how to mesuret the stiffness of an optical trap.
Here are a few links to articles for Optical trapping that may help.
For measuring trap stiffness: http://www2.bioch.ox.ac.uk/~oubsu/ebjknight/stiffness.html
Optical Trapping: http://www.pubmedcentral.nih.gov/articlerender.fcgi?&pubmedid=16878180
Optical Tweezers: Measuring Piconewton Forces
Mark C. Williams
Department of Physics and Center for Interdisciplinary Research on Complex Systems
Optical tweezers instruments use the forces of laser radiation pressure to trap small particles. Using various techniques, these trapped particles can then be manipulated and forces on the objects in the trap can be measured. The forces that such an instrument is capable of measuring are of the order of one to 100 piconewtons (pN). While this technique has been used for over 20 years to manipulate and study the
properties of micron-sized dielectric particles, it is only recently that this precise force measurement instrument has been applied to the study of biological systems. This chapter will outline the physics responsible for the trapping and force-measuring capabilities of optical tweezers instruments. The designs of two major types of optical tweezers will be described and the limitations and advantages of each type will be discussed. Detailed designs of instruments and instructions
for building optical tweezers will not be given. However, this information can be found in the references at the end of the chapter. After discussing the general properties of optical tweezers, we will describe a few of the many types of experiments that have used these instruments to study the properties of biological systems.
The ability to manipulate single molecules with nanometer precision and to measure forces on these molecules with piconewton accuracy using optical tweezers has opened up several important new areas of study in biophysics. These new single molecule manipulation experiments have allowed us to test physical models describing the properties of DNA in a way that was never before possible. We can now observe the activity of a single enzyme acting on a DNA molecule and watch tiny molecular motors exert forces on biological molecules. We can directly test the effects of DNA binding proteins on the properties of DNA and use these measurements to determine their energy of interaction. A detailed knowledge of individual interactions between molecules is essential for understanding the complex mechanisms involved in real biological processes. Single molecule measurements of these interactions have generated significant new insights into these processes. A few examples of how optical tweezers have been used to study these
interactions will be discussed in Section 5 of this chapter.