This study investigates the characteristics of detonation structure and droplet behavior in $n$-heptane spray detonations where droplets experience a thermodynamic critical event. Using a one-dimensional Eulerian-Lagrangian simulation, this investigation seeks to understand the influence of droplet criticality on spray detonation dynamics. When a droplet reaches the critical point, it transitions into a combustible flow, strengthening the detonation by reducing evaporation time and shortening both the induction zone length and post-detonation dispersion distance. The thermophysical critical event is identified when the initial pressure ($p_0$) exceeds 0.5 atm, and the initial droplet diameter ($d_0$) exceeds 1 µm. At $p_0=0.5$ atm, where droplets cannot reach the critical state, a longer time is required to stabilize the detonation. Evaluation of the critical droplet fraction reveals that droplet criticality contributes substantially to detonation in off-stoichiometric mixtures compared to stoichiometric ones. This liquid-to-gas transition at the critical point generates locally high $n$-heptane mass fraction, elevates the heat release rate, and creates a criticality-induced shock (CIS). The CIS causes noticeable fluctuations in the induction zone length and post-detonation dispersion distance, particularly at $d_0=30\mu \mathrm{m}$. A critical droplet diagram is constructed, showing that the droplet thermophysical critical event is observed for $d_0$ ranging from 5 to 100 µm and $p_0$ between 1.0 and 1.5 atm.

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• Combustion
• Detonation
• Reacting multiphase flows
• Shock waves