Electrowetting-Tuned Liquid Droplets on Lubricated Superhydrophobic Surfaces for Whispering-Gallery-Mode Sensing

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Miniaturized, portable, sensitive, and low cost sensing systems are important for medical and environmental diagnostic and monitoring applications. Chip scale integrated photonic sensing systems that combine optical, electrical, and fluidic functions are especially attractive for sensing applications due to the high sensitivity of optical sensors, the small form-factor of chip scale systems, and the low-cost processing possible for systems fabricated with well-developed mass production techniques. While optical sensing with a detection limit down to single nanoparticles has been achieved by various methods, such as scattering interferometric and photothermal microscopy and nanofiber sensors, microcavity sensing attracts much attention because their high quality factors (Q factor, which physically represents the rate of energy loss relative to the total stored energy) and small mode volumes enable significant enhancement of light-matter interactions. Microcavity sensing has seen tremendous progress and the sensing performance has been demonstrated by detecting single nanoparticles and single biological molecules. However, detection in liquids with whispering gallery mode (WGM, i.e., closed circular beams supported by total internal reflections at the external cavity interface) cavities was achieved only in rare cases using dielectric micro-resonators that were immersed or filled with liquids in a closed environment. No real and stable high Q-factor sensing experiment with non-solid optical resonators has been reported to date. In this program, the team formed at Virginia Tech aims to perform optical sensing with micro-resonators made directly of liquid droplets. New research outcomes from this project will be integrated with educational endeavors. Bio-inspired nanoscience and mechanical engineering will be integrated with all levels of K-12 education. Participation in the proposed researches will motivate the underrepresented groups for advanced degrees. In addition to graduate students, undergraduate and female students will participate in the research through thesis, project-based courses, or multidisciplinary senior design projects.The research objective of this project is to assist in realizing on-chip detection and sizing systems with ultra-high sensitivity and Q-factor, laying the groundwork to investigate the properties and dynamics of single particle analyte and single biological molecules with an accuracy that cannot be achieved using ensemble measurements. The research tasks are below: (1) The lubricated superhydrophobic surfaces with engineered micro/nanostructures can make the liquid droplet stand with a sufficiently large contact angle for WGM sensing and the lubricant cloaking can help prevent fast evaporation of water droplet; (2) The novel WGM system configuration enables resilient coupling of the liquid microcavity with the built-in waveguide on the substrate; (3) An ideal detection system requires not only the ability of trace analyte response or single-particle-level response, but also the rapid detection of targets. Electrowetting will be applied to actuate liquid droplets in a programmable path and transport to the target with built-in waveguide for WGM sensing; (4) For the first time, ultra-sensitivity WGM will be employed to detect the existence of depletion shell on a liquid droplet; (5) The overarching goal is to develop chip-scale integrated photonic sensing systems that combine optical, electrical, and fluidic functions. This proposed high Q-factor approach will pave the way for sensors that have unprecedented sensitivity to tiny changes in their environments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.