Poster Z4 - Lift-off of a single flat-plate microchip particle in straight rectangular microchannel flows

Introduction: The continued advancement in semiconductor manufacturing and miniaturization of integrated circuits have created opportunities to integrate microchip particles into microfluidics for many applications including particle-based multiplexed bioassays and combinatorial synthesis. Although there have been studies on the flow of spherical particles, cells, and beads [1] and non-spherical, neutrally-buoyant particles [2] in microfluidic channels, further understanding of the particulate flow dynamics of non-neutrally buoyant, flat-plate particles in microchannels is necessary for the design of microfluidic devices for manipulating and sorting microchips. An essential area for understanding the particulate flow of non-neutrally buoyant particles through microchannels is the lift-off and suspension of the particles through hydrodynamic forces. In this study, we investigated the lift-off of a single sedimented flat-plate, non-neutrally buoyant microchip in rectangular cross-section microchannels with Poiseuille flow in solvents with different kinematic viscosities. We examine the effects of the channel dimensions and the Archimedes number on the critical shear Reynolds number for particle lift-off. Towards understanding the hydrodynamic contributions towards the incipient particle motion, we performed 3-D computational fluid dynamics simulations to evaluate the drag and lift forces acting on the particle under the different flow and channel configurations studied. 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.

Materials and

Methods: We developed a microfluidic particulate flow monitoring system (Figure 1) consisting of a LabVIEW-controlled (NI, Austin, TX) syringe pump (Model 44, Harvard Apparatus, Holliston, MA) for fluid delivery and a high-speed (1500 FPS) camera (MQ003MG-CM, Ximea, Münster, Germany) for monitoring particle movement. Poly(methyl methacrylate) sheets were milled using a desktop computer numerical control mill (Desktop PCB Milling Machine, Bantam Tools, Peekskill, NY) and sealed to create microchannels using an ethanol-assisted thermal bonding procedure [3]. Flow adapters with an inlet particle trap 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 the need for controlled particle orientation for light-activated encoding/identification [4]. Through experimental particulate flow studies of a single p-Chip beginning at rest near the inlet of microchannels of different dimensions and aspect ratios, and in various solvents (water, isopropanol, and methanol), we identified the minimum flow rates (recorded at 1 mL/min resolution) for particle lift-off. To investigate the hydrodynamic forces acting on the particle at the observed lift-off flow rates, we performed 3-D steady-state computational fluid dynamics simulations (COMSOL Multiphysics® and LiveLink™ for SOLIDWORKS®, COMSOL, Inc., Burlington, MA) with a stationary particle positioned near the bottom channel wall at different axial distances along the microchannels. Particle positioning along all channel configurations were handled using SOLIDWORKS® (Dassault Systèmes, Waltham, MA).

Results, Conclusions, and Discussions: From our experimental particulate flow characterization studies of a p-Chip, we observed, in agreement with previous literature on lift-off of bulk particles [5], that the Archimedes number (Ar), the ratio of the gravitational forces and the viscous forces, has a significant contribution on the observed critical shear Reynolds number (R_C) needed for incipient particle movement. We found that R_C for single particle lift-off increases as Ar increases (Figure 2). While previous 2-D numerical studies on lift-off of a single spherical particle suggested that R_C increases as the ratio of the channel height and the rotational diameter (H/D_max) increases [6], we determined through dimensional analysis using Buckingham π theorem that the ratio of the channel area and the rotational diameter squared (HW/D_max^2) is the significant factor affecting R_C of a flat-plate particle. While previous studies of spherical particles observed that the relationship between R_C and Ar at various H/D_max may be conveniently expressed in terms of power-law models, we observed that these expressions are not applicable for flat-plate particles at a higher range of Ar studied in our experiments. Furthermore, we found that the channel aspect ratio (H/W) must also be considered alongside HW/D_max^2 in predicting R_C. In Figure 3, we categorize our experimental data for R_C vs. Ar in terms of H/W and show that at each investigated Ar, an increase in HW/D_max^2 resulted in a decrease in R_C for channels with aspect ratios ≠ 1. 3-D CFD models of the fluid flow around a stationary particle positioned along the axial length of the microchannel revealed that the particle trap introduces entrance effects (Figure 4). However, from the evaluated lift forces, which was found to be dominated by pressure, we observed that the evaluated lift forces converge to a singular value of (6.7 ± 0.4) × 10-7 N downstream of the microchannel, therefore suggesting that the inclusion of the particle trap does not influence R_C and that lift, rather than drag, contributes to the incipient particle motion (Figure 5). 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.