Integrated optofluidic particle manipulation




Blakely, Justin Thomas

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Optical confinement and manipulation of matter, or optical trapping, is widely adopted at micro-scales as a research tool in disciplines of biology, engineering, and physics. Microfluidic systems arc attractive from the standpoint of low sample volumes, laminar flow, and viscous damping and offer an ideal environment for trapping of miniaturized objects and microorganisms. Various trapping configurations are presented in this thesis using a custom fabricated consumer-grade optofluidic chip and are of significant scientific and practical importance. Microfluidics and fiber optics are integrated in-plane to achieve several flow-dependent particle trapping mechanisms on-chip. Each mechanism results from a combination of fluid drag and optical scattering forces. Parallel and offset fibers, orthogonally oriented to the flow, show cyclic cross-stream particle transit with flow-dependent particle trajectories and loss. Upstream-angled fibers with flow result in circulatory particle trajectories. Asymmetric angled fibers result in continuous particle circulation whereas symmetry with respect to the flow axis enables both stable trapping and circulation modes. Stable trapping of single particles, self-guided multi-particle arrays and stacked particle assemblies are demonstrated with a single upstream-oriented fiber. Size tuning of trapped multiple particle assemblies is also presented. The planar interaction of fluid drag and optical forces results in novel possibilities for cost-effective on-chip diagnostics, mixing, flow rate monitoring, and cell analysis. An opto-hydrodynamic theory is adopted to verify experimentally observed particle array dynamics in a dual-beam fiber-optic trap. When applied to dielectric microsphere particles, the theory confirms the inhomogeneous self-organization and the spontaneous emergence of self-sustained oscillations in particle arrays. In the presence of small-scale symmetry-breaking, self-sustained oscillations are shown to occur spontaneously from an exchange between the optical scattering and the gradient optical forces, in the absence of inertia that is central to the dynamics of ion traps. Experimental results show non-uniform equilibrium particle spacing and spontaneous self-sustained oscillation for large particle numbers. Self-organization and oscillation is of general interest to other systems involving multi-particle optical interactions.



Microfluidics, Fiber optics