Particle dynamics in magneto-fluidic microsystems

Abstract

The trend in microfluidics and lab-on-a-chip is to miniaturize and integrate many functions in a single chip, while achieving a high functional performance. To reach fast processing and a high sensitivity at the same time, recent lab-on-a-chip approaches use high-volume preparation steps together with micro-scaled detection techniques. One of the challenges is to solve transport limitations within microfluidic processes. Using superparamagnetic particles as actuation vehicles in lab-on-a-chip systems appears to be a promising approach. However, the full control of particle motion in direction and velocity remains complicated. In this thesis we investigate the interactions between neighboring particles, surrounding fluid and nearby walls. We found that these effects highly influence the dynamics of the particle loaded fluid. First particle dynamics in open fluid volumes was studied using an experimental setup containing a sub-microliter fluid volume surrounded by four miniaturized electromagnets for particle actuation. On the basis of optical velocity measurements, the induced motion of single particles and ordered particle chains was analyzed. Experiments on single particles revealed velocities that highly vary between particles and also the average measured velocity was found to deviate from theoretical predictions, which we attributed to non-uniform magnetic particle properties. Equations for the influence of particle chain formation on magnetization and hydrodynamics have been established, and show an increasing logarithmic dependence of the velocity as function of the chain length. Experimental studies on rotating particle chains showed transient regimes for the chain shape including chain rupture events, which could be reconstructed with a mechanistic pin-joint model based on magnetic and hydrodynamic inter-particle forces. Furthermore, within spatial confinements of a microsystem, we studied the interactions between particles, fluid, and nearby walls. An experimental setup was built providing a constant magnetic force on individual particles dispersed in a microchannel. The hydrodynamic interactions appeared to generate unforeseen self-organization phenomena. Superparamagnetic particles aligned on the channel axis successively organize towards a stable poly-twin system, which could be explained by a 1-dimensional model based on the flow profile along the axis. In addition, particles traveling close to a channel wall show complex rotation transitions that result in s-shaped trajectories while focusing towards the channel center, which could be explained by self-induced fluid velocity gradients within the channel. Using micro-scaled flux guides to generate high magnetic field gradients, the particles reached amplified velocities and could be controlled in their circular pathways within the channels. On system level, the fluid driving efficiency of the observed particle configurations were evaluated with numerical simulation models. Axially aligned particles appear to be very efficient for fluid pumping through channels. The efficiency can be tuned by the particle to channel radius and the particle spacing. The off-axis counter-rotating particles appear to enhance near-surface mixing. Integrated fluid actuation by magnetic particles is demonstrated in micro pore systems, where pressure-driven techniques are ineffective for the exchange of fluids. Our experimental investigations and theoretical analyses lead to a better understanding of particle dynamics, in order to improve the functional performance of magneto-fluidic microsystems