Binary collision between a smaller N, N, N', N'- tetramethylethylenediamine (TMEDA) droplet and a larger white fuming nitric acid (WFNA) droplet was investigated experimentally and computationally for understanding the influence of off-center collision on the hypergolic ignitability of the system, which is controlled by the mass interminglement and mixing subsequent to the droplet coalescence. The ignition delay time was experimentally found to non-monotonically vary with the impact parameter, which measures the degree of off-center collisions. This phenomenon was hypothetically attributed to the non-monotonicity of mass interminglement of colliding droplets with increasing the impact parameter-the increased droplet stretching by slightly off-center collision promotes mass interminglement, but the stretching separation by significantly off-center collision reduces mass interminglement. This hypothesis was computationally verified by a validated volume-of-fluid (VOF) simulation of a simplified problem, in which the transport phenomena and chemical reactions are neglected and the controlling physics of droplet mass interminglement is emphasized. Furthermore, a parametric study for wide ranges of controlling non-dimensional parameters, such as the collision Weber number of 20-140 and the droplet size ratio of 1.3-2.0, further confirms that the non-monotonicity of ignition delay time with the impact parameter is a general characteristic of the present hypergolic system. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.