[Spm] Force of Adhesion Variations

[Robert W. Carpick]

Clare - the pull-off force is a function of tip radius and tip shape 
as well as the tip and sample surface chemical composition and the 
resulting interfacial forces. If your surface is rough, then you will 
also see significant variation from one measurement to the next.

Most likely, your pull-off force is varying because the tip radius is 
different from one lever to the next; we have seen that this is very 
common. You can account for this by measuring the tip radius in TEM 
or using blind reconstruction.

Also, you need to be sure your force calibration is accurate for each 
lever. If you are using the manufacturer's quoted spring constants, 
you will likely have errors, which again will differ from one lever 
to the next.

So the procedure is:

- calibrate the force constant and deflection sensitivity of each 
cantilever you use
- measure the tip shape and radius before and after in TEM, or during 
your run using blind reconstruction
- use contact mechanics to convert your pull-off force to a work of 
adhesion value, which will be intrinsic to the materials

If you can, using a procedure to clean the tip, or coat it with a 
controlled material, can be helpful. There has been significant 
discussion in the SPM digest on this in the past.

There are several papers that discuss these issues; this paper of 
ours reviews the procedures we recommend:

Accounting for the JKR-DMT transition in adhesion and friction 
measurements with AFM
	D.S. Grierson, E.E. Flater, and R.W. Carpick, J. Adhes. Sci. 
Technol., 19 (3-5) 291-311(2005)

Good luck,
Rob


>Our biggest problem is when we measure forces in liquid with one tip 
>and then when a different tip is tried the forces can be two, three, 
>or four times larger or smaller than the initial measurements.  Each 
>time we are using brand new tips from the same wafer.  We are also 
>using clean new surfaces.  We are also very careful in controlling 
>the environment and purity of the solvent.  What can we do to 
>minimize this discrepancy between tips?
>
>Thanks
>
>Clare Aubrey Medendorp
>University of Kentucky
>Pharmaceutical Sciences
>ceaubr2 at email.uky.edu


-- 
----------------------------------------------------------------------------------------------------
Robert W. Carpick, Associate Professor, University of Pennsylvania
Dept. of Mechanical Engineering and Applied Mechanics
271 Towne Bldg., 220 S. 33rd St., Philadelphia, PA  19104-6315  USA
Tel: 215-898-4608  Fax: 215-573-6334   email: carpick at seas.upenn.edu
http://www.seas.upenn.edu/meam/faculty/carpick.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://spmlist.di.com/pipermail/spm/attachments/20070621/2e4cca34/attachment-0002.html