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(a) Sketch of the experimental platform with pole tips that create an isodynamic magnetic field over the microchannel. (b) Traveling superparamagnetic particles , focused on the axis of a microchannel by hydrodynamic wall interactions. A twin is formed at the front of the row and travels away, but keeps a small intraspacing caused by repelling magnetic forces. (c) Steel particles experiment where gravity provides a constant particle force. Only hydrodynamic particle interactions occur that allow twins to fully come in contact, as particles 4 and 5 show. Particles 3 and 4 are still at the onset of twinning.
(a) Calculated velocity profiles of two particles (thin lines), showing that the fluid velocity decrease is inversely proportional to the distance from the particle. The resulting axis velocity profile (bold line) is obtained by superposition of the two velocity curves, which demonstrates an even increase of both particle velocities: a constant spacing over time. (b) Hydrodynamic interactions of three particles give a higher velocity enhancement for the center particle by contributions of both neighbors. (c) The difference in velocities leads to formation of a twin that increases velocity with respect to the left particle and travels away.
Simulation of 15 particles in a microchannel ( , ), starting as a full contact chain (top) and develops into a polytwin system (bottom). The pump efficiency per particle (ratio between fluid outflow velocity and average particle velocity) is given as a function of average particle spacing, and increases as twins are formed. Moreover, the induced pressure drop over the channel reduces by the increasing particle spacing.
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