Micro droplets and coated micro droplets serve as well defined tiny reaction chambers or basis for capsules in chemistry and microbiology. However, the initially separated droplets tend to form groups (or coalesce) whenever geometrical defects occur in the channel. By introducing a defined perturbation in form of a sudden channel expansion, we can study this group formation systematically. We get beautiful images of very regular trains of coated micro droplets turning into well defined groups of different sizes. Using a purely kinematic model we can even predict, how many droplets will join a group, depending on initial distance, length and coating thickness. Thus, an initially undesired effect became a well defined and visually appealing phenomenon.
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