  A system of ferromagnetic particles trapped at a liquid-liquid interface and
subjected to a set of magnetic fields (magnetocapillary swimmers) is studied
numerically using a hybrid method combining the pseudopotential lattice
Boltzmann method and the discrete element method. After investigating the
equilibrium properties of a single, two and three particles at the interface,
we demonstrate a controlled motion of the swimmer formed by three particles. It
shows a sharp dependence of the average center-of-mass speed on the frequency
of the time-dependent external magnetic field. Inspired by experiments on
magnetocapillary microswimmers, we interpret the obtained maxima of the swimmer
speed by the optimal frequency centered around the characteristic relaxation
time of a spherical particle. It is also shown that the frequency corresponding
to the maximum speed grows and the maximum average speed decreases with
increasing inter-particle distances at moderate swimmer sizes. The findings of
our lattice Boltzmann simulations are supported by bead-spring model
calculations.
