Effects of the Wettability Gradient of the Flow Structure Inside a Sessile Droplet Carrying a Hydrophobic Microparticle on Solid Substrate

Manipulation of microparticles by sessile droplets is important for the control of multi-step biological and industrial processes in wide range of applications e.g., in disease diagnosis, cell separation, and so on. Here, we report on the flow structure inside a sessile droplet containing a hydrophobic microparticle using many-body dissipative particle dynamics (MDPD) simulations. The droplet is actuated by a linear wettability gradient on the solid substrate to pick up, transport, and deliver a hydrophobic microparticle. For quantitative analysis, the droplet velocity at different locations was presented using a modified “quasi-stationary” post-processing method. Detailed flow structures are presented at different time sequences of high interest, such as when the droplet touches the microparticle, when the droplet starts to pick up the microparticle, when droplet transports the microparticle, and finally when droplet delivers the microparticle. Due to the existence of the microparticle, the flow structure inside the sessile droplet is significantly altered compared to the case without a presence of the microparticle inside it. More importantly, in presence of the microparticle droplet velocity follows a nonmonotonic trend. The friction between the microparticle and the substrate not only can retard the motion of the droplet, but also may cause the microparticle to be delivered. This is due to the fact that when the friction force increases, although the driving force exposed on the microparticle by the droplet is increased however its amount is limited by the cohesive attraction forces of the droplet. Hence, when the required driving force to transport the microparticle gets larger than the maximum of it provided by the droplet, the microparticle will be dropped off. More importantly, it is proved that the critical velocity for the delivery of the microparticle is mainly affected by the cohesive forces inside the droplet, not by the droplet morphology.