main cause to the problem is that in fluid, the added mass around the fluid tip holder tends to cause an oscillation which can result in the system seeing it as a fault and hence create a beeping noise and halt any engaging approach to a sample.When we minimize the gains,especially the one used for during engage, this oscillation can be prevented and a steady engage is possible.
Hello Anketene,
With a very soft lever as you describe, at k=0.02N/m, it is possible that the cantilever is deflecting 2V or more before the lever actually reaches the surface. From your description of behavior, it certainly sounds like false engage is occuring, where the system reaches the setpoint but the lever is not on the surface. I would try using a larger delta between your pre-engage vertical deflection and deflection setpoint. Hope this helps.
Adam
I think the problem is in the focusing IN the liquit. The way I do it without any problems: First allign and focus the tip and surface in air, add the liquit, realign the laser ( only the x-as a little couterclockwise) to find the maximum sum back. Then Approach! You will make good contact, but the surface will not be in focus. In the newer software you can adjust the focus after making contact with the navigate window.
Joop