In-line interaction between two spherical particles due to a laminar wake effect

► An equation to predict the hydrodynamic force on a trailing sphere is formulated.
► The formulation is based on the flow velocity in the wake behind a leading sphere.
► The agreement of the predictions with respect to numerical data is achieved.
► The hydrodynamic force increases with s/dp; it diminishes with Re.
► The inertial force is significant at small inter-particle distances.