Thermophoresis as a cause of periodic change of direction of magnetic fluid convection in vertical channels

Abstract

The results of experiments with magnetic fluid based on undecane with magnetite particles size of of 6 nm and their volume fraction of 16 % are analyzed. Convection in two vertical connected channels with a square section of 3.2×3.2 mm2 and a height of 50 mm was studied. Two equally probable stationary convection directions are observed in the undecane when heated from below. In the magnetic fluid, instead of a stationary flow, a periodic change of direction was observed: the flow upward in one channel and downward in the other, spontaneously and periodically (~ 1 hour) changed to the flow in the opposite direction. The oscillations are associated with the instability of the convective flow due to the thermodiffusion separation of particles. A formula for the characteristic separation time, which is determined by the thermodiffusion parameters of the medium, was obtained. In the channel, with upward flow, the trajectories of colloidal particles are deviate towards the wall due to thermophoresis, and part of the particles falls into the "trap" – a thin (~0.01 mm) diffusion boundary layer. In another channel, the fluid flows down, and the trajectories of colloidal particles deviate from the wall due to thermophoresis, and the boundary layer loses particles. The concentration difference between the channels increases and ends with a change in the convection direction. The experimental thermograms show a linear change in the buoyancy force in the channels over time. The formula for the rate of accumulation (loss) of concentration in the boundary layer is obtained. The Soret coefficient of the magnetic fluid by processing the experimental thermograms using the found formula was measured 0.25 K–1.