Increase Of Slip Length Research Articles

Anomalous friction of supercooled glycerol on mica

Although friction of liquids on solid surfaces is traditionally linked to wettability, recent works have unveiled the role of the solid’s internal excitations on interfacial dissipation. In order to directly evidence such couplings, we take advantage of the considerable variation of the molecular timescales of supercooled glycerol under mild change of temperature to explore how friction depends on the liquid’s molecular dynamics. Using a dedicated tuning-fork AFM, we measure the slippage of glycerol on mica. We report a 100 fold increase of slip length upon cooling, while liquid-solid friction exhibits a linear scaling with molecular relaxation rate at high temperature. This scaling can be explained by a contribution of mica’s phonons which resonate with density fluctuations in the liquid, allowing efficient momentum transfer to mica. These results suggest that engineering phononic spectra of materials could enhance flow performance in nanofluidic channels and industrially relevant membranes.

Stability of viscoelastic film on a slippery inclined plane

The linear and weakly nonlinear stability of viscoelastic film flowing down a slippery inclined plane is investigated analytically. Under the assumption of the long wave approximation, the first-order Benny equation of Oldroyd-B fluid thin film with slip condition is obtained. Through the normal mode analysis, the neutral stability curve and the temporal growth rates are calculated to explore the linear stability of the film. Linear results show that the critical Reynolds number decreases with the increase in slip length and viscoelastic parameter and that the liquid film may exhibit pure elastic instability. For the nonlinear stability analysis, both hydrodynamic instability and elastic instability are discussed. The primary bifurcations in the phase plane are identified by calculating the Landau coefficient, i.e., the unconditional stable region, the supercritical region, the subcritical region, and the explosive region. The dependence of primary bifurcation regions upon the slip length and Deborah number are studied, and the results indicate that the slip boundary and viscoelasticity destabilizes the flow. According to the Ginzburg–Landau equation, the threshold amplitude of the nonlinear equilibrium solution is analyzed as well.

Channel width-dependent viscosity and slip length in nanoslits and effect of surface wettability

The channel width-dependent behaviors of viscosity (μ) and slip length (ls) in nanoslits are investigated using many-body dissipative particle dynamics simulation in both Poiseuille and Couette flow systems. In both systems, the viscosity and slip length increase as the channel width (w) grows in smaller channels, while they reach bulk values in larger channels. Moreover, as the surface wettability decreases, the slip length is found to increase, while the viscosity remains the same. The channel width-dependent behavior in nanoslits can be explained by the unique structure of the confined fluid. As the channel width narrows, the uniform density profile in the central region diminishes, and an oscillation pattern appears throughout the system. The change in the microstructure with the channel width alters friction between layers of fluid in laminar flow and fluid-solid friction, leading to a w-dependent μ and ls. Nonetheless, the alteration of surface wettability influences only fluid–solid interactions but not the friction between layers of fluid.

Lattice Boltzmann simulation of neutrally buoyant circular slip particle motion in a clockwise double-lid-driven square cavity

This paper is on the motion of a neutrally buoyant but circular slip particle in a clockwise double-lid-driven square cavity. The slip flow at the particle surface is implemented by the lattice Boltzmann method with corrected slip boundary schemes. The effects of slip length (Ls), initial particle position, Reynolds number (Re), and particle size (D) are studied on the migration of the slip particle. The motion of the circular slip particle is dominated by the centrifugal and boundary-repulsion forces. The results show that the cavity center is the unique fixed point, and once the slip particle initially deviates from the cavity center, it is always stabilized at the same limit cycle. With the increase in slip length, the limit cycle of the circular slip particle is closer to the cavity boundaries, which brings a stronger centrifugal force to balance the increased boundary-confinement effect. As the slip length, Ls, exceeds 0.02D, the limit cycle forms more quickly than the circular no-slip particle. When Re increases to within 1000, the limit cycle is squashed along the leading diagonal of the cavity and pushed toward the boundaries; however, when Re increases beyond 1000, two developing secondary vortices confine the limit cycle to shrink toward the cavity center. With the increase in particle size, the enhanced boundary confinements lead to the shrinkage of the limit cycle toward the cavity center.

