(G-244) Characterization of Particulate Flow Dynamics of Non-Neutrally Buoyant, Flat-Plate Microchips in Rectangular Microchannel Flows

Introduction:: The continued advancement in semiconductor manufacturing and miniaturization of integrated circuits have created an opportunity to incorporate flowing microchip particles into microfluidic devices for a variety of applications including particle-based multiplexed bioassays and combinatorial synthesis. There have been numerous studies on the inertial focusing and ordering of neutrally buoyant, spherical droplets, cells, and beads in microfluidic channels [1]. However, additional understanding of the particulate flow dynamics of non-neutrally buoyant, flat-plate particles flowing in rectangular microchannels is necessary for the design and optimization of microfluidic devices for manipulating and sorting microchips. In this study, we investigated the particle lift-off and inertial focusing behavior of non-neutrally buoyant, flat-plate microchips in rectangular microchannel flows.

Materials and Methods:: We developed a microfluidic particulate flow monitoring system (Figure 1) consisting of a syringe pump (Model 44, Harvard Apparatus, Holliston, MA) for fluid delivery and an integrated high-speed (1500 FPS) camera (MQ003MG-CM, Ximea, Münster, Germany) to capture videos of the flowing microchips. To create rectangular microchannels, poly(methyl methacrylate) sheets were milled using a desktop computer numerical control mill (Desktop PCB Milling Machine, Bantam Tools, Peekskill, NY). We sealed the microfluidic devices using an ethanol-assisted thermal bonding procedure [2]. Flow adapters were printed using a stereolithography 3-D printer (Form 3B, FormLabs, Somerville, MA). A 600 μm × 600 μm × 100 μm p-Chip® microtransponder (p-Chip Corporation, Chicago, IL, USA) was selected as the microchip particle for flow studies due to its viability as a platform for biotechnological applications as well as its requirement for controlled particle orientation for light-activated encoding/identification [3]. We conducted experimental particulate flow studies of a single p-Chip flowing through a rectangular microchannel in various solvents (water, isopropanol, methanol) with different kinematic viscosities and at different channel aspect ratios (AR: 0.67, 1.00, 1.50) and operating flow rates. Custom image processing and computer vision algorithms were developed in MATLAB (Mathworks, Natick, MA, USA) to perform automatic channel wall detection for image calibration (Figure 2), video summarization, and particle position/rotation tracking during video analysis (Figure 3).

Results, Conclusions, and Discussions:: Figure 4 shows the plot of the critical shear Reynolds number, R, as a function of the Archimedes number, Ar, for particle lift-off at different values of the ratio of the channel width and the maximum particle diameter, W/D­_max, and channel aspect ratio, AR. It was observed that the R necessary for particle lift-off increase for larger Ar which suggests that gravity plays a dominant role over viscous in the lift-off of the microchip particle. It was observed that an increase in W/D­_max results in a decrease in R needed for particle lift-off. Unlike previous studies which demonstrated that R necessary to lift heavier spherical particles decreases with increasing W/D­_max [4], our current findings on critical lift-off of flat-plate shape show that the critical R for lift off of flat-plate particles decreases with increasing HW/D­_max² . Different particle motion including translating, rotating, lateral bouncing, tumbling, and rolling were observed. The different categorized particle motion for the non-neutrally buoyant, flat-plate particle are similar to that observed for non-spherical, neutrally buoyant particles [5]. Tumbling and rolling occurred more frequently at higher flow rates (Re > 80). Further analysis of the categorized particle motion, particle flow rates, rotational periodicity, lateral bouncing, and focused lateral equilibrium positions would be important to understand the particulate flow behavior of non-neutrally buoyant, flat-plate microchips. The characterization of the flow of a single non-neutrally buoyant, flat-plate particle provides guidance on operating flow conditions and channel configurations of microchip-integrated microfluidics. This study will further our understanding of particulate flow dynamics and help realize the potential applications of microchip particle-laden microfluidic systems.


Acknowledgements (Optional): : We thank Richard Morris, Wlodek Mandecki, and Wesley Kopacka (p-Chip Corporation, formerly PharmaSeq) for their insightful discussions. This work was financially supported by the National Science Foundation under the Cyber-Physical Systems Award CPS-1740052 and the Jacques S. Yeager, Sr. Professorship.


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