A drop or droplet is a small column of liquid bounded by free surfaces. The subject of droplets has been studied by many researchers, and the main issues of concern for drops and droplets vary according to engineering fields such as wetting, colloid motions, and spreading and evaporation. This book is to addresses droplet phenomena, especially droplet spreading and coalescence containing drops, droplet spreading, and coalescence. Presenting several years of collaborative work on droplet spreading, coalescence, and similar work done by other researchers, readers would be well served to find a suitable application for droplet and coalescence.