Here we disclose a particle separator or fractionator. This separator/fractionator works by driving particles into the bottom of a conduit or pipe with flow obstacles in the upper portion of the pipe. After the obstacles (which may block particles directly or divert particles due to flow patters), the flow can be divided with the larger particles in the lower stream and the particle depleted flow exiting the upper stream. The attached manuscript in the publications section presents images of the proof of principle flow obstacles tested under high throughput conditions. Although the precise details of the separation mechanism require further effort to confirm, two mechanisms may be operative. First, the modular inline particle fractionator may work by driving all large particles into the express lane with no recovery of the larger particles back into the rest of the flow stream. Second, the modular inline particle fractionator may work by laminarizing the flow into the express. When the flow re-expands there is a vertical upward flow that may compete with the gravitational force on the particles to set their position or distribution of positions within the flow streams. This later mechanism opens up the potential to determine the cutoff location. Likely both mechanisms have some influence that may vary with flow rate. (Please note that these flow obstacles are distinctive from those of the mesofluidic separators that were disclosed and on which patent applications have been filed.) In essence, these separators are analogous to asymmetric flow field flow fractionation (AFFFF) in which particles in the flow are driven to one side of a flow channel and then the flow field is changed allowing the smaller particles to diffuse away from the side of the channel first and then the larger particle so that progressively larger particles exit the separator. Here in this flow through design, the temporal dimension is exchanged for a spatial dimension so that the degree of separation varies as a function of distance from the end of the flow obstacles with smaller particle exiting on top and larger particles preferentially exiting closer to the express lane.