Chuanjin Lan

Numerical Simulations of the Digital Microfluidic Manipulation of Single Microparticles.

Single-cell analysis techniques have been developed as a valuable bioanalytical tool for elucidating cellular heterogeneity at genomic, proteomic, and cellular levels. Cell manipulation is an indispensable process for single-cell analysis. Digital microfluidics (DMF) is an important platform for conducting cell manipulation and single-cell analysis in a high-throughput fashion. However, the manipulation of single cells in DMF has not been quantitatively studied so far. In this article, we investigate the interaction of a single microparticle with a liquid droplet on a flat substrate using numerical simulations. The droplet is driven by capillary force generated from the wettability gradient of the substrate. Considering the Brownian motion of microparticles, we utilize many-body dissipative particle dynamics (MDPD), an off-lattice mesoscopic simulation technique, in this numerical study. The manipulation processes (including pickup, transport, and drop-off) of a single microparticle with a liquid droplet are simulated. Parametric studies are conducted to investigate the effects on the manipulation processes from the droplet size, wettability gradient, wetting properties of the microparticle, and particle-substrate friction coefficients. The numerical results show that the pickup, transport, and drop-off processes can be precisely controlled by these parameters. On the basis of the numerical results, a trap-free delivery of a hydrophobic microparticle to a destination on the substrate is demonstrated in the numerical simulations. The numerical results not only provide a fundamental understanding of interactions among the microparticle, the droplet, and the substrate but also demonstrate a new technique for the trap-free immobilization of single hydrophobic microparticles in the DMF design. Finally, our numerical method also provides a powerful design and optimization tool for the manipulation of microparticles in DMF systems.

Symmetry boundary condition in dissipative particle dynamics

Dissipative particle dynamics (DPD) is a coarse-grained particle method for modeling mesoscopic hydrodynamics. Most of the DPD simulations are carried out in 3D requiring remarkable computation time. For symmetric systems, this time can be reduced significantly by simulating only one half or one quarter of the systems. However, such simulations are not yet possible due to a lack of schemes to treat symmetric boundaries in DPD. In this study, we propose a numerical scheme for the implementation of the symmetric boundary condition (SBC) in both dissipative particle dynamics (DPD) and multibody dissipative particle dynamics (MDPD) using a combined ghost particles and specular reflection (CGPSR) method. We validate our scheme in four different configurations. The results demonstrate that our scheme can accurately reproduce the system properties, such as velocity, density and meniscus shapes of a full system with numerical simulations of a subsystem. Using a symmetric boundary condition for one half of the system, we demonstrate about 50% computation time saving in both DPD and MDPD. This approach for symmetric boundary treatment can be also applied to other coarse-grained particle methods such as Brownian and Langevin Dynamics to significantly reduce computation time.

Wall Effect on Separated Flow Around an Inclined Flat Plate at High Incidence

The flow around bluff bodies has received a great deal of attention due to its practical importance in engineering and scientific relevance in fluid mechanics. The largely separated turbulent flow can be triggered by an inclined flat plate with sharp leading and trailing edge and the presence of wall can alternate the flow structures greatly. This paper applied the Unsteady Reynolds-averaged Navier-Stokes (URANS) model to simulate the vortex shedding phenomenon over the inclined flat plate for a Reynolds number Re = 20 000 with different angle of attack α and gap ratio D/L, the ratio of the distance from the plate to the wall (D) to the plate length (L). Vortex generation from the leading and trailing edge was captured clearly and the transportation and development of vortex structures were shown for different parameters. The smaller the gap ratio is, the more the flow characteristics are affected by the wall presence. The integral quantities, such as lift and drag coefficients show different peak values for different angle of attack.Copyright

Numerical Study of Sand Deposition and Control by Flat Solar Panels

To make the best use of solar energy, most solar plants are located in deserts or dry and sandy areas, where most of the sand originate in sandstorms and desertification. For large scale solar plants, the structure of the solar panels can reduce the mean wind speed greatly, thus having a great effect on the deposition and entrainment process of the sand and dust. To study the effect of installment of solar panels on wind flow, numerical simulations are applied to get the turbulent flow field in the lee of the solar panels, with inclination angles ranging from 15° to 30° and at different spacing. The results show that 30° is the optimal choice and the performance with larger spacing at 2.5 times panel length is better than the case at 1.5 times.Copyright

Effects on Dust Emission From an Inclined Flat Solar Panel

Flat solar panels array is one of the typical geometries used in photovoltaic (PV) power plants. The presence of these inclined solar panels can significantly accelerate or decelerate wind speed and distort the wind velocity profiles near the ground when wind flows around, which leads to considerable changes in dust emissions determined by shear stress at ground surface. To understand the fundamental principle of the effects of a flat solar panel on the dust emission rate at ground surface, the incompressible viscous flow past an inclined flat plate with ground effect was numerically investigated based on finite volume method. Results indicate that the presence of an inclined plate has little impact on the ground when the gap between the plate and ground is larger than twice plate length at tilt angle of 25° and Reynolds number of 300. However, as the gap is decreased the vortex clusters behind the plate are flattened and restrained by the ground, which results in the change of the shear stress at the ground surface. Both the magnitude and the distribution of shear stress at the ground surface can be significantly changed by varying the distance between the plate and the ground. The results demonstrate the possibility to adjust the strength and distribution of surface shear stress hence change the dust emission rate on the ground by an inclined flat plate.Copyright