Rapid onset of molecular friction in liquids bridging between the atomistic and hydrodynamic pictures

Abstract

Friction in liquids arises from conservative forces between molecules and atoms. Although the hydrodynamics at the nanoscale is subject of intense research and despite the enormous interest in the non-Markovian dynamics of single molecules and solutes, the onset of friction from the atomistic scale so far could not be demonstrated. Here, we fill this gap based on frequency-resolved friction data from high-precision simulations of three prototypical liquids, including water. Combining with theory, we show that friction in liquids emerges abruptly at a characteristic frequency, beyond which viscous liquids appear as non-dissipative, elastic solids. Concomitantly, the molecules experience Brownian forces that display persistent correlations. A critical test of the generalised Stokes-Einstein relation, mapping the friction of single molecules to the visco-elastic response of the macroscopic sample, disproves the relation for Newtonian fluids, but substantiates it exemplarily for water and a moderately supercooled liquid. The employed approach is suitable to yield insights into vitrification mechanisms and the intriguing mechanical properties of soft materials.

Fluctuation–dissipation relations far from equilibrium: a case study

“…The same observations hold for the memory kernel K(t). In equilibrium, the memory kernel also has an initial exponential decay, however for larger times it becomes negative and approaches zero with the same power law as the VACF but different sign, [52,62]. The oscillatory dependence on t discussed in the VACF for large P e is reflected in the memory kernel by a strong initial damping, followed by a very pronounced minimum with negative friction (see Fig.…”

“…Using Fourier transform techniques in the long-time regime as described in Refs. [51,52] might improve these values.…”

Fast increase of nanofluidic slip in supercooled water: the key role of dynamics

“…[22][23][24][25] On the modeling side, several efforts have been pursued in order to understand the molecular mechanisms that control friction, with special interest on the discussion of the relation between the friction coefficient and the time autocorrelation of the force exerted by the liquid on the wall. [26][27][28][29][30][31][32][33] Further work has been performed to study the impact on friction of different wall features such as wettability, 34,35 roughness, 36 crystallographic orientation, 37 electronic structure, [38][39][40] or electrostatic interactions. 41 Yet a large number of questions with regard to the interface properties, such as its viscoelastic or purely viscous nature [42][43][44] or the possible link with its interfacial thermal transport equivalents via wall's wetting properties, [45][46][47] remain open nowadays, limiting the perspectives for a rational search of optimal interfaces.…”

Continuous Demixing Transition of Binary Liquids: Finite‐Size Scaling from the Analysis of Sub‐Systems

“…For values of y = 𝓁∕𝜉 ≳ 20, the data indeed converge and, in principle, one could read off 𝜒 0 directly. However, a more robust approach should allow for corrections to scaling, Equation (8), which suggests to fit this form to the data with 𝜒 0 , z ∞ and a 1 as parameters. Even though the range of values for large y is severely limited, we performed an asymptotic fit as y → ∞ to estimate the amplitude 𝜒 0 , yielding 𝜒 0 = 0.080 ± 0.002 and thereby improving our earlier estimate of 𝜒 0 = 0.092 ± 0.011.…”

“…In the past decade, the seemingly never ending growth in computing power was, among other factors, driven by a shift to massive parallelization, making molecular simulations of unprecedented system sizes and run lengths broadly available. [7][8][9][10][11][12][13] The use of huge systems mitigates artifacts due to a finite simulation box and allows, in principle, probing the critical divergences directly as one would do in an experiment. Yet, an elegant and conceptually superior alternative exploits the scale invariance of the critical fluid and turns the limitation of a finite system size into an advantage by explicitly following the divergences of certain fluid properties as a function of the system size.…”