Particle separation is a technological area where microfluidics shows promises towards miniaturization, specificity, and throughput. We study here the mechanisms for particle separation in deterministic lateral displacement (DLD), a size-based microfluidic particle separation method. The experiments are also designed to be a model system for colloidal transport on solid-water (SWI) and air-water (AWI) in subsurface. To mimic particle transport around the obstacles in DLD we developed a simple but versatile microfluidic platform in which the particles’ trajectories are tracked during their motion around an individual solid (PDMS) or fluid (bubble) obstacle.
The trajectories of individual particles passing an obstacle are analyzed using a collision model1. In this model there are two types of particle–obstacle collisions. The hydrodynamic collisions are reversible with symmetric trajectories around the obstacle. The touching collisions are irreversible with asymmetric trajectories. We characterize the type of collision for particles transport via both pressure-driven flow and gravity-driven transport. Only hydrodynamic collisions are observed with pressure-driven flow as the particles follow symmetric trajectories with respect to the obstacle. We also do not observe adsorption of the particles to either the AWI or SWI. In contrast, we observe both symmetric and asymmetric particle trajectories for gravity-driven particle transport. We observe a transition from symmetric to asymmetric trajectories as the impact point between the particle and the obstacle moves from the top to closer to the center of the obstacle. We find that the transition between symmetric and asymmetric trajectories depends on the particle size and show that we can rely on this size dependence for particle separation. In addition, we find that particles around fluid obstacle have smaller transitioning impact point than that of solid obstacle even if the obstacles have nearly same size and shape
