Dynamics of particle lane formation in confined viscoelastic fluids under shear

Simple shear flow can induce flow-aligned chain formation of particles suspended in viscoelastic fluids. Although this phenomenon has been reported for decades, direct in situ measurements of the alignment dynamics and particle trajectories during chain formation remain limited. Here, we develop an in situ observation platform based on a parallel-plate geometry with a fixed top plate and a rotating bottom plate, separated by a gap comparable to the particle diameter, enabling simultaneous observation of particle alignment under radially varying shear rates. The narrow gap strongly confines particle motion, thereby enhancing hydrodynamic interactions and collision events between particles. Using a viscoelastic fluid embedding zircon particles as the sample, we find that alignment occurs once the local particle Weissenberg number exceeds unity (Wi_{p}≥1), defined using an effective shear rate based on the wall velocity and the available gap width. Particle tracking further reveals an intermittent back-and-forth motion along the flow direction, observed in a reference frame corotating at half the angular velocity of the bottom plate, where the fluid velocity vanishes at the channel midplane. This motion arises from switching between two stable particle heights and is hereafter referred to as "shuttling," which we define explicitly to avoid ambiguity. Using the image brightness in a dyed fluid as a proxy for out-of-plane position, we demonstrate that this shuttling motion originates from vertical displacement of the particles. We further construct a minimal agent-based model in which the particle height follows a Ginzburg-Landau-type double-well potential, and show that collision-driven accumulation emerges in simulations. In the strongly confined geometry, alignment occurs via an effective attraction induced by collisions, which is qualitatively reminiscent of clustering phenomena in active matter.