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If I have a silicon substrate and I want to measure normal forces, I take the change in setpoint x the cantilever stiffness x the sensitivity from the force displacement (fd) curves. However, if I am scanning in a liquid (fluid cell) do I use the sensitivity from the fd curves from the liquid or the fd curves from the silicon substrate in air?
Hi Dave,
The deflection sensitivity needs to be calibrated each time the probe itself or the laser alignment is changed. This includes switching between air and fluid operation, which usually means switching probe holders. But even if the same probe holder were used for both air and liquid there is always some shift in the laser path due to the different refractive index of air and water.
A related question would be if it's ok to use a spring constant that was determined in air. Here there's no problem because the spring constant is intrinsic to each each probe and doesn't change when switching between air and water.
The practical answer to your question might depend on what you're trying to accomplish too. If you only want a rough sense of the tip-sample force then you may find that using a nominal deflection sensitivity is good enough. For a given probe type, on a single instrument, and with a consistent alignment procedure you will likely find that the calibrated deflection sensitivity doesn't change by that much. But if you are attempting to measure any forces truly quantitatively then it's essential to calibrate both the deflection sensitivity and spring constant each time.
Regards,
-Ben
Thanks!!