Droplet coalescence kinetics: thermodynamic non-equilibrium effects and entropy production mechanism

TL;DR

This study investigates the thermodynamic non-equilibrium effects and entropy production during droplet coalescence using the discrete Boltzmann method, revealing key physical criteria and force dynamics that govern the process.

Contribution

It provides a detailed analysis of TNE effects and entropy production in droplet coalescence, highlighting their roles as physical criteria and elucidating the force mechanisms involved.

Findings

Total TNE strength and entropy production rate peak at coalescence stages.

Surface tension promotes coalescence, NOMFs inhibit it, with effects varying by droplet size.

Larger curvature increases TNE intensity and forces in smaller droplets.

Abstract

The thermodynamic non-equilibrium (TNE) effects and the relationships between various TNE effects and entropy production rate, morphology, kinematics, and dynamics during two initially static droplet coalescence are studied in detail via the discrete Boltzmann method. The temporal evolutions of the total TNE strength ($D^*$) and the total entropy production rate ($\dot S$) can both provide concise, effective and consistent physical criteria to distinguish the stages of droplet coalescence. Specifically, when $\bar D^*$ and $\dot S$ reach their maxima, it corresponds to the time when the liquid-vapor interface length changes the fastest; when $D^*$ and $\dot S$ reach their valleys, it corresponds to the moment of the droplet being the longest elliptical shape. During the merging process, the force contributed by surface tension in the coalescence direction acts as the primary promoting force for droplet coalescence and reaches its maximum concurrently with coalescent acceleration. In contrast, the force contributed by non-organized momentum fluxes (NOMFs) in the coalescing direction inhibits the merging process and reaches its maximum at the same time as $D^*$. For the coalescence of two unequal size droplets, the smaller droplet exhibits larger values for TNE intensity, merging velocity, driving force contributed by surface tension, and resistance contributed by NOMFs. Moreover, these values gradually increase with the initial radius ratio of the large and small droplets due to larger curvature. However, non-equilibrium components and forces related to shear velocity in the small droplet, are all smaller than those in the larger droplet and gradually decrease with the radius ratio.