Numerical simulation of heat and mass transfer in heat exchange tubes based on action of interfacial slip

The liquid flow close to the interface has received much attention recently, that is determined by interfacial slip action. In heat exchange tube, the thermal diffusion in the flowing boundary layer is also related to the interfacial flow state. However, the study of convective heat transfer mostly ignores the interfacial slip and medium thermal diffusion. In this study, the variation law of heat transfer efficiency and the actual interfacial concentration under the action of interfacial slip and thermal diffusion in the heat exchange tubes are reported. We find that the heat transfer efficiency of no slip reaches the highest, compared to no slip, that of negative slip significantly reduces (at −0.99 mm, copper: −79.56%; aluminum alloy: −78.60%; carbon steel: −75.85%), and that of positive slip slightly reduces (at 0.58 mm, copper: −16.87%; aluminum alloy: −15.66%; carbon steel: −9.82%). As the slip length increases, the interfacial concentration of positive slip tends to the initial concentration, while the concentration changes rate of negative slip increases (at −0.99 mm, H2SO4: 110.29%, NaCl: 45.90%, C2H6O: −28.37%). These results show that the interfacial slip not only affects heat transfer performance but also interfacial concentration, and further affecting the metal corrosion behavior at the heat transfer interface.

Control of boundary slip by interfacial nanobubbles: A perspective from molecular dynamics simulations

Enhancing boundary slip using interfacial nanobubbles (INBs) has gained significant interest in nanofluidic transport. In this study, we conducted a comprehensive investigation on the influence of INBs on boundary conditions for both smooth and rough substrates using molecular dynamics simulations. We analyzed the impact of INB protrusion angle, coverage percentage, quantity, and fluidity on the slip length. Our results showed that INBs always increase the slip length on a smooth substrate, with a linear increase in slip length observed with increasing surface coverage. On a rough substrate, we found that the protrusion angle, quantity, and fluidity of INBs play a crucial role in determining the slip length. Smaller protrusion angles and fewer quantities of INBs were found to be more favorable for enhancing the slip length when the INB coverage is fixed, while the correlation between boundary slip and INB quantity depended on the wetting state of the substrate when the size of the INBs was fixed with a low protrusion angle. Additionally, we revealed that the fluidity of gas molecules inside the INBs dominated the enhancement of slip length by INBs. Overall, our findings are expected to provide valuable insight into drag reduction based on INBs.

Laser direct writing of rose petal biomimetic micro-bulge structure to realize superhydrophobicity and large slip length

Large slip length at the solid-liquid interface is key factor in the performance of hydrostatic bearings. A superhydrophobic biomimetic-structured surface enables the surface with partial slip. Inspired by rose petal micron papillae, a superhydrophobic surface with biomimetic micro-bulge structures was fabricated by laser direct writing. The prepared surface of tin-bronze alloy shows superhydrophobic property, the maximum water contact angle (WCA) was 163°. The structured surfaces exhibited slip lengths of 20 µm and 37 µm for deionized water and VG5 lubricant, and the original smooth surface has no-slip. The slip length increases with the height of the micro-bulge and the viscosity of working fluid, a maximum slip length measured was 165 µm. It was found that the layered structure contributed to the increase in slip length, demonstrating the good drag reduction properties of the prepared superhydrophobic/oleophobic layer surface. This simple and inexpensive method can fabricate superhydrophobic surfaces with large-slip length, which applied to the solid-liquid interface of hydrostatic bearings can improve dynamic and static properties.

A Three-Dimensional Slip Velocity Model for Water-Lubricated Hydrodynamic Journal Bearings

Hydrodynamic journal bearings, coated with polytetrafluoroethylene (PTFE) and lubricated by water, have been widely used in ships and large-scale pumps, and the function is to maintain the stability of rotor system. However, slip velocity exists on the PTFE-coated surface, whose effect is still an open question. This study aims to investigate the static characteristics of water-lubricated hydrodynamic journal bearings under three-dimensional slip velocity boundary conditions. Firstly, under the non-slip boundary condition, the CFD (computational fluid dynamics) method with ANSYS Fluent is verified based on the Reynolds lubrication equation and the open literature. Then, a three-dimensional slip velocity equation that is based on the Navier slip velocity boundary condition is proposed and embedded into Fluent. Finally, the effects of slip length on the static characteristics are analyzed. Under the same eccentricity ratio, with the increase in slip length, the load capacity decreases due to the decrease of the pressure circumferential gradient, and the friction power decreases. Under the same eccentricity ratio and the same slip length, with the increase in the attitude angle, the load capacity and friction power increase. However, under the non-slip boundary condition, the effects of attitude angle on the load capacity and friction power are insignificant. This paper could provide a reference for studying slip velocity in the hydrodynamic journal bearing.

