Trapping of Superparamagnetic Particles With a Single Current-Conducting Micro-Ring

In this paper, we introduce a new device based on a single current-carrying micro-ring that allows to transport and trap superparamagnetic particles at a pre-defined position. The technique is based on the superposition of the magnetic field gradient from the conductor ring and an additional homogeneous magnetic field. The functionality of the trap is discussed based on the magneto-static energy of the particle inside the area of the micro-ring and experimentally confirmed by trapping single superparamagnetic beads inside the ring. By observing the fluctuations of individual trapped Dynabeads particles, the viscosity of the surrounding medium can be measured on a microscopic scale from an evaluation of the mean square displacement (MSD) as a function of time. The viscosity obtained in this way differs for particles with different diameters, which is a consequence of the close proximity of the particles to the surface of the SiO2-passivated Si substrate. The long-time behavior of the MSD clearly reveals the confined Brownian motion as expected for trapped particles and allows to extract the trap stiffness as a function of current I and external magnetic field Bext. Our results show that the trap stiffness can be systematically tuned by varying I and Bext. The experimentally observed trap stiffness values are in good agreement to the predictions from numerically modeling the energy landscape in the vicinity of the energy minimum. For the trap used in this paper, a single superparamagnetic particle can be reliably positioned with a lateral accuracy of $<;1~\mu \text{m}$ over length scales $>100~\mu \text{m}$ . This technique can be beneficially used for lab-on-chip systems in combination with magneto-resistive field sensors where particles have to be transported to the active sensor area, which is typically on the range of approximately 1 $\mu \text{m} \times 1~\mu\text{m}$ .