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1. Particle trapping in merging flow junctions by fluid-solute-colloid-boundary interactions

   https://doi.org/10.1103/PhysRevFluids.5.024304  

   Shin, Sangwoo ; Ault, Jesse T. ; Toda-Peters, Kazumi ; Shen, Amy Q. ( February 2020 , Physical Review Fluids)
2. Ion Trapping and Acceleration at Dipolarization Fronts: High-Resolution MHD and Test-Particle Simulations

   https://doi.org/10.1029/2018JA025370  

   Ukhorskiy, A. Y. ; Sorathia, K. A. ; Merkin, V. G. ; Sitnov, M. I. ; Mitchell, D. G. ; Gkioulidou, M. ( July 2018 , Journal of Geophysical Research: Space Physics)
3. Particle trapping, size-filtering, and focusing in the nonthermal plasma synthesis of sub-10 nanometer particles

   https://doi.org/10.1088/1361-6463/ac57de  

   Xiong, Zichang ; Lanham, Steven ; Husmann, Eric ; Nelson, Gunnar ; Eslamisaray, Mohammad Ali ; Polito, Jordyn ; Liu, Yaling ; Goree, John ; Thimsen, Elijah ; Kushner, Mark J ; et al ( March 2022 , Journal of Physics D: Applied Physics)

   Abstract Low-pressure nonthermal flowing plasmas are widely used for the gas-phase synthesis of nanoparticles and quantum dots of materials that are difficult or impractical to synthesize using other techniques. To date, the impact of temporary electrostatic particle trapping in these plasmas has not been recognized, a process that may be leveraged to control particle properties. Here, we present experimental and computational evidence that, during their growth in the plasma, sub-10 nm silicon particles become temporarily confined in an electrostatic trap in radio-frequency excited plasmas until they grow to a size at which the increasing drag force imparted by the flowing gas entrains the particles, carrying them out of the trap. We demonstrate that this trapping enables the size filtering of the synthesized particles, leading to highly monodisperse particle sizes, as well as the electrostatic focusing of the particles onto the reactor centerline. Understanding of the mechanisms and utilization of such particle trapping will enable the design of plasma processes with improved size control and the ability to grow heterostructured nanoparticles. 

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4. Particle-in-Cell Simulation of Quasi-Neutral Plasma Trapping by RF Multipole Electric Fields

   https://doi.org/10.3390/physics1030028  

   Hicks, Nathaniel K. ; Bowman, Amanda ; Godden, Katarina ( December 2019 , Physics)

   Radio-frequency (RF) charged particle traps, such as the Paul trap or higher order RF multipole traps, may be used to trap quasi-neutral plasma. The presence of positive and negative plasma species mitigates the ejection of particles that occurs due to space charge repulsion. For symmetric species, such as a pair plasma, the trapped particle distribution is essentially equal for both species. For plasma with species of disparate charge-to-mass ratio, the RF parameters are chosen to directly trap the lighter species, leading to loss of the heavier species until sufficient net space charge develops to prevent further loss. Two-dimensional (2D) electrostatic particle-in-cell simulations are performed of cases with mass ratio m+/m− = 10, and also with ion–electron plasma. Multipole cases including order N = 2 (quadrupole) and higher order N = 8 (hexadecapole) are considered. The light ion-heavy ion N = 8 case exhibits particles losses less than 5% over 2500 RF periods, but the N = 8 ion–electron case exhibits a higher loss rate, likely due to non-adiabaticity of electron trajectories at the boundary, but still with low total electron loss current on the order of 10 μA. The N = 2 ion-electron case is adiabatic and stable, but is subject to a smaller trapping volume and greater initial perturbation of the bulk plasma by the trapping field. 

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5. Amplification factor in DC insulator-based electrokinetic devices: a theoretical, numerical, and experimental approach to operation voltage reduction for particle trapping

   https://doi.org/10.1039/d1lc00614b  

   Ruz-Cuen, Rodrigo ; de los Santos-Ramírez, J. Martin ; Cardenas-Benitez, Braulio ; Ramírez-Murillo, Cinthia J. ; Miller, Abbi ; Hakim, Kel ; Lapizco-Encinas, Blanca H. ; Perez-Gonzalez, Victor H. ( November 2021 , Lab on a Chip)

   Insulator-based microfluidic devices are attractive for handling biological samples due to their simple fabrication, low-cost, and efficiency in particle manipulation. However, their widespread application is limited by the high operation voltages required to achieve particle trapping. We present a theoretical, numerical, and experimental study that demonstrates these voltages can be significantly reduced (to sub-100 V) in direct-current insulator-based electrokinetic (DC-iEK) devices for micron-sized particles. To achieve this, we introduce the concept of the amplification factor—the fold-increase in electric field magnitude due to the presence of an insulator constriction—and use it to compare the performance of different microchannel designs and to direct our design optimization process. To illustrate the effect of using constrictions with smooth and sharp features on the amplification factor, geometries with circular posts and semi-triangular posts were used. These were theoretically approximated in two different systems of coordinates (bipolar and elliptic), allowing us to provide, for the first time, explicit electric field amplification scaling laws. Finite element simulations were performed to approximate the 3D insulator geometries and provide a parametric study of the effect of changing different geometrical features. These simulations were used to predict particle trapping voltages for four different single-layer microfluidic devices using two particle suspensions (2 and 6.8 μm in size). The general agreement between our models demonstrates the feasibility of using the amplification factor, in combination with nonlinear electrokinetic theory, to meet the prerequisites for the development of portable DC-iEK microfluidic systems. 

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