Combined Pressure-Driven and Electroosmotic Slip Flow through Elliptic Cylindrical Microchannels: The Effect of the Eccentricity of the Channel Cross-Section

Electroosmotic force has been used extensively to manipulate fluid flow in a microfluidic system with various channel shapes, especially an elliptic cylinder. However, developing a computational domain and simulating fluid flow for a system involving an elliptic channel consumes a large amount of time. Moreover, the mathematical expression for the fluid velocity of electroosmotic flow in an elliptic channel may be given in the form of the Mathieu functions that have difficulty in achieving the numerical result. In addition, there is clear scientific evidence that confirms the slippage of fluid at the solid-fluid interface in a microscale system. In this study, we present the mathematical model of combined pressure-driven and electroosmotic flow through elliptic microchannels under the slip-fluid condition. From the practical point of view in fluidics, the effect of the eccentricity of the channel cross-section is investigated on the volumetric flow rate to overcome the difficulty. The results show that the substitution of the equivalent circular channel for an elliptic channel provides a valid flow rate under the situation that the areas of both channel cross-sections are equal and the eccentricity of the elliptic cross-section is less than 0.5. Additionally, the flow rate obtained from the substitution is more accurate when the slip length increases or the pressure-gradient-to-external-electric-field ratio decreases.

A Fractal Permeability Model of Tight Oil Reservoirs Considering the Effects of Multiple Factors

The prediction of permeability and the evaluation of tight oil reservoirs are very important to extract tight oil resources. Tight oil reservoirs contain enormous micro/nanopores, in which the fluid flow exhibits micro/nanoscale flow and has a slip length. Furthermore, the porous size distribution (PSD), stress sensitivity, irreducible water, and pore wall effect must also be taken into consideration when conducting the prediction and evaluation of tight oil permeability. Currently, few studies on the permeability model of tight oil reservoirs have simultaneously taken the above factors into consideration, resulting in low reliability of the published models. To fill this gap, a fractal permeability model of tight oil reservoirs based on fractal geometry theory, the Hagen–Poiseuille equation (H–P equation), and Darcy’s formula is proposed. Many factors, including the slip length, PSD, stress sensitivity, irreducible water, and pore wall effect, were coupled into the proposed model, which was verified through comparison with published experiments and models, and a sensitivity analysis is presented. From the work, it can be concluded that a decrease in the porous fractal dimension indicates an increase in the number of small pores, thus decreasing the permeability. Similarly, a large tortuous fractal dimension represents a complex flow channel, which results in a decrease in permeability. A decrease in irreducible water or an increase in slip length results in an increase in flow space, which increases permeability. The permeability decreases with an increase in effective stress; moreover, when the mechanical properties of rock (elastic modulus and Poisson’s ratio) increase, the decreasing rate of permeability with effective stress is reduced.

Slippery electrokinetic flow of viscoelastic fluids with pressure-dependent viscosity and relaxation time

In this paper, the pressure-driven electrokinetic flow of incompressible viscoelastic fluids with exponential type pressure-dependent viscosity and relaxation time is investigated in a slippery nanochannel. Based on the exponential type pressure-dependent viscosity and relaxation time, the 2D momentum equation has been solved. By regular perturbation expansion method, asymptotic analytical solutions of flow velocity and pressure are obtained up to second order. The results show the magnitude of streaming potential decreases and fluid viscosity increases with an increase of pressure relaxation coefficient ω. When slip length increases, the viscosity of viscoelastic fluids becomes gradually weak. The increase in ω can cause a decrease in electrokinetic energy conversion (EKEC) efficiency. In addition, slip length can enhance EKEC efficiency of viscoelastic fluids with pressure-dependent viscosity and relaxation time.

Friction reduction in a nanochannel with grafted poly(N-isopropylacrylamide) oligomers: A molecular dynamics study

Superhydrophobic surfaces have been used for reducing friction in micro- and nanochannels. In the present work, water flow between two carbon walls with nanostructures made of poly(N-isopropylacrylamide) via the molecular dynamics method has been studied. The structure of this polymer can change based on the temperature of the environment, so that by increasing the temperature the structure becomes hydrophobic. This property has been studied and the effect of multiple factors on the slip length is presented. The effects of the number of monomers in the polymer, the distance between the polymers, and the temperature on the flow field are investigated. The results reveal that the slip length and the flow rate increase with the temperature but both have a maximum with respect to the distance between the polymers and the number of monomers. For different temperatures considered in the study, the minimum friction occurs when the number of monomers and the distance are equal to 30 and 80, respectively. In this condition, the slip length increases by 180% and the flow rate rises by 200%. We also study the effect of air fraction in the fluid. The results indicate that by increasing the air fraction to 30%, the slip length and the flow rate increase by 250% and 310%, respectively.

