Flows laden with particles are frequently found in thermal and flow processes. Fuel sprays in automotive and gas-turbine engines, evaporators, cooling towers, pulverized-coal/air flow in power plants, and flow with internal boiling (gas bubbles) are some of the examples of the so-called particle-laden, two-phase flows. Many industrial processes involve mixing of liquid/solid, gas/solid, liquid/gas combinations of materials. Also, atmospheric processes such as aerosol, raindrops, and cloud particles have a profound effect on local and global meteorology. For these two-phase flow processes, it is of interest to determine the particle size, velocity and, in some cases, chemical compositions so that one can assess the effects of the particles on the overall thermal and flow properties.