Diffusion-controlled, Size-based Separation of Narrowedly Distributed Suspensions in Pressure-driven Flows Through Microfluidic Bumper Arrays

Microfluidic bumper arrays (also referred to as Deterministic Lateral Displacement devices, DLD) have been proposed in the last fifteen years as a simple and effective mean to implement the label-free, size-based separation of a suspension of micrometric-sized particles. The separation resolution in these devices resolution is significantly higher than that associated with more traditional separation techniques such as, e.g., SEC columns. In DLD devices, the suspended particles are dragged by a pressure-driven flow through a periodic array of obstacles, typically hosted in a channel with rectangular cross-section. Experiments have proven that if a focused current entraining a suspension of particles of different size is continuously introduced upstream the obstacle array, size-sorted populations of particles can be collected at different locations of the device outlet. This is because, as a consequence of the fluid drag and of the hydrodynamics-mediated collisions with the obstacles, particles of different size follow on the average differentmigration pathts, which are at an angle with respect to the average direction of the carrier flow. Based on the DLD separation mechanism, prototypes have been constructed and used for sorting and isolating suspensions of clinical and biological interest, ranging from the size of red blood and circulating tumor cells, down the nanometric scale of exosomes. Recently, a novel chromatographic use of DLD devices has been proposed, where the suspension is separated in time and space coordinates by exploiting the dependence on particle mobility on particle size. By investigating particle transport in a idealized setting (associated with point-sized obstacles), it has been argued that the separation-enhancing effect due to mobility should allow, in principle, to effectively separate particles suspensions characterized by a narrow size distribution to very high degrees of resolution. In this contribution, the possibility of reproducing the same phenomenon in a classical pressure-driven flow in the presence of finite-sized obstacles is explored. Specifically, it is shown that (ideal) conditions similar to the type of particle motion predicted for point-size obstacles can be obtained even in the presence of obstacles of finite dimension, provided that the obstacle shape near the collision point is accurately tuned to obtain a diffusion-controlled behavior of particle dynamics.