Dynamic behavior of droplet through a confining orifice:A lattice Boltzmann study

The motion of gravity-driven deformable droplets passing through a confining orifice in two-dimensional (2D) space is numerically studied by the phase-field-based multiple-relaxation-time (MRT) lattice Boltzmann (LB) model, and the ratio of orifice-to-droplet diameter is less than 1. Droplets are placed just above a sink with an orifice in the middle, accelerate under gravity and encounter the orifice plate. In this work, we mainly consider the effects of the Bond number (Bo), orifice-to-droplet diameter ratio (r=d∕D), plate thickness (Ht), wettability (or contact angle) and the diameter ratio of two droplets (rd=D1∕D2) on the dynamic behavior of droplet through the orifice. The results show that these issues have great influences on the typical flow patterns (i.e., release and capture). With the decrease of contact angle, the droplet is more easily captured, and there exists a critical equilibrium contact angle θeq when the Bond number and the orifice-to-droplet diameter ratio as well as the thickness of the plate are specified. For the case with θ>θeq, the droplet can finally pass through the orifice, otherwise, the droplet cannot pass through the orifice. In addition, the droplet is more likely to pass through the orifice as the thickness of the obstacle increases. Actually, when the obstacle thickness is large enough, droplet breaks into three segments and a liquid slug is formed in a hydrophilic orifice. Finally, for the evolution of two droplets with a larger diameter ratio (rd=1.0), the combined droplet finally passes through the orifice due to greater inertia than the cases with rd=0 and rd=0.43. Besides, we also establish the relation r=0.5723Bo−13 which can be used to separate droplet release from capture at Ht=1.2mm.