Defects, Corrugation and Temperature Govern Rarefied-Air Drag on Graphene Coatings

Abstract

In rarefied atmospheric environments, where continuum fluid dynamics breaks down, aerodynamic drag is governed by gas-surface momentum exchange, making surface structure and chemistry key design knobs. Using molecular dynamics simulations, we show that coating the $α$-Al2O3(0001) surface with graphene markedly reduces the tangential momentum accommodation coefficient (TMAC) of N2, shifting scattering toward more specular reflection and thereby lowering drag; we further benchmark this response against graphite. The reduction strengthens up to 900 K. While structural defects can increase TMAC via defect-induced corrugation and local atomic and electronic rearrangements, graphene retains its performance at experimentally relevant defect densities.

Figures (5)

• Figure 1: Schematic representation of the simulated systems. Nitrogen molecule impinging on (a) multilayer graphite surface; (b) $\alpha$-Al$_2$O$_3$ (0001) slab; (c) graphene-coated $\alpha$-Al$_2$O$_3$ (0001) surface. Panels (d–g) show the pristine graphene lattice and the atomic configurations of the key defects studied: (d) pristine graphene; (e) single vacancies; (f) divacancies; (g) Stone–Wales defects.
• Figure 2: (a) Schematic representation of gas beam scattering at fixed angle. (b-d) Angular distribution of scattered nitrogen molecules on selected surfaces as a function of reflection angle $\alpha$ for three different incidence angles (b) $\theta_{in}=30^{\circ}$; (c) $\theta_{in}=45^{\circ}$; (d) $\theta_{in}=70^{\circ}$. Our results for $\mathrm{graphite}$, $\alpha$-$\mathrm{Al_{2}O_{3}}$, $\mathrm{graphene}$/$\alpha$-$\mathrm{Al_{2}O_{3}}$ are shown as green line with circles, blue line with squares, orange line with triangles, respectively. Experimental mehta_2018andric_2018 and simulation andric_2018 results on $\mathrm{graphite}$ from the literature are marked with red lines with circles and black lines with circles, respectively.
• Figure 3: Correlations between incoming (horizontal-axis) $v_x^i$ and outgoing final $v_x^f$ (vertical-axis) tangential velocity of nitrogen molecules on three different surfaces: alumina, graphene-coated alumina and graphite, for a gas flow at room temperature with bulk velocity of 280 m/s. The solid red lines demonstrate the least-square linear fit of the tangential velocity data to obtain the tangential momentum accommodation coefficients $\alpha_t$ (TMACs).
• Figure 4: (Top) TMAC as a function of temperature for bare alumina (solid blue line with square markers) and graphene-coated alumina (solid orange line with triangular markers). (Bottom) Temperature dependence of additional accommodation coefficients: energy accommodation coefficient (EAC), normal energy accommodation coefficient (ENAC) and rotational energy accommodation coefficient (E$_{\mathrm{rot}}$AC) for alumina and graphene-coated alumina.
• Figure 5: (a) TMACs and (b) CAFs at different density of defects on graphene coated alumina; vacancies (black squares), divacancies (green triangles) and Stone-Wales (light blue circles). The orange and light-orange dashed lines correspond respectively to pristine and compressed graphene.