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1. 3D printed imaging platform for portable cell counting

   https://doi.org/10.1039/d1an00778e  

   Awate, Diwakar M. ; Pola, Cicero C. ; Shumaker, Erica ; Gomes, Carmen L. ; Juárez, Jaime J. ( June 2021 , The Analyst)

   null (Ed.)

   Despite having widespread application in the biomedical sciences, flow cytometers have several limitations that prevent their application to point-of-care (POC) diagnostics in resource-limited environments. 3D printing provides a cost-effective approach to improve the accessibility of POC devices in resource-limited environments. Towards this goal, we introduce a 3D-printed imaging platform (3DPIP) capable of accurately counting particles and perform fluorescence microscopy. In our 3DPIP, captured microscopic images of particle flow are processed on a custom developed particle counter code to provide a particle count. This prototype uses a machine vision-based algorithm to identify particles from captured flow images and is flexible enough to allow for labeled and label-free particle counting. Additionally, the particle counter code returns particle coordinates with respect to time which can further be used to perform particle image velocimetry. These results can help estimate forces acting on particles, and identify and sort different types of cells/particles. We evaluated the performance of this prototype by counting 10 μm polystyrene particles diluted in deionized water at different concentrations and comparing the results with a commercial Beckman-Coulter Z2 particle counter. The 3DPIP can count particle concentrations down to ∼100 particles per mL with a standard deviation of ±20 particles, which is comparable to the results obtained on a commercial particle counter. Our platform produces accurate results at flow rates up to 9 mL h −1 for concentrations below 1000 particle per mL, while 5 mL h −1 produces accurate results above this concentration limit. Aside from performing flow-through experiments, our instrument is capable of performing static experiments that are comparable to a plate reader. In this configuration, our instrument is able to count between 10 and 250 cells per image, depending on the prepared concentration of bacteria samples ( Citrobacter freundii ; ATCC 8090). Overall, this platform represents a first step towards the development of an affordable fully 3D printable imaging flow cytometry instrument for use in resource-limited clinical environments. 

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2. 3D manipulation and dynamics of soft materials in 3D flows

   https://doi.org/10.1122/8.0000600  

   Tu, Michael Q. ; Nguyen, Hung V. ; Foley, Elliel ; Jacobs, Michael I. ; Schroeder, Charles M. ( July 2023 , Journal of Rheology)

   Flow-based manipulation of particles is an essential tool for studying soft materials, but prior work has nearly exclusively relied on using two-dimensional (2D) flows generated in planar microfluidic geometries. In this work, we demonstrate 3D trapping and manipulation of freely suspended particles, droplets, and giant unilamellar vesicles in 3D flow fields using automated flow control. Three-dimensional flow fields including uniaxial extension and biaxial extension are generated in 3D-printed fluidic devices combined with active feedback control for particle manipulation in 3D. Flow fields are characterized using particle tracking velocimetry complemented by finite-element simulations for all flow geometries. Single colloidal particles (3.4 μm diameter) are confined in low viscosity solvent (1.0 mPa s) near the stagnation points of uniaxial and biaxial extensional flow for long times (≥10 min) using active feedback control. Trap stiffness is experimentally determined by analyzing the power spectral density of particle position fluctuations. We further demonstrate precise manipulation of colloidal particles along user-defined trajectories in three dimensions using automated flow control. Newtonian liquid droplets and GUVs are trapped and deformed in precisely controlled uniaxial and biaxial extensional flows, which is a new demonstration for 3D flow fields. Overall, this work extends flow-based manipulation of particles and droplets to three dimensions, thereby enabling quantitative analysis of colloids and soft materials in complex nonequilibrium flows. 

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3. The Impact of Stern‐Layer Conductivity on the Electrohydrodynamic Flow Around Colloidal Motors under an Alternating Current Electric Field

   https://doi.org/10.1002/aisy.201900096  

   Yang, Xingfu ; Johnson, Sophia ; Wu, Ning ( October 2019 , Advanced Intelligent Systems)

