(741c) Tuning the Isotropic Interaction Potential Between Paramagnetic Particles Using a Rotational Magnetic Field

We present a novel method to control the attractive interaction potential between colloidal particles, from 5kT to 40kT using an AC magnetic field. This system consists of negatively charged superparamagnetic colloidal particle suspension which are confined between two coverslips with certain separation. A highly tunable long-range attractive interaction is induced by a high frequency rotating magnetic field generated by two pairs of coaxial air-core solenoids connecting with multi-frequency AC power supply. The solenoids are calibrated to be orthogonal to each other..A local field theory is put forward to characterize the attractive pair interaction and account for the effect of different parameters. The repulsive component to the interaction potential is the short-range electrostatic interactions, which can be described using DLVO theory.

We first study the frequency effect on the pair interaction by increasing the frequency up to 100Hz. At low frequencies, an attractive potential between particles is generated causing them to form chain-like structures and the magnetic torque causes the chain to rotate with the magnetic field. The torque will vanish at frequency above a critical frequency which is a function of field strength. There is a maximum threshold frequency to also consider which is related to relaxation time of the paramagnetic particles. At frequencies above this threshold frequency, the particles are not able to relax via either Néel relaxation or Brownian relaxation. Both frequencies are determined by simulation or experiment and only the critical frequency is shown to be dependent on field strength. The frequency used in our experiment is fixed on 20Hz due the trade-off between torque elimination and particle relaxation.

We measure the pair potential and three-body potential experimentally. Quantitative aggrement is found between experiments and local field pair potential theory at various field strengths. The effective three-body potential of a dilute trimer system is measured and compared with the superposition of exact pair potential to study the many-body effect on the system. The same deviation is observed for various field strengths. We believe this system provides an ideal platform to study phase behavior and dynamics of atomic system in two dimensions.