Manipulation of microparticles by droplets is a very useful and important technique for many microfluidics applications. Due to the large specific surface necessary for chemical binding and easy recovery from a dispersion, utilization of nanospheres or microspheres has become more and more popular for different medical, biological, and optical applications. The goal of this research is to understand the mechanism for the manipulation of microparticles by droplets. Dissipative particle dynamics (DPD), which is extensively used to model mesoscale flow phenomena, is applied as the numerical tool for this study. A model for solid microparticles is designed to study the interactions among microparticles, liquid droplets, and solid substrates. A spherical shell is used to represent the microparticle, and the shell surface is packed by dense enough beads to avoid undesired penetration of liquid beads into solid microparticles, conserving the momentum automatically. After that, the interaction between a rigid microparticle and a solid substrate is modeled based on contact mechanics, including adhesion forces, normal forces, and friction forces. After the model for microparticles is built, a baseline case simulating the pickup and transport of a hydrophobic microparticle by a droplet is demonstrated and compared with experimental observations. Then, the flow structures within a droplet containing a hydrophobic microparticle are revealed.

With this developed numerical tool, parametric studies are conducted to investigate the effect on the manipulation processes (including pickup, transport, and drop off) of a microparticle by droplet sizes, wetting properties of microparticles, and particle-substrate friction coefficients. The increase of droplet size can speed up the transport of microparticles. However, the increase of particle-substrate friction coefficients can lead to drop-off of a hydrophobic microparticle. The mechanism for the drop-off, or delivery, is analyzed by checking the development of the friction force and driving force on the microparticle during the transport process. The critical velocity, defined as the instantaneous velocity of the microparticle right before the occurrence of delivery, is measured, and it is found that the critical velocity is about same for different sizes of droplets. Based on the numerical results, two different designs, namely passive delivery and active delivery, have been demonstrated to be capable of controlling the location for the delivery of single hydrophobic microparticle without any trap design or external field forces. These numerical results provide a fundamental understanding of interactions among the microparticle, the droplet and the substrate to facilitate the optimal experimental design of digital microfluidic system utilizing microparticles.