Slip of Liquid Flow at a Hydrophobic Surface : A Simulation Model for Apparent Slip Velocity

2003 ◽  
Vol 2003.2 (0) ◽  
pp. 231-232
Author(s):  
Takao FUJITA ◽  
Keizo WATANABE
Keyword(s):  
Simulation Model ◽  
Liquid Flow ◽  
Slip Velocity ◽  
Hydrophobic Surface ◽  
Apparent Slip

Near-Surface Velocimetry Using Evanescent Wave Illumination

2003 ◽  
Author(s):  
Songwan Jin ◽  
Peter Huang ◽  
Jinil Park ◽  
Jung Yul Yoo ◽  
Kenneth S. Breuer
Keyword(s):  
Slip Velocity ◽  
Finite Thickness ◽  
Fluorescent Microscopy ◽  
Direct Consequence ◽  
Microfluidic System ◽  
Wall Region ◽  
Near Surface ◽  
Shear Rates ◽  
Seed Particles ◽  
Apparent Slip

Total internal reflection fluorescent microscopy (TIRFM) is used to measure particle motion in the near wall region of a microfluidic system. TIRFM images have minimum background noise and contain only particles that are very close to channel surface, where slip velocities may be present. Submicron sized fluorescent particles suspended in water are used as seed particles and images are analyzed with a PTV algorithm to extract information about apparent slip velocity. At relatively low shear rates (less than 2500 sec−1), an apparent slip velocity, proportional to the shear rate was observed. However, numerical simulations show that this observation is a direct consequence of the small, but finite thickness of the illuminated region, and most likely not due to physical slip at the surface. The statistical difference in apparent slip velocities measured over hydrophilic and hydrophobic surfaces is found to be minimal. Issues associated with the experimental technique and the interpretation of the experimental results are also discussed.

Flow at the interface of a model fibrous porous medium

2001 ◽  
Vol 426 ◽  
pp. 47-72 ◽  
Author(s):  
DAVID F. JAMES ◽  
ANTHONY M. J. DAVIS
Keyword(s):  
Porous Medium ◽  
Slip Velocity ◽  
Volume Fraction ◽  
Solid Volume Fraction ◽  
Circular Cylinders ◽  
Interfacial Region ◽  
Filling Fraction ◽  
Apparent Slip ◽  
Square Array ◽  
Pressure Driven Flow

Planar flow in the interfacial region of an open porous medium is investigated by finding solutions for Stokes flow in a channel partially filled with an array of circular cylinders beside one wall. The cylinders are in a square array oriented across the flow and are widely spaced, so that the solid volume fraction ϕ is 0.1 or less. For this spacing, singularity methods are appropriate and so they are used to find solutions for both planar Couette flow and Poiseuille flow in the open portion of the channel. The solutions, accurate to O(ϕ), are used to calculate the apparent slip velocity at the interface, Us, and results obtained for Us are presented in terms of a dimensionless slip velocity. For shear-driven flow, this dimensionless quantity is found to depend only weakly on ϕ and to be independent of the height of the array relative to the height of the channel and independent of the cylinder size relative to the height of the channel. For pressure-driven flow, Us is found to be less than that under comparable shear-flow conditions, and dependent on cylinder size and filling fraction in this case. Calculations also show that the external flow penetrates the porous medium very little, even for sparse arrays, and that Us is about one quarter of the velocity predicted by the Brinkman model.

Apparent slip velocity of paper coatings in viscometric flows

2000 ◽  
Vol 15 (5) ◽  
pp. 502-508 ◽  
Author(s):  
Annaleena Kokko ◽  
Tom Grankvist ◽  
Nick Triantafillopoulos
Keyword(s):  
Slip Velocity ◽  
Paper Coatings ◽  
Apparent Slip

Analysis of Laminar Heat Transfer on Super-Hydrophobic Surface With Slip Velocity

2009 ◽  
Author(s):  
Wei Sun ◽  
Yuhong Zhou ◽  
Xinxin Fan ◽  
Tianqing Liu
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Thermal Resistance ◽  
Transfer Coefficient ◽  
Slip Velocity ◽  
Structural Parameters ◽  
Hydrophobic Surface ◽  
Trapped Air ◽  
Effective Heat ◽  
Effective Heat Transfer Coefficient

It is still in suspense for the effects of slip velocity and structural parameters on the heat transfer on a super-hydrophobic surface. It is thus necessary to study it in both theory and experiments. In this paper, the convective heat transfer with constant heat flux condition inside a circular microchannel was investigated. The velocity and temperature profiles when slip velocity exists were derived firstly, and then the heat transfer coefficient and Nusselt number were obtained. Furthermore, an effective conduction model for the super-hydrophobic surface with different structural parameters was proposed and the thermal resistance of the surface with trapped air was calculated. Finally, the effective heat transfer coefficient of super-hydrophobic surface was found with the integration of heat transfer coefficient and surface thermal resistance. The calculation results show that 1) the slip of fluid on a super-hydrophobic surface makes the temperature profile inside the channel more uniform, and the heat transfer coefficient or Nusselt number increased, 1.8 times higher maximally under constant heat flux condition; 2) the thermal resistance of super-hydrophobic surface increases with trapped air volume; 3) the effective heat transfer coefficient on super-hydrophobic surface declines seriously with trapped air volume, especially with the trapped air area; 4) there exists a critical thickness for the trapped air on a super-hydrophobic surface with given surface structural parameters, under which the effective heat transfer coefficient is not less than that on normal surfaces without slip. Therefore, it is necessary to consider the structural parameters of super-hydrophobic surfaces, such as rib height and distance between ribs, so that the heat transfer on the super-hydrophobic surface will not be impacted by the trapped air.

