Numerical Simulations of the Digital Microfluidic Manipulation of Single Microparticles

Abstract

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.

Droplet Sliding: The Numerical Observation of Multiple Contact Angle Hysteresis

“…As a mesh-free and coarse-graining numerical approach, dissipative particle dynamics , (DPD) is designed to address problems at the mesoscale. More details of the theoretical aspects and applications of DPD can be found in some recent reviews. − Many-body DPD (MDPD) is one of the variants of the original DPD and has been intensively employed to simulate the fluid system with liquid/vapor interfaces; thus, it is especially suitable for the simulation of wetting phenomena. − MDPD inherits the main interaction scheme from original DPD, so it still satisfies the fluctuation–dissipation theorem. However, the conservative force is modified from a pure repulsive interaction to a force-loop interaction, and thus, the free liquid/vapor interface can be mimicked.…”

“…Table lists the main parameters used in the simulation. By adopting these parameters, the number density of the droplet is 9 and is higher than those in some other MDPD simulations. ,,, The reason for choosing such a compact droplet is to avoid the fluid particle evaporation from the droplet, as the evaporation can introduce difficulty to the calculation of the contact angle.…”

“…Thus, although the following discussion is in a nondimensional way, similar phenomena are expected to happen in experiments. Also, for the simplification of the calculation and comparison with other MDPD studies, ,,,, the reduced MDPD units are used in the following simulations. According to the unit mapping, the radius of the following used droplet is R d [ L ] = 9 × 10 –5 m. It is a very small droplet; thus, the gravity effect can be neglected in the simulations.…”

Six Stages of Microdroplet Detachment from Microscale Fibers

“…A dissipative particle dynamics (DPD) method is a mesh-free particle based method that has been successfully applied to simulate complex fluids, especially at mesoscales that are complementary to atomic and continuum scales. − In the mesoscale regime, continuum theories often fail to capture Brownian motions, while atomistic simulation techniques such as molecular dynamics (MD) become computationally prohibitive. To tackle the mesoscale problems, DPD method tracks coarse-grained particles that are composed of a cluster of atoms or molecules.…”

Low voltage driven surface micro-flow by Joule heating

“…Studies on wetting dynamics show a complicated process where the capillary and precursor diffusion play crucial roles. 27,28 Typically the wetting distance L increases with time as a power law L ¼ Dt c , where D is the prefactor and t is the time. According to different material systems used, c ranging from 1/10 to 1/2 is theoretically predicted and experimentally observed.…”