In order to position single beads at predefined places, some conditions
have to be adjusted. First of all, the concentration of the beads in the
water drop has to be right. A too high concentration results in more
than one bead at every position, and with a too low concentration, there
is a high probability that there is no bead at all in the vicinity of
the generated magnetic field. For the used MICROMOD particles
(confer section 1.2), concentrations of
2-5
g/ml worked very well.
As another problem, clustering was observed during several experiments. The magnetic particles often clumped together, although the particles are superparamagnetic and their concentration in water was very low. To get rid of this clustering effect, the water-particle mixture has to be thoroughly shaken directly before the experiment. Luckily, these two requirements can easily be met, but they cannot be ignored.