Hypergolic ignition by the head-on collision of a smaller N,N,N',N' tetramethylethylenediamine (TMEDA) droplet and a larger white fuming nitric acid (WFNA) droplet was experimentally investigated by using a droplet collision experimental apparatus equipped with a time-resolved shadowgraph, a photodetector and an infrared detector. The investigation was focused on understanding the influence of droplet collision and mixing, which vary with the collisional Weber number (We = 20-220) and the droplet size ratio (Delta = 1.2-2.9) while have a fixed Ohnesorge number (Oh = 2.5x10(-3)), on the hypergolic ignitability and the ignition delay times. The hypergolic ignition was found to critically rely on the heat release from of the liquid-phase reaction of TMEDA and nitric acid, which is subsequent to and enhanced by the effective mixing of the droplets of proper size ratios. Consequently, the ignitability regime nomogram in the We-A space shows that the hypergolic ignition favors small Delta s and large Wes; the ignition delay times tend to decrease with either decreasing Delta, or increasing We, or both. A non-monotonic variation of the ignition delay times with We was observed and attributed to the non-monotonic emergence of jet-like mixing patterns that enhance the droplet mixing and hence the liquid-phase reaction. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.