(140g) Superheated Homogeneous and Heterogeneous Bubble Nucleation Rates Using Npt and NpZz T Molecular Dynamics: Effects of Surface Interactions

Performance of gas-evolving electrodes and boiling heat transfer surfaces could be substantially improved if it were possible to control the gas bubble nucleation process. While surface geometry is known to play an important role, at least at some length scales, if gas or vapor can be trapped, the effect of chemical interactions between liquid and solid is not known. This study examines this question by comparing vapor nucleation rates of for Lennard-Jones argon near a Lennard-Jones fcc solid with different solid-fluid interaction strengths and with the purely homogenous fluid rates.

A molecular dynamics approach is used employing an NPT ensemble with 2500 atoms for the homogenous case and the NP_zzT ensemble with 2302 fluid and 648 solid atoms for the heterogeneous case. The normal stress (P_zz) on an upper (?opposing?) wall potential is controlled by moving the position of the wall potential. Nucleation is achieved by quickly ramping up the set temperature of equilibrated configurations. The method used by Bartell¹ to calculate nucleation rates from sets of nucleation times is used.

In all calculations, the set pressure or normal stress is P* = 0.01 (4.23 bar). Dimensionless values use the fluid-fluid parameters. For the smooth solid surfaces, three cases are considered by varying ε_fs, the solid-fluid interaction parameter, from ½ the value of the fluid-fluid parameter ε_ff (most repulsive) up to 1.518 times the fluid-fluid value. For ε_fs/ε_ff = 0.5, nucleation is fastest due to the the relative solid-fluid repulsion which enables void formation. The nucleation rates vary from 2.5 X 10³⁴ m^-3s^-1 to 5.0 X 10³⁵ m^-3s^-1 for T* from 1.091 to 1.124 (132 to 136 K) and the free energy of forming bubbles larger than the critical size is ΔG* = 157 (158 kJ/mol). For ε_fs/ε_ff = 1.0 the nucleation rates vary from 1.2 X 10³³ m^-3s^-1 to 1.8 X 10³⁵ m^-3s^-1 for T* from 1.107 to 1.124 and the free energy is 372. For ε_fs/ε_ff = 1.518, the nucleation rates vary from 1.7 X 10³³ m^-3s^-1 to 5.0 X 10³⁴ m^-3s^-1 for T* from 1.111 to 1.124 and the free energy is 355. The nucleation rates and energies in the second and third case are nearly the same which suggests that the limit of the effect of the fluid-solid interactions on nucleation has been reached.

Nucleation is easier in all three heterogeneous cases compared with the homogenous case. The nucleation rates vary from 1.6 X 10³³ m^-3s^-1 to 1.1 X 10³⁵ m^-3s^-1 for temperatures between 1.186 and 1.194 and the free energy is 720.

¹L.S. Bartell, J. Phys. Chem. A, 106(45), 10893 (2002).