The hypergolic ignition induced by the head-on collision of TMEDA and WFNA droplets was experimentally investigated with emphasis on the effect of droplet size on the ignitibility and the ignition delay time. The ignitibility regime nomogram in We - d(o) space indicates that the reduction of droplet size tends to suppress the hypergolic ignition. The ignition delay time, which was precisely determined by using grayscale level analysis, becomes shorter for smaller droplets. The seemingly conflicting size effects were resolved by means of time scaling analysis to reveal the size dependence of the three pre-ignition processes, which were identified as the first stage of droplet collision, deformation and intermixing, the second stage of droplet heating from interior to surface, and the third stage of droplet vaporization subject to heat loss by convective cooling. (C) 2019 Elsevier Ltd. All rights reserved.