Identifying two regimes of slip of simple fluids over smooth surfaces with weak and strong wall-fluid interaction energies

Abstract

The slip behavior of simple fluids over atomically smooth surfaces was investigated in a wide range of wall-fluid interaction (WFI) energies at low shear rates using non-equilibrium molecular dynamics simulations. The relationship between slip and WFI shows two regimes (the strong-WFI and weak-WFI regimes): as WFI decreases, the slip length increases in the strong-WFI regime and decreases in the weak-WFI regime. The critical value of WFI energy that separates these regimes increases with temperature, but it remains unaffected by the driving force. The mechanism of slip was analyzed by examining the density-weighted average energy barrier (ΔE¯) encountered by fluid atoms in the first fluid layer (FFL) during their hopping between minima of the surface potential. We demonstrated that the relationship between slip and WFI can be rationalized by considering the effect of the fluid density distribution in the FFL on ΔE¯ as a function of the WFI energy. Moreover, the dependence of the slip length on WFI and temperature is well correlated with the exponential factor exp(-ΔE¯/(kT)), which also determines the critical value of WFI between the strong-WFI and weak-WFI regimes.

Effects of viscous heating and wall-fluid interaction energy on rate-dependent slip behavior of simple fluids

“…5(a)]. This behavior is intuitively expected since interfaces with lower WFI energies result in larger slip lengths, if the WFI is sufficiently strong [13]. However, as shown in Fig.…”

“…III C]. Another contributing factor leading to smaller values of the shear viscosity in the case ε WF = 0.005ε is that the FFL is displaced closer to the walls, which effectively increases the channel height and, consequently, reduces viscosity [13].…”

“…Over the past decades, experiments [1][2][3][4][5] and molecular dynamics (MD) simulations [6][7][8][9][10][11][12][13] have demonstrated that the traditional no-slip boundary condition (BC) does not necessarily hold in small-scale systems that involve either hydrophobic surfaces or high shear rates. Application of the slip BC, instead of no-slip BC, in numerical models remediates the singular or unphysical behavior of fluid in some situations, such as droplet spreading [14][15][16][17] and corner flows [18,19].…”

“…Typical values of slip length observed in experiments on fluids over smooth nonwetting surfaces are of the order of ten nanometers [1][2][3][4]. Complimentary to the experimental approach, MD simulations [6][7][8][9][10][11][12][13] were extensively used to study the slip behavior, since this method provides the detailed information about structure and dynamics of fluids near solid interfaces at the atomic level.…”

The local slip length and flow fields over nanostructured superhydrophobic surfaces

“…Helmholtz related the magnitude of slip length to be on the order of the mean free path. During the past decades, a number of experimental ( Vinogradova, 1999;Bonaccurso et al, 2002;Granick et al, 20 03;Leger, 20 03;Spikes and Granick, 2003;Vinogradova and Yakubov, 2006;Ho et al, 2011;Vinogradova and Belyaev, 2011 ) and simulation studies ( Thompson and Troian, 1997;Barrat and Bocquet, 1999;Lichter et al, 20 04;Priezjev and Troian, 20 04;Priezjev, 2007;Huang et al, 2008;Liu and Li, 2010;Bao et al, 2017;Hu et al, 2017 ) have demonstrated that the slip length can be on the order of tens nanometers for liquids flowing over smooth nonwetting surfaces, and the μm scale slip lengths cannot be achieved without gas or air trapped at the surface.…”