Coupling effect of wall slip and spanwise oscillation on drag reduction in turbulent channel flow

The coupling effect of the isotropic wall slip and the spanwise oscillation boundary conditions on the drag reduction and turbulence properties in a turbulent channel flow is investigated using direct numerical simulations. As the slip length increases, the drag reduction gradually changes from the oscillation dominated to the slip dominated. For turbulent dynamics, in terms of amplitude, the wall slip condenses the envelope range of the phase fluctuations caused by the oscillatory wall motion, while in terms of phase, the hysteresis of the turbulent dynamics leads to the lag phase of the phase-averaged skin-friction drag, which is larger than that of the Stokes strain.

Wall Slip Behaviour of Polymers Based on Molecular Dynamics at the Micro/Nanoscale and Its Effect on Interface Thermal Resistance.

Taking the Poiseuille flow of a molten polymer in parallel plates as the research object and polymethyl methacrylate (PMMA) as the research material, an all-atom analysis model of the molecular dynamic flow of polymer macromolecules is established according to the Navier slip law. The effects of wall wettability and external pressure on the wall slip behaviour of polymer macromolecules, as well as the spatial evolution process of the entanglement–unentanglement process of polymer chains near the wall under different shearing effects, were studied. The interface thermal resistance rule was explored, and an interface thermal resistance model considering the wall slip behaviour was established. Finally, a micro-injection experiment was used to verify the validity and accuracy of the model. The results show that when the wall is hydrophobic, the polymer melt exhibits significant wall slip. As the external pressure increases, the wall slip speed and the slip length increase. However, after a certain pressure is exceeded, the growth rate of the slip length is basically zero. As the external pressure increases, the PMMA molecular chains gradually start to separate, the single molecular chain becomes untangled from the entangled grid, and the chain detaches from the wall after exceeding a certain threshold. Wall slip reduces the interface thermal resistance between the solid–liquid interface and enhances the interface heat transfer performance. The interface thermal resistance value calculated by molecular dynamics can more accurately reflect the heat conduction rule of the solid–liquid interface at the micro/nanoscale than that measured by the thermal resistance experiment, indicating that the micro/nano interface thermal resistance obtained by molecular dynamics simulation is reliable.

The Effects of Asymmetric Slip Flow Between Parallel Plates of a Microchannel Under Uniform Heat Flux

Abstract Microchannels have many applications in the field of modern technologies. Today, it is necessary to increase the efficiency of these systems due to the increasing rate of thermal loads. Recent researches have shown that liquid flows in the microchannels depend on their size and surface properties. Consequently, there is no symmetric flow condition in a symmetric geometric channel necessarily. In this study, the effect of slip length on the amount of asymmetric heat transfer in a microchannel is investigated numerically. By calculating the parameters such as Nusselt number and local pressure drop coefficient, it is observed that the asymmetric slip flow affects the flow profile and could lead to a decrease or increase in the heat transfer in the microchannel. According to the results, asymmetric slip flow can lead to a 20–40% reduction or increase in the Nusselt number. The most important point in the design of micro-scale asymmetric cooling systems is the increase of slip length at high temperatures.

Molecular Dynamics Study of Friction Reduction of Two-Phase Flows on Surfaces Using 3D Hierarchical Nanostructures

The use of superhydrophobic surfaces is one the most promising methods for reducing the friction and increasing the flow rate in fluid transfer systems. Because in such systems the surface structure plays a key role, in this study, we explore the performance of the hierarchical nanostructures. These nanostructures are inspired by the superhydrophobic surface of the lotus leaf. We consider a flow between two walls with hierarchical nanostructures and simulate the system via the molecular dynamics method. The size of the nanostructures and the distance between them have been studied to find whether a design with a maximum flow rate exists. The nanostructures have two parts, a bigger part on the wall which is a half-sphere and a smaller part which is a cylinder on top of the bigger part. Twenty-four different nanostructures are placed on the two walls and three different distances are selected. The effect of wall materials was also examined by considering four different materials, namely, carbon, silicon, and two other hypothetical materials. In the second part of this study, a two-phase flow consisting of water and air have been simulated to study the effect of the trapped airs in the performance. The results show that in the carbon-made walls for the design with minimum pressure drop, the slip length increases by 97% and the flow rate increases by 200%. The increases for silicon-made walls and similar sizes are 99 and 183%, for the slip length and the flow rate, respectively. The slip length for carbon-made walls is almost 3 times larger than the silicon-made walls. Finally, by increasing the air fraction up to 30% in the carbon-made walls, the slip length increases by 430% and the flow rate increases by 310%, and also, for the silicon-made walls, the increases are 380 and 360% for the slip length and the flow rate, respectively.

