Handling of micrometer sizes particles, such as biological cells or coated beads, plays a relevant role in the field of life science. A number of devices have been presented in the last years, in which acoustic forces generated by coupling the vibration of a solid structure excited by a piezoelectric transducer to the particle suspension are used to collect particles in lines or position them in clumps on a grid. Following the trend of lab-on-a-chip devices, efforts have been made to shrink the size of such systems, aiming at less reagent consumption and shorter reaction times. The majority of these systems consist of closed fluid filled volumes, typically channels. Here the use of an open fluid volume, a droplet, is examined. By exciting resonances into the droplet positioned on a surface, particles can be gathered into a line, two parallel lines or, as the frequency of excitation is increased, into more complex patterns. Such a concentration process will have useful applications in improved detection sensitivity of low concentration particulate solutions.