Liquid mediated surface-particle interactions in near-surface motion of superparamagnetic particles and magnetic Janus microparticles in quiescent liquids

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Precisely controlled actuation of small magnetic particles (MP) opens new avenues for the development of new strategies for analyte detection and diagnostics, e.g., for disease marker or virus detection. MP are promising as there is as yet no indication that the magnetic fields used for their actuation would modify biological matter. Since the early 2000s external time-dependent magnetic fields have been superposed for this purpose with static magnetic field landscapes (MFLs) emerging from arrays of magnetic charges (in domains, micromagnets, or from currents in micro wires). With this approach MP-transport, fluid mixing, MP-separation, MP-sorting, and even sensing of biologically functionalized MPs, e.g., via giant magnetoresistance and Hall-effect sensors has been achieved. A central characteristic of the MFLs are strong field gradients over small distances close to the surface from which the MFL emerges. In most of the known work in this context the interactions between MP and surface via the liquid, except for the magnetic interactions, have been neglected and in most of the studies superparamagnetic particles have been used. With this proposal we want to extend knowledge on these important micro systems by two aspects: We want to (1) systematically investigate the influence of the liquid mediated interactions between the MP and a close surface on the MP motion and explore (2) the different motion types (rotation and translation) of spherical non-magnetic micro particles with a magnetic cap (magnetic Janus particles, MJPs) in MFLs. These particular particles possess a much larger magnetic moment as compared to their superparamagnetic counterparts and may not only be translated but also oriented and rotated. In particular, we will establish a technique to automatically track not only the 2D-MP positions and MJP-orientations as functions of time (of all particles in a recorded video) but also their height over the substrate surface (3D tracking). This experimentally determined quantity will be used to quantify the influence of surface – MP interactions on the MP motion. The attained understanding will then be used for a proof-of-concept application, where a virus replacement (a surface-functionalized nano particle) shall be detected by a change in MP height over the surface.