The objective of this project is to create a new method for selective motion control of micrometer-sized particles suspended in liquids. The method is based on an optical counterpart to "whispering gallery" resonance observed in certain structures, in which a sound made at one point may be heard clearly in certain other special spots, even though it may be inaudible everywhere else. In this project, a suspended spherical particle plays the role of the whispering gallery, with the resonance determined by its diameter and optical properties relative to the surrounding liquid. Laser light with wavelength matched to the whispering gallery resonance will apply significant optical forces to the particle, while particles of even slightly different diameters will feel no effect. This allows extremely selective particle manipulation and separation based on size and optical characteristics, enabling compact and robust micro-opto-fluidic devices for use in industry, healthcare, and environmental applications. The project will engage students from Gordon College, an undergraduate institution, in state-of-the-art collaborative research and advance the growth of an undergraduate-focused research infrastructure. Gordon College undergraduates will develop hands-on expertise in optics, microfabrication, microfluidics, and control. The project will also engage graduate students from Boston University, a major research university, who will obtain valuable teaching and research mentoring skills.
This project will investigate a method of separation using evanescent optical fields around an optical waveguide to exert forces on microparticles that are being carried in a flow field. These microparticles possess whispering gallery mode resonances that are known to be strongly dependent on the shape and size of the particles, ensuring an unprecedented selectivity of particles with narrowly defined geometric characteristics. Careful control of the forces and flow velocities allows for the design of particle trajectories, directing the selected ones into reservoirs to accomplish separation. To increase throughput, a networked control system model will be leveraged and communication and control policies designed that leverage the single actuation channel to simultaneously steer particles of different specific sizes. The project's approach has several desirable aspects: it has exquisite and tunable selectivity; it is applicable to solid particles as well as suspended liquid droplets; and it can be used in a compact and robust micro-opto-fluidic device. This work will advance fundamental science and engineering in several directions. It will provide detailed parametric study of the light propelling forces due to whispering gallery mode resonances and help to develop a fundamental understanding of the phenomenon. The particle separation application allows for the development of technology useful across various fields while also providing a setting to develop the field of networked control theory. Because a single actuator is employed to control the movement of multiple particles, this work requires a novel application of networked control theory and a new domain for exploring and resolving challenges in this field. The research will also push the boundaries in experimental fluid dynamics and optical science leading to the fabrication and test of a micro-opto-fluidic device.
