Effect of size on supercritical fluid phase distribution under confinement
Abstract
<p indent="0mm">Various studies have shown that supercriticai fluid (SF) has a nonuniform molecular structure. Here, we start with the molecular dynamics (MD) simulation of argon. Argon has been widely used as a model fluid for MD simulations. An SF film is simulated in a cuboid with two solid walls. In the simulation system, the pressure is kept constant, and the temperature is varied, including the LL, TPL, and GL states of SF. We examine the effect of the size and wall on the SF phase distribution in different states. Evidently, the number density near the wall has an obvious stratification for LL, which gradually reduces with increasing temperature (TPL and GL). The fluid density in the near-wall region has strong dependence on the wall wettability, and the density peak decreases with decreasing wall wettability. The vapor mass of the system increases with the temperature, while it first increases and then decreases with decreasing wall wettability. Different channel heights have different turning points, and different fluid states display different size effects. The three-dimensional atomic distribution suggests that LL and GL atoms are populated together on strongly and weakly wetted surfaces, respectively. The lesser potential energy, the greater the depth of the potential well, which depth has a strong binding effect on atoms, leading to the easy formation of a high-density region. In contrast, the binding degree of atoms decreases with increasing potential energy, and atoms can easily escape to form low-density regions. These findings may help in understanding the structure and phase distribution of SF under confinement and inspire its practical application.</p>
