Optically trapped dielectric particles experience a linear restoring force due to the trapping laser beam for small displacements of the trapped beads from the center of the optical trap. The optical force adds an apparent contribution to the measured shear elastic modulus of the solution in microrheology experiments. Traditional methods of correcting for the effect of the trap and obtaining the true shear elastic modulus of the solution involved taking measurements in separate purely viscous solutions using similar but not identical trapped beads under the same experimental conditions. A new method is proposed in this research to do this correction. It is based on finding the apparent response of the system including solution and trap at two different laser powers then extracting the true medium response from the measured responses. It was found to be an effective method to correct for the optical trap effect for samples of purely viscous solutions that theoretically should have zero elastic shear modulus. It was also tested in a solution of worm-like micelles to check its viability in solutions that have an inherent elastic component of shear modulus. Measurements in water yielded a zero elastic shear modulus and those in micelle solution agreed with previously published data for worm-like micelles solutions. The new method requires less labor and avoids possible sources of error involved in the traditional methods.


Physics Department

Degree Name

MS in Physics

Graduation Date


Submission Date

May 2014

First Advisor

Addas, Karim

Committee Member 1

Elmeseiry, Medhat

Committee Member 2

Swillam, Mohamed


59 p.

Document Type

Master's Thesis

Library of Congress Subject Heading 1


Library of Congress Subject Heading 2



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All experiments for this research were conducted at the Third Institute of Physics-Biophysics (DPI) at the Georg-August University in Göttingen, Germany. The institute also covered all the costs for the materials and equipment used in the project. Funds for the travel and accommodation to work on this project were available through the Graduate Student Grant (Office of the Dean of Graduate Studies at AUC) and the generous support of the institute in Germany.