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5 September 2007 Microrheology of microlitre samples: probed with rotating optical tweezers
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Microrheology is the study of fluid flows and material deformations on a microscopic scale. The study of viscoelasticity of microscopic structures, such as cells, is one application of microrheometry. Another application is to study biological and medical samples where only a limited volume (microlitres) of fluid is available. This second application is the focus of our work and we present a suitable microrheometer based on optical tweezers. Optical tweezers are an optical trap created by a tightly focused laser beam. The gradient force at this focus acts to trap transparent micron sized particles, which can be manipulated within the surrounding environment. We use the polarisation of the incident field to transfer angular momentum to a trapped spherical birefringent particle. This causes the particle to rotate and measuring the polarisation of the forward scattered light allows the optical torque applied to the sphere to be calculated. From the torque, the viscosity of the surrounding liquid can be found. We present a technique that allows us to perform these measurements on microlitre volumes of fluid. By applying a time-dependent torque to the particle, the frequency response of the liquid can also be determined, which allows viscoelasticity to be measured. This is left as a future direction for this project.
© (2007) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Simon J. W. Parkin, Gregor Knöner, Timo A. Nieminen, Norman R. Heckenberg, and Halina Rubinsztein-Dunlop "Microrheology of microlitre samples: probed with rotating optical tweezers", Proc. SPIE 6644, Optical Trapping and Optical Micromanipulation IV, 66440O (5 September 2007);


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