Dynamic wetting of single particles at fluid-fluid interfaces quantified using magnetic tweezers

The miniaturization and integration of assay processes in lab-on-chip devices is a key challenge for point-of-care diagnostics. Assay processes based on magnetic particles are particularly suited for miniaturization and integration, because the particles can be actively controlled using external magnetic fields. Here we study the interaction of magnetic particles with fluid-fluid interfaces. We use a magnetic tweezers setup with five electromagnets to apply a well-controlled mechanical torque using out-of-plane rotating magnetic fields. The particle position at the interface is controlled by the particle-surface functionalization and it is measured using z-stack particle imaging. Out-of-plane rotation induces an asymmetric deformation of the interface due to contact line pinning and eventually a contact line displacement. By reconstructing the angular trajectory of the particle and by comparison with numerical simulations, we quantify the interface deformation and contact line friction forces on the microparticle surfaces. The results show different particle dynamics for the different surface functionalizations. This technique represents a novel tool to quantify the interaction of particles with fluid-fluid interfaces, with relevance for particle-based biosensing, colloid science, active rheology, and mesoscopic flow phenomena.