SELF-ORGANIZED STRUCTURES IN MAGNETIC LIQUIDS

Amazing patterns occur when a droplet of an oily suspension of finely divided magnetic particles, a magnetic liquid is confined within a Hel e-Shaw cell and is subject to a magnetic held perpendicular to the sur face of the cell. The pattern is macroscopic and the typical linear di mension of the pattern is about six orders of magnitude bigger than th e particles in the magnetic liquid. In a series of experiments we have investigated the effect on the pattern of varying the height of the c ell between 0.5 mm and 2 mm The height of the cell was shown to drasti cally influence both the characteristic sizes and the nature of the pa ttern The pattern changes character from being chaotic to become labyr inthine and finally droplet-like as the height is increased. The effec t of varying the amount of magnetic liquid in the Hele-Shaw cell while maintaining the total volume of the cell was also studied In this cas e, the typical pattern consists mainly of columns; we have investigate d how the size distribution of the columns, viewed from above, change as the fraction of ferrofluid in the cell is increased When increasing the fraction of ferrofluid, the number of columns remains approximate ly the same, but the bigger ones grow on the expense of the smaller. F rom the center of mass of the columns a 2D-Veronoi pattern is construc ted in the plane of the cell, and the edge statistics are found as a f unction of the fraction of magnetic liquid in the Hele-Shaw cell. The average number of edges in the Veronoi cells is always between 5 and 6 , and this number is approaching 6 as the fraction of ferrofluid is in creased. The edge statistics give information about the spatial arrang ement of the droplets. Finally, experiments have been carried out in w hich we focused on how the pattern depends on the applied magnetic fie ld.