  The radial relative velocity between particles suspended in turbulent flow
plays a critical role in droplet collision and growth. We present a simple and
accurate approach to RV measurement in isotropic turbulence - planar 4-frame
particle tracking velocimetry - using routine PIV hardware. This study
demonstrates the feasibility of accurately measuring RV using routine hardware
and verifies, for the first time, the path-history and inertial filtering
effects on particle-pair RV at large particle separations experimentally.
