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I am a biologist who has very limited knowledge in biophysics. My student has recently used a Bioscope Catalyst to generate some really nice images. We have read your application notes but still couldn't understand what some of the parameters mean in layman's terms.
In short, we are comparing the properties of two materials. Their "DMT modulus" values are similar, but the "deformation" parameters are very different.
Our question is: what does this mean? Our local Bruker AFM specialist told us that we could imagine the DMT modulus parameter as a measurement of the material's stiffness. If true, how could the probe cause more deformation on one of the materials, even the two have similar stiffness.
I hope this question makes sense to you, and thatI you are not laughing at our ignorance. We have just been introduced to the wonders of AFM and are trying to understand what we are doing...
Thank you in advance for your help and patience.
Your question is very precise. There is a confusion between stiffness and rigidity moduli. These are two different things. Briefly, the stiffness is the derivative of the load force with respect to the deformation (incline of the force curve). It has the dimension of force/length. And it does depend on the load force and the AFM probe used. Rigidity modulus (DMT modulus is one of those) is a property of the material, and should not depend on either the probe or deformation. It has dimension of force/length^2.
A bit more detailed answer: DMT modulus is the Young's modulus (the rigidity modulus of ideal elastic isotropic material) which is calculated by using the load force plus adhesion. In many situations, it is the correct way to calculate the rigidity modulus when adhesion is noticeable (but not the only one.. the other JKR method is not implemented in the SMP software because it is more difficult). In the ideal case, this modulus should not depend on the load (or deformation). However, in many measurements soft materials researchers use excessively sharp probes. It results in a highly non-linear deformation. As a result, the derived DMT modulus can depend on the deformation. If you observed something like that, you can be in the non-linear regime. I do not know references where this is described for AFM in details (although it is described in the general textbooks on elasticity of materials).We are preparing two manuscripts demonstrating all these for the case of the AFM study polymers and cells. If this work is of interest, please check with me in ~6 months.
Best regards,
Igor
_____________________________________
Igor Sokolov, Ph.D.
Professor,
Department of Physics,
Department of Chemistry and Biomolecular Science,
Director of Nanoengineering and Biotechnology Laboratories Center (NABLAB),
NY Center for Advanced Material Processing (CAMP) ,
Clarkson University,
Potsdam, NY 13699
Phone: 315-268-2375
Fax: 315-268-6610
http://www.clarkson.edu/~isokolov
http://www.clarkson.edu/~nablab