   Particles under a perpendicularly applied alternating current (AC) electric field assemble into complex structures and exhibit tunable locomotion. Although they possess very similar physical properties, silica particles form two‐dimensional (2D) close‐packed crystals in deionized water, whereas polystyrene (PS) spheres repel each other. Using nanoparticle tracers, it is shown that the electrohydrodynamic (EHD) flow around silica particles is contractile, whereas it is extensile around PS particles. The Stern‐layer conductivities of PS and silica spheres are further measured experimentally and used in theoretical models to calculate the EHD flow surrounding them, which matches well with experiments. Therefore, the incorporation of Stern‐layer conductivity resolves the puzzle that EHD flow surrounding a particle with moderate zeta potentials is extensile. The impacts of zeta potential, Stern‐layer conductivity, salt concentration, and particle size on the EHD flow are examined herein. It is found that particles with high zeta potential, small diameter, or immersed in low salt concentration solutions tend to have extensile EHD flow surrounding them because the enhanced surface conductivity in the double layer makes the particles effectively more polarizable than the solvent. Herein, it is further shown that asymmetric EHD motors made from PS and silica particles exhibit behaviors that are consistent with the model predictions.

    

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4. Modelling ultrafine particle growth in a flow tube reactor

   https://doi.org/10.5194/amt-15-4663-2022  

   Taylor Jr., Michael S. ; Higgins, Devon N. ; Johnston, Murray V. ( January 2022 , Atmospheric Measurement Techniques)

   Abstract. Flow tube reactors are often used to study aerosolkinetics. The goal of this study is to investigate how to best representcomplex growth kinetics of ultrafine particles within a flow tube reactorwhen the chemical processes causing particle growth are unknown. In atypical flow tube experiment, one measures the inlet and outlet particlesize distributions to give a time-averaged measure of growth, which maybe difficult to interpret if the growth kinetics change as particles transitthrough the flow tube. In this work, we simulate particle growth forsecondary organic aerosol (SOA) formation that incorporates both surface-and volume-limited chemical processes to illustrate how complex growthkinetics inside a flow tube can arise. We then develop and assess a methodto account for complex growth kinetics when the chemical processes drivingthe kinetics are not known. Diameter growth of particles is represented by agrowth factor (GF), defined as the fraction of products from oxidation ofthe volatile organic compound (VOC) precursors that grow particles during aspecific time period. Defined in this way, GF is the sum of all non-volatileproducts that condensationally grow particles plus a portion of semi-volatilemolecules that react on or in the particle to give non-volatile products thatremain in the particle over the investigated time frame. With respect toflow tube measurements, GF is independent of wall loss and condensationsink, which influence particle growth kinetics and can vary from experimentto experiment. GF is shown to change as a function of time within the flowtube and is sensitive to factors that affect growth such as gas-phase mixingratios of the precursors and the presence of aerosol liquid water (ALW) onthe surface or in the volume of the particle. A method to calculate GF from theoutlet-minus-inlet particle diameter change in a flow tube experiment ispresented and shown to accurately match GFs from simulations of SOAformation. 

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5. Collision and breakup of fractal particle agglomerates in a shear flow

   https://doi.org/10.1017/jfm.2018.959  

   Dizaji, Farzad F. ; Marshall, Jeffrey S. ; Grant, John R. ( March 2019 , Journal of Fluid Mechanics)

   A computational study was performed both of a single agglomerate and of the collision of two agglomerates in a shear flow. The agglomerates were extracted from a direct numerical simulation of a turbulent agglomeration process, and had the loosely packed fractal structure typical of agglomerate structures formed in turbulent agglomeration processes. The computation was performed using a discrete-element method for adhesive particles with four-way coupling, accounting both for forces between the fluid and the particles (and vice versa ) as well as force transmission directly between particles via particle collisions. In addition to understanding and characterizing the particle dynamics, the study focused on illuminating the fluid flow field induced by the agglomerate in the presence of a background shear and the effect of collisions on this particle-induced flow. Perhaps the most interesting result of the current work was the observation that the flow field induced by a particle agglomerate rotating in a shear flow has the form of two tilted vortex rings with opposite-sign circulation. These rings are surrounded by a sea of stretched vorticity from the background shear flow. The agglomerate rotates in the shear flow, but at a slower rate than the ambient fluid elements. In the computations with two colliding agglomerates, we observed cases resulting in agglomerate merger, bouncing and fragmentation. However, the bouncing cases were all observed to also result in an exchange of particles between the two colliding agglomerates, so that they were influenced both by elastic rebound of the agglomerate structures as well as by tearing away of particulate matter between the agglomerates. Overall, the problems of agglomerate–flow interaction and of the collision of two agglomerates in a shear flow are considerably richer in physical phenomena and more complex than can be described by the common approximation that represents each agglomerate by an ‘equivalent sphere’. 

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