## 3 Results  
### Validation of the Rotational Magnetomotive Concept  
To validate the hypothesis mentioned above, we first analyzed the transverse and longitudinal magnetic field components of our rMM-OCT setup during the off- and on-states of the solenoid. For this experiment, we operated the solenoid in DC (direct current) mode and successively measured the field components (Figure 3a,b) using a conventional magnetometer (KOSHAVA 5, Wuntronic GmbH, Munchen, Germany). In order to prevent the permanent magnet from being pushed off the solenoid during the on-state, we limited the supply voltage to 30 V. As it can be seen in Figure 3c, switching off the solenoid results in a varying magnetic field vector in the sample region, i.e., the focus region of the OCT beam. When the solenoid is switched on, the tilt angle of the field vector changes from 32\({}^{\circ}\) to 83\({}^{\circ}\), thus, there is a dominant vertical field during the on-state compared to a rather horizontal field during the off-state. The magnitude of the field vector amounts to 17 mT in the off-state and 101 mT in the on-state. This significant difference between the two states is most likely due to the empirical and non-optimized character of our dual-magnet configuration. Ideally, one would assume that the best results will be obtained with equal field strengths in both states. However, the setup presented in this work is considered more a proof-of-principle rather than an optimized system. Related, the corresponding field gradient (in the direction of the respective tilt angle) is approximately 4 T/m and 9 T/m in the off- and on-states, respectively. However, we believe that the field gradient plays at least a minor role in this context, as the particles are supposed to rotate rather than move via a magnetic gradient force-induced displacement. Figure 2: Principle of rotational magnetomotive OCT (rMM-OCT) using an electromagnet (solenoid) and a laterally displaced permanent magnet. (**a**) Top view and (**b**,**c**) side view of the dual-magnet configuration. When the solenoid is turned off (**b**), anisotropic magnetic particles in a soft sample are aligned horizontally (\(y\)-direction) towards the field of the permanent magnet; when the solenoid is turned on (**c**), particles are aligned vertically (\(z\)-direction) towards the field of the solenoid. Drawn magnetic field lines are a guide to the eye only. The indicated coordinate system (\(x,y,z\)) is identical to Figure 1b. The coordinate origin is at the optical axis of the OCT probe (\(x=y=0\)) and the surface of the solenoid (\(z=0\)) (color online). In a second step, we proved magnetic particle alignment and rotation in the switchable field. We dispersed magnetic particles in glycerin (0.5 mg/mL) and investigated them using the bright-field microscopy channel of our multimodal imaging setup in combination with a high-magnification objective (Leica C Plan L 40\(\times\)/0.50). The particles were identical to the ones used for cell experiments in our previous study [6]; they are sub-micron particles with a nominal size of 200 nm (nano-screen MAG/R-DXS, Chemicell GmbH, Berlin, Germany) consisting of a magnetite core, a fluorescent shell emitting in the red visible spectral range, and a coating made of dextran-sulfate for biomolecule coupling. As shown in Figure 4a, the magnetic particles are rather homogeneously distributed in the freshly prepared sample. However, after 30 min exposure to the permanent magnetic field, the particles have formed long chains that are transversally aligned towards the permanent magnet (\(y\)-direction, Figure 4b). Finally, when the electromagnet is switched on, these chains rotate and align longitudinally towards the solenoid (\(z\)-direction, Figure 4c). A corresponding time-lapse video of the particle movement is shown in Video S1. This type of chaining, alignment and rotation behavior is also reported in the literature for magnetic particles inside cells. [PERSON], [PERSON] et al.