Trapping markers in a ring

Figure 3.7 presents an approach to trap several magnetic markers inside a ring shaped structure. The ring has an outer diameter of 37.1$\mu$m and an inner diameter of 14.9$\mu$m, so the conducting line is 11.1$\mu$m wide. A potential is applied to both top rings with a current of 100mA. Two images of the video are presented here, after 23secs (a) and 114secs (b).

Figure 3.7: Trapping magnetic markers inside a ring shaped conducting line. See the CD for the complete video.

Figure 3.8: Trapping many magnetic markers inside a ring shaped conducting line, from [85]

Attracted by the magnetic field of the conducting rings, the magnetic markers follow the gradient to the nearest local maximum. From figure 3.7(a), it is clear that there are three local maxima. Two maxima are at both inner sides of the two supply lines above the ring, and the third is at the bottom of the inner side of the ring. After 114secs, a lot more beads are attracted and populate the conducting rings (b). The beads at the two maxima between the supply lines build up one big crowd, and inside the ring structure, the beads align to the local maximum at the lower edge of the inner ring.

As expected from EARNSHAWS theorem (confer page ) the beads do not cumulate in the center of the ring, but at the inner edge. This is also true for the inner side of the supply lines. The local maxima are directly at the edges, and not in the middle between the supply lines.

LEE et al. made a similar experiment in 2001 [85]. They structured a ring shaped trap for magnetic particles using the electro-plating technique (see figure 3.8). Although they used similar magnetic markers (BANGS LABORATORIES, diameter 1-2$\mu$m), the ring structure is much bigger and, due to the electro-plating technique, the ring is much higher (the height is not given exactly, but probably about 3$\mu$m). Applying a high current of 350mA, they cumulated hundreds or thousands of particles inside the ring structure. Figure 3.8(b) does not show the expected maxima at the edges, but a nearly uniform distribution inside the ring. There are several possible explanations for this differing result. First of all, the ring structure elevates several micro-meter above the surface, and therefore, the ring is a high wall for the particles inside the ring. When the current is turned off, the particles move away from the wall to the middle of the ring. The elevated structure is also the reason why there are no particles between the supply lines. Another possible explanation is that there are so many particles inside the ring that they show clustering effects. The results can not really be explained without seeing the complete process in a video (unfortunately the video is not available). Because we are more interested in manipulating very few or only single particles, this example is not investigated further, and other approaches were tried.