Droplet coalescence kinetics: Coalescence mechanisms and thermodynamic non-equilibrium effects under isothermal and non-isothermal conditions

Droplet coalescence is a multiscale phenomenon driven by multiple thermodynamic non-equilibrium (TNE) effects. This study investigates the coalescence mechanisms and the complex interplay between various TNE effects in both isothermal and non-isothermal cases kinetically. The main findings include: (i) Coalescence initiation and cut-through mechanisms: In the non-isothermal case, latent heat release results in a temperature rise that slightly increases the surface tension gradient (driving force) near the contact point, while substantially enhancing the pressure gradient (resistance). This leads to a significantly prolonged initiation of coalescence compared to the isothermal case. The additional temperature field effects introduce greater spatial complexity to both the flow field and morphology. For the cut-through mechanism, in both cases, pressure extends the liquid-vapor interface in opposite directions, promoting liquid bridge growth. (ii) TNE effects: The latent heat-induced temperature rise in the thermal case considerably dampens the intensity of TNE effects. Before and after droplet contact, TNE quantities driven by the temperature gradient ( Δ 3,1∗ and Δ3∗) and those driven by the velocity gradient ( Δ 2∗ and Δ 4,2∗), alternate in dominating the coalescence process. This alternating dominance results in a more complex spatiotemporal evolution of TNE effects compared to the isothermal case, where the TNE intensity is dominated by Δ2∗ and Δ 4,2∗. (iii) Entropy production mechanisms: In the non-isothermal case, entropy production is contributed by both Δ 2∗ and Δ3 ,1 ∗, with the former being the dominant contributor. The temperature field reduces the entropy production rate, but extends its duration, thereby increasing the total entropy production. The entropy production rates from Δ2 ∗ and Δ3 ,1 ∗ alternate before and after the droplet cut-through. Further investigation into effects of the initial droplet distance rd revealed that it increases droplet cut-through time tc, the time when total TNE reaches its maximum tD m ax, and the time when total entropy production rate reaches its maximum tS˙prmax in a power-law manner: ( tc,t Dmax,tS˙prmax) ∼rd α. This research provides kinetic insights into droplet coalescence, offering mesoscopic support for dynamic, cross-scale regulation and multifunctional integration of coalescence processes in industrial applications.