Stability of slip channel flow revisited

In this work, we revisit the temporal stability of slip channel flow. Lauga and Cossu [“A note on the stability of slip channel flows,” Phys. Fluids 17, 088106 (2005)] and Min and Kim [“Effects of hydrophobic surface on stability and transition,” Phys. Fluids 17, 108106 (2005)] have investigated both modal stability and non-normality of slip channel flow and concluded that the velocity slip greatly suppresses linear instability and only modestly affects the non-normality. Here, we study the stability of channel flow with streamwise and spanwise slip separately as two limiting cases of anisotropic slip and explore a broader range of slip length than previous studies did. We find that, with a sufficiently large slip, both streamwise and spanwise slip trigger three-dimensional leading instabilities. Overall, the critical Reynolds number is only slightly increased by streamwise slip, whereas it can be greatly decreased by spanwise slip. Streamwise slip suppresses the nonmodal transient growth, whereas the spanwise slip enlarges the nonmodal growth, although it does not affect the base flow. Interestingly, as the spanwise slip length increases, the optimal perturbations exhibit flow structures different from the well-known streamwise rolls. However, in the presence of equal slip in both directions, the three-dimensional leading instabilities disappear and the flow is greatly stabilized. The results suggest that earlier instability and larger transient growth can be triggered by introducing anisotropy in the velocity slip.

Numerical study of droplet formation in the T-junction microchannel with wall velocity slip

Effect of hydrophobicity on droplet formation in the T-junction microfluidic devices is numerically studied. A Navier slip model is adopted. Results reveal that, as increasing of slip length, the velocity profile across the channel becomes flattened. With the increase of slip length, the shear stress on dispersed phase exerted by the continuous phase decreases, thus, the deformation of the dispersed phase weakens while the final droplet size increases. The pressure distributions along the central axis of main channel show a smaller pressure drop when the wall velocity slip length increases.

Effect of nano-confinement on high pressure methane flow characteristics

High pressure gas flow in shale nanoporous media plays an important role in shale gas recovery. The objective of this work is to study the effect of confinement on high pressure methane flow in nanoscale pores. We used dual control volume grand canonical molecular dynamics for different temperatures and pore sizes and compared our results with conventional hydrodynamic equation. We observed that when pore size is larger than 2 nm, flux increases with pore size. But when pore size is less than 2 nm, flux shows varying behaviors depending on the pore size due to methane molecular configurations and structures under different nano-confinements. We also observed that enhancement over the prediction from Hagen-Poiseuille equation can be more than two orders of magnitude in nanopores of around 1 nm pore size. Similar to previous computer simulation and experiments on liquid flow in nanopores, the dependences of the enhancement factor ε and the calculated slip length Ls on the pore sizes show discontinuity within three distinct regions: R1) pore size less than 0.8 nm; R2) between 0.8 and 1.0 nm; and R3) larger than 1.0 nm. Within R2 and R3, ε and Ls monotonically decrease with pore size. The methane molecular flux, enhancement factor, and calculated slip length increase from 328.15 K to 368.15 K, but remain comparable for 368.15 K and 408.15 K.

Molecular Dynamics Study of Fluid Solid Interfacial Slip and Its Effect on Aerodynamic Drag

Interfacial hydrodynamic slippage is controlled by two factors say physical structure and chemical composition. Various studies have been conducted experimentally which try to connect the physical structure of the surface and its chemical property on the interfacial wettability. One such example is the Tunable wettability in surface-modified ZnO-based hierarchical nanostructures [2]. In which vertically aligned Nanoneedles and Nanonails were employed as a platform to determine the effect of surface structure. According to which a variation in static contact angles were observed as the cap size the nanonails constantly increased. Starting with a contact angle of 104° the contact angle first increases and then decreases, which means that the slip length first increases and then decreases. The increase in slip length reduces the drag, which has immense application in the aerodynamic field. This paper investigates the relation between the chemical wettability and aerodynamic drag by performing MD simulations of couette flow with varying fluid-surface interaction.