Droplet collisions and impacts are so spectacular that they have come to symbolize the beauty and fascination of fluid mechanics. Although simulations of two dimensional and axisymmetric systems go back to the early times of two-phase flow simulation, those of fully three-dimensional configurations have become possible only recently. It remains difficult, however, to perform realistic simulations of laboratory experiments.

Introduction

Droplet impacts are of major industrial interest. In what is perhaps the most significant application, fuel droplets impact on the walls of pipes and combustion chambers. There they may spread and form thin films or shatter into a spray of smaller droplets. Impacts also have an obvious relevance to ink-jet printing and spray coating. In other industrial processes, droplet impacts are of interest in metallurgy (Liow et al., 1996; Bierbrauer, 1995) and gas-injection processes. High speed droplet impacts may damage turbines operating with multiphase flows. In hypothetical severe nuclear reactor accidents, molten-core debris may impact on containment walls, splashing at very large velocity. In agriculture, impacts are related to the effect of rain on soil erosion (Farmer, 1973), or the spread of pesticides as they are sprayed on plants. Rain also influences air–sea interactions, enhancing the gas exchange and perhaps damping sea waves (Sainsbury and Cheeseman, 1950; Tsimplis and Thorpe, 1989). Droplet breakup, atomization, impacts, and splashes also cause the accumulation of charge in droplets, as shown by the 1905 Nobel physics laureate Philip Lenard following the work of Hertz (Lenard, 1892).