Large variation in the boundary-condition slippage for a rarefied gas flowing between two surfaces

Surface-Induced Near-Field Scaling in the Knudsen Layer of a Rarefied Gas

We report on experiments performed within the Knudsen boundary layer of a low-pressure gas. The noninvasive probe we use is a suspended nanoelectromechanical string, which interacts with ^{4}He gas at cryogenic temperatures. When the pressure P is decreased, a reduction of the damping force below molecular friction ∝P had been first reported in Phys. Rev. Lett. 113, 136101 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.136101 and never reproduced since. We demonstrate that this effect is independent of geometry, but dependent on temperature. Within the framework of kinetic theory, this reduction is interpreted as a rarefaction phenomenon, carried through the boundary layer by a deviation from the usual Maxwell-Boltzmann equilibrium distribution induced by surface scattering. Adsorbed atoms are shown to play a key role in the process, which explains why room temperature data fail to reproduce it.

Viscoelastic Drag Forces and Crossover from No-Slip to Slip Boundary Conditions for Flow near Air-Water Interfaces

The "free" water surface is generally prone to contamination with surface impurities, be they surfactants, particles, or other surface active agents. The presence of such impurities can modify flow near such interfaces in a drastic manner. Here we show that vibrating a small sphere mounted on an atomic force microscope cantilever near a gas bubble immersed in water is an excellent probe of surface contamination. Both viscous and elastic forces are exerted by an air-water interface on the vibrating sphere even when very low doses of contaminants are present. The viscous drag forces show a crossover from no-slip to slip boundary conditions while the elastic forces show a nontrivial variation as the vibration frequency changes. We provide a simple model to rationalize these results and propose a simple way of evaluating the concentration of such surface impurities.

Slippage and boundary layer probed in an almost ideal gas by a nanomechanical oscillator

We measure the interaction between ⁴He gas at 4.2 K and a high-quality nanoelectromechanical string device for its first three symmetric modes (resonating at 2.2, 6.7, and 11 MHz with quality factor Q>0.1×10⁶) over almost 6 orders of magnitude in pressure. This fluid can be viewed as the best experimental implementation of an almost ideal monoatomic and inert gas of which properties are tabulated. The experiment ranges from high pressure where the flow is of laminar Stokes-type presenting slippage down to very low pressures where the flow is molecular. In the molecular regime, when the mean-free path is of the order of the distance between the suspended nanomechanical probe and the bottom of the trench, we resolve for the first time the signature of the boundary (Knudsen) layer onto the measured dissipation. Our results are discussed in the framework of the most recent theories investigating boundary effects in fluids (both analytic approaches and direct simulation Monte Carlo methods).

Friction between solids and adsorbed fluids is spatially distributed at the nanoscale

The widespread developments in the use of nanomaterials in catalysis, adsorption, and nanofluidics present significant new challenges in achieving optimal adsorbed fluid flow characteristics. Here we demonstrate, using molecular dynamics simulations of nanoconfined fluids, that at nanoscales, fluid-solid friction is not restricted to a sharp interface as is commonly assumed; instead it is distributed over the whole adsorbed fluid phase, and is strongest in an interfacial region that is not negligible in comparison to the system size. Our simulations yield position-dependent dynamical fluid-solid friction coefficients, and lead to a modification of conventional hydrodynamics, incorporating distributed momentum loss in the fluid due to fluid-solid interaction. The results demonstrate that the usual concepts of slip length or interfacial friction coefficient are meaningful only for uniform fluids, and lose their significance for adsorbates in nanospaces, which are intrinsically inhomogeneous. We show that static friction coefficients, based on equilibrium density distributions, follow the same spatial dependence as the dynamical coefficients. These results open up possibilities for tailoring nanomaterials and surfaces to engineer low friction pathways for adsorbed fluid flow by tuning the potential energy landscape.

In Situ control of gas flow by modification of gas-solid interactions

The boundary condition for gas flow at the solid-gas interface can be altered by in situ control of the state of a thin film adsorbed to the solid. A monolayer of ocatadecyltrichlorosilane (OTS) reversibly undergoes a meltinglike transition. When the temperature of an OTS-coated particle and plate is moved through the range of OTS "melting" temperatures, there is a change in the lubrication force between the particle and plate in 1 atm of nitrogen gas. This change is interpreted in terms of a change in the flow of gas mediated by the slip length and tangential momentum accommodation coefficient (TMAC). There is a minimum in slip length (290 nm) at 18 °C, which corresponds to a maximum in TMAC (0.44). The slip length increases to 590 nm at 40 °C which corresponds to a TMAC of 0.25. We attribute the decrease in TMAC with increasing temperature to a decrease in roughness of the monolayer on melting, which allows a higher fraction of specular gas reflections, thereby conserving tangential gas momentum. The importance of this work is that it demonstrates the ability to control gas flow simply by altering the interface for fixed geometry and gas properties.

Crossover from hydrodynamics to the kinetic regime in confined nanoflows

We present an experimental study of a confined nanoflow, which is generated by a sphere oscillating in the proximity of a flat solid wall in a simple fluid. Varying the oscillation frequency, the confining length scale, and the fluid mean free path over a broad range provides a detailed map of the flow. We use this experimental map to construct a scaling function, which describes the nanoflow in the entire parameter space, including both the hydrodynamic and the kinetic regimes. Our scaling function unifies previous theories based on the slip boundary condition and the effective viscosity.