Interaction of particles with fluid-fluid interfaces quantified using magnetic tweezers

A key challenge in point-of-care diagnostics is the miniaturization and integration of assay processes in lab-on-chip devices. Assay processes based on magnetic particles are particularly suited for miniaturization and integration, because the particles can be actively controlled using external magnetic fields [1,2]. The sensitivity and specificity of the assays crucially depend on the interaction of the particles with surfaces, interfaces and biological materials. Therefore we are investigating particle-based methodologies to characterize interactions of protein-coated particles with surfaces [3], protein-ligand systems [4] and cells [5]. Here, we extend the magnetic particle based toolbox to the characterization of interaction between particles and fluid-fluid interfaces, providing a novel tool for colloidal science studies [6]. In our experiments we use a magnetic tweezers setup with five electromagnets. Magnetic particles are first suspended in an aqueous phase and then trapped at a fluid-fluid interface. In a rotation experiment, a well controlled mechanical torque is applied to the trapped particles, using in-plane and out-of-plane rotating magnetic fields. The resulting particle rotation is tracked using optical imaging with fluorescent tags. In-plane rotation gives information about the in-plane mobility of the particles in the interface, thereby probing the local micro-rheology. Out-of-plane rotation induces an asymmetric deformation of the interface (capillary dipole) due to contact line pinning. By reconstructing the angular displacement and comparison with numerical simulations we are able to quantify the interface deformation. As a next step we will record data of contact line pinning and contact line friction forces on the microparticle surfaces. This novel technique represents a versatile tool to quantify and unravel the interactions of particles with fluid-fluid interfaces, with relevance for colloid science, active rheology, mesoscopic flow phenomena, and particle-based biosensing. References [1] D.M. Bruls, et.al., Lab Chip 9, 3504, (2009). [2] R.C. den Dulk, et.al., Lab Chip 13, 106, (2013). [3] M. Kemper, et.al., Langmuir 28, 8149, (2012). [4] A. Jacob, et.al., Anal Chem 84, 9287, (2012). [5] M. Irmscher, et.al., J. R. Soc. Interface 10, 1048, (2013). [6] V.Garbin, et.al., J. Colloid and Interface Science 387, 1, (2012).