Undergraduate Thesis Or Project
 

Optical Tweezer Trapping of Colloidal Polystyrene and Silica Microspheres

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  • Optical trapping is a tool used throughout a wide variety of disciplines rang- ing from precisely probing and manipulating sub-micron organisms in biol- ogy to analyzing fundamental charge transfer in colloidal physics. This thesis presents research involving optical tweezer force measurements of polystyrene and silica microspheres. In addition, preliminary work in tweezer-based sur- face charge measurements is developed to examine surface charge at the solid-liquid interface of silica microspheres. The main purpose of constructing, and testing the tweezer apparatus is to utilize its trapping capability for experiments involving charge measurements. The optical tweezer related research provides information pertaining to force measurements of a single beam optical tweezer trap equipped with an inde- pendent back focal-plane laser position detection system. Calibration meth- ods and experimental power dependence for 1μm-diameter polystyrene and silica microspheres provide proof-of-principle results that exhibit expected linear power dependence to within an experimental error of 10.5%. The research involving charge measurements provide preliminary the frame- work for studying charging/discharging dynamics of 1μm-diameter colloidal silica spheres. Studying (dis)charging dynamics on nano-scale levels is dif- ficult, however, the methods presented in this thesis uses an ultrasensitive technique adopted from Roberts et al. [1] to measure the effective surface charge of particles dispersed in water. This is performed by conducting electrophoresis experiments designed to track the movement of an optically trapped particle in the presence of an applied (AC) electric-field. Experimen- tal results demonstrate the ability to measure the effective surface charge. The proof-of-principle experiments show that for a 1μm silica microsphere the surface charge is found to be in the range of 66qel − 113qel, as compared to Roberts et al. nonpolar suspended PMMA particles 14qel, yet similar to Behrens et al. [2] measurements of silica in deionized water 471qel − 2200qel. The preliminary work documented in this thesis sets forth the necessary groundwork to examine charging dynamics of molecular-coated silica micro- spheres, and further experiments are being designed to study ADT-TES-F coatings at varying electric-field frequencies. REFERENCES [1] G Seth Roberts, Tiffany A Wood, William J Frith, and Paul Bartlett. Direct measure- ment of the effective charge in nonpolar suspensions by optical tracking of single particles. The Journal of chemical physics, 126(19):194503–194503, 2007 [2] S. H. Behrens and D. G. Grier. The charge of glass and silica surfaces. Journal of chemical physics, 115(14):6716–6721, 2001.
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  • This project was partially funded by the National Science Foundation.
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