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When perform the force spectroscopy in liquid, why the “snap-in” and” pull-off” still happen on hard substrate (HOPG)? Does the contaminant on the tip account for this effect?
Hi Wang Shuo,
The instabilities in the force curve where the cantilever suddenly accelerates towards the surface ("snap in") and releases from the surface ("pull off") are caused by attractive forces overcoming the spring force of the cantilever. This can happen in air and fluids. In air these regions are more pronounced as you have capilliary forces acting in between the tip and surface. In fluids, the tip is fully immersed at all times, so there is no (water) meniscus that can pull the tip down.
Regards,
Stefan
Thanks for your comment
You mean that the van der Waals’ force account for these effects in liquid? But in air, these effects mainly caused by capillary force.
The forces related to the (liquid) adhesion layer on surface in air are quite large indeed and therefore often dominate your signal.
Hi Wang Shuo, electrostatic forces also play a part in solution.
Look up "DLVO theory" for more information.
Adrian