Simulation of thin film formation using magnetic properties of nanoparticles

Abstract

We have investigated the sedimentation phenomenon in a colloidal dispersion composed of spherical ferromagnetic particles using Brownian dynamics simulations. We have attempted to clarify the conditions under which all particles settle down and a thin film is formed, by varying the applied magnetic field, magnetic force between particles, mass density, and temperature of the solution. The particle concentration was set so that the average inter-particle distance was 2.5 times the particle diameter. The main results obtained are summarized as follows. When the mass density is increased, the sedimentation rate increases due to the influence of gravity. The sediment is deposited at the bottom layer immediately after being dropped, and the particles display a stronger tendency to settle down at the bottom layer. However, as the magnetic force between the particles increases, the bonding between the particles becomes strong and clusters are formed, which makes it difficult to obtain the desired thin film in which all particles precipitate at the bottom layer and exist separately. Even in such a situation where the influence of the applied magnetic field is dominant, a thin film can be obtained if the magnetic moment of the particle is strongly restrained in the direction of the magnetic field, thus preventing the formation of clusters. In addition, when the density ratio is as large as 30 as is in this case, a planar cluster may be formed in the lowermost layer even when all the particles have settled down. On the other hand, if the temperature of the liquid is too low, the clusters remain intact without collapsing because of the low random force. As a result, the desired thin film cannot be obtained. However, there is a limitation where in the number of conditions under which the thin film formation occurs does not increase when the random force value exceeds a certain level.