The Nanoscale World

Colloidal probe calibration

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BenSD posted on Tue, Nov 27 2012 2:25 PM

Hello,

 

I am trying to calibrate a colloidal probe (2 um beads) and I am getting very high deflexion sensitivity (around 1000 nm/V) and then very low spring constant (0.012 instead of 0.6N/m).

I calibrated the deflexion sensitivity in liquid and then the thermal tune in air...

 

Anyone could tell me what s wrong?

 

Thanks,

Benoit.

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Answered (Not Verified) Ang Li replied on Tue, Nov 27 2012 10:04 PM
Suggested by Ang Li

Hi, a wrong deflection sens will lead to a wrong spring constant calibration, and your deflection sens of 1000nm/v looks very likely wrong (usually deflectioni sens <100nm/v). you may try deflection calibration again. if you still get an odd number like this, you may share more information on your calibration procedure for further discussion.

Ang Li

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Try obtaining your deflection sensitivity in the same medium as you are thermally tuning your lever. The optical sensitivity is likely to be different in air and water due to the different refractive indices. I'd perform both measurements in air.

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You should definitely calibrate both the deflection sensitivity and the thermal tune in the same conditions (air or liquid). The deflection sensitivity does vary somewhat from one to the other. I'm guessing that you see a huge adhesion in air and you are trying to avoid that by calibrating the deflection sensitivity in liquid. But then in liquid, the resonance frequency drops, made still worse by the added mass of the 2um particle, which makes doing the thermal tune in liquid difficult or impossible.

But the deflection sensitivity of ~1000 nm/V is surely incorrect. That will lead to overestimation of the thermal noise and therefore the underestimation of the spring constant. Maybe post images of the force curves that you are using to calibrate the deflection sensitivity and would could see what's happening. The Catalyst has a very large Z range, so it should be possible to capture a good force curve in air, even if the adhesion is huge and you need to use a very large ramp size. Just use closed loop Z, make sure to use lots of points in the curve (like 4096 or more), and keep the non-contact deflection near 0V. Don't worry if the deflection rails at -12V, you don't need that part of the curve anyway. You just want to calibrate over ~200nm of positive deflection once you are in contact.

You might also consider looking into the added mass method if you are making the particle probes yourself and you know the density of the particle material. Some background information can be found here.

-Ben

 

 

 

 

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BenSD replied on Wed, Nov 28 2012 11:51 AM

Hello,

Thanks for your answer!

Like you said, the adhesion was too high and the deflexion I was using too low, I switch to sapphire surface in liquid which reduced the adhesion and I increased the deflexion to 2V. I got a def sens of 14 nm/v this way, then I did the thermal tune in air anf got a spring constant of 0.65N/m...

Benoit.

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OK, that's a definite improvement. But I will caution again that the deflection sensitivity in air varies from the deflection sensitivity in fluid. Since this enters into the spring constant calibration as squared term, any error in deflection sensitivity becomes a bigger error in spring constant.

-Ben

 

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BenSD replied on Wed, Nov 28 2012 1:21 PM

OK, so if thermal tune is more accurate in air, to do elasticity measurement in liquid I need to measure the def sens in air, then do the thermal tune in air and finally, measure the def sens again in liquid.

Is that correct?

The reason I didn't measure the def sens in air is because I couldn't get any good force curve due to the huge adhesion...

Thanks,

Benoit.

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Correct.

Yes, I understand that it might be difficult to get a good force curve in air due to the high adhesion. But you should be able to use a ramp size of up to nearly 20um, if that's what it takes to overcome the adhesion.

-Ben

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BenSD replied on Wed, Nov 28 2012 1:46 PM

OK, thanks I will try!

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