| Summary: | 碩士 === 國立中正大學 === 光機電整合工程所 === 98 === Optical tweezers has been being a very popular tool for precision metrology because it is an active method and can sense force variation down to several pico-Newtons. In this study, this tool is employed to investigate the micro-rheology of a dilute CTAB/SS (Centyltrimethylammonium bromide/Sodium salicylate) solution, a solution of cationic surfactant with wormlike micelle structure. Measurements and characterizations are based on two sets of optical tweezers for detecting the motion signals of the optical trapped particle. Two types of motions, oscillation and rotation, are studied. An optical tweezers with two quadratic photodiodes (QPDs) is arranged for the oscillation measurement. The merit of using the 2nd QPD is to enhance the measurement accuracy which is especially favorable for the optical tweezers using a relatively low-powered laser as the light source because that exhibits smaller tweezers stiffness and allows for larger particle’s oscillation amplitude. Although the minute motion signals can be analyzed using a lock-in amplifier, the implementation of the 2nd QPD, however, is able to detect the oscillating information contributed by the motion table that should be excluded from the resultant signal. The torque for rotation is generated by irradiating a circular polarized light on the trapped particle which is spherical and birefringent. The torque magnitude is obtained by a polarization analyzer. The rheological characteristics obtained by these two approaches are examined and discussed and their fundamental difference from those measured using macro-rheometers is also addressed.
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