Hydrodynamically interacting droplets at small Reynolds numbers
Droplets moving at close distances interact hydrodynamically. We present the experimental and theoretical results of a study for monodisperse 73 ?m geometric diameter droplets with initial velocities between 1.5 and 3 m/s and variable generation rate (ranging from 125 to 2000 s-1) emitted horizontally and vertically. During the decelerated motion the droplet Reynolds number varied between the value <1 and 10. The hydrodynamic interaction depends on the spatial distance, and we observed a considerable increase of the droplet trajectory endpoints with increasing generation rate. Reviewing the various approaches for the description of hydrodynamic interactions, we adapted a theory for a stationary Stokes flow past a linear array of spheres and found a surprisingly good agreement between our experiments and the above model. Even better agreement was obtained by additionally taking the Basset force into account.