UC San Diego

The growing interest in acoustic manipulation of particles in micro to nanofluidics using surface acoustic waves, together with the many applications of magnetic nanoparticles—whether individual or in arrays—underpins our discovery of how these forces can be used to rapidly, easily, and irreversibly form 1D chains and 2D films. These films and chains are difficult to produce by other methods yet offer many advantages over suspensions of individual nanoparticles by making use of the scale of the structures formed, 10−9 to 10−5 m, and by taking a balance of the relevant external and interparticle forces, the underlying mechanisms responsible for the phenomena become apparent. For loosely connected 1D chains, the magnetic field alone is sufficient, though applying an acoustic field drives a topology change to interconnected loops of ∼10–100 particles. Increasing the acoustic field intensity drives a transition from these looped structures to dense 2D arrays via interparticle Bjerknes forces. Inter-particle drainage of the surrounding fluid leaves these structures intact after removal of the externally applied forces. The self-evident morphology transitions depend solely upon the relative amplitudes of the Brownian, Bjerknes, and magnetic forces.