On the measurement of slip length for liquid flow over super-hydrophobic surface

2009 ◽  
Vol 54 (24) ◽  
pp. 4560-4565 ◽  
Author(s):  
Jian Li ◽  
Ming Zhou ◽  
Lan Cai ◽  
Xia Ye ◽  
Run Yuan
Keyword(s):  
Liquid Flow ◽  
Slip Length ◽  
Hydrophobic Surface

Roughness Effects on Continuous and Discrete Flows in Superhydrophobic Microchannels

2011 ◽  
Vol 9 (5) ◽  
pp. 1094-1105 ◽  
Author(s):  
Junfeng Zhang ◽  
Daniel Y. Kwok
Keyword(s):  
Surface Roughness ◽  
Contact Angle ◽  
Slip Length ◽  
Contact Angle Hysteresis ◽  
Hydrophobic Surface ◽  
Contact Angles ◽  
Lattice Boltzmann Model ◽  
Roughness Effects ◽  
Apparent Slip ◽  
Velocity Changes

AbstractThe dynamic behaviors of continuous and discrete flows in superhydrophobic microchannels are investigated with a lattice Boltzmann model. Typical characters of the superhydrophobic phenomenon are well observed from our simulations, including air trapped in the surface microstructures, high contact angles, low contact angle hysteresis, and reduced friction to fluid motions. Increasing the roughness of a hydrophobic surface can produce a large flow rate through the channel due to the trapped air, implying less friction or large apparent slip. The apparent slip length appears to be independent to the channel width and could be considered as a surface property. For a moving droplet, its behavior is affected by the surface roughness from two aspects: the contact angle difference between its two ends and the surface-liquid interfacial friction. As a consequence, the resulting droplet velocity changes with the surface roughness as firstly decreasing and then increasing. Simulation results are also compared with experimental observations and better agreement has been obtained than that from other numerical method. The information from this study could be valuable for microfluidic systems.

Digitized Heat Transfer for Thermal Management of Compact Microsystems

2005 ◽  
Author(s):  
Kamran Mohseni ◽  
Eric Baird ◽  
Hongwu Zhao
Keyword(s):  
Heat Transfer ◽  
Substrate Temperature ◽  
Thermal Management ◽  
Liquid Flow ◽  
Slip Velocity ◽  
Active Management ◽  
Contact Lines ◽  
Electrowetting On Dielectric ◽  
Wall Boundary ◽  
Receding Contact

Active thermal management of compact microsystems by a periodic array of discrete liquid metal droplets is proposed and referred to as “digitized heat transfer.” This is in contrast to convective heat transfer by a continuous liquid flow. Two methods of droplet actuation, electrowetting on dielectric and continuous electrowetting, are described. Liquid metals or alloys support significantly higher heat transfer rates than other fluids, such as water or air. In addition, electrowetting is an efficient method of microscale fluid control, requiring low actuation voltages and very little power consumption. These concepts are used in this investigation to design an active management technique for high-power-density electronic and integrated micro systems. Preliminary calculations indicate that this technique could potentially offer a viable cooling strategy for achieving some of the most important objectives of electronic cooling, i.e., minimization of the maximum substrate temperature, reduction of the substrate temperature gradient and removing substrate hot spots. Numerical simulation of a droplet in a microchannel is also investigated. We propose a technique for dynamically calculating the slip velocity at the wall boundary including both the advancing and receding contact lines. The technique is based on the observed non-Newtonian behavior of a continuous liquid flow at high shear rates and its associated slip velocity (Thompson and Trioan 1997). While most of the wall boundary has negligible slip, significant slip at the advancing and receding contact lines are calculated from the data itself.

scholarly journals Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions

2018 ◽  
Vol 843 ◽  
pp. 450-478 ◽  
Author(s):  
Pedro Costa ◽  
Francesco Picano ◽  
Luca Brandt ◽  
Wim-Paul Breugem
Keyword(s):  
Particle Size ◽  
Newtonian Fluid ◽  
Size Effects ◽  
Slip Velocity ◽  
Finite Size ◽  
Particle Dynamics ◽  
Wall Layer ◽  
Shear Stresses ◽  
Finite Size Effects ◽  
Apparent Slip

We use interface-resolved numerical simulations to study finite-size effects in turbulent channel flow of neutrally buoyant spheres. Two cases with particle sizes differing by a factor of two, at the same solid volume fraction of 20 % and bulk Reynolds number are considered. These are complemented with two reference single-phase flows: the unladen case, and the flow of a Newtonian fluid with the effective suspension viscosity of the same mixture in the laminar regime. As recently highlighted in Costa et al. (Phys. Rev. Lett., vol. 117, 2016, 134501), a particle–wall layer is responsible for deviations of the mesoscale-averaged statistics from what is observed in the continuum limit where the suspension is modelled as a Newtonian fluid with (higher) effective viscosity. Here we investigate in detail the fluid and particle dynamics inside this layer and in the bulk. In the particle–wall layer, the near-wall inhomogeneity has an influence on the suspension microstructure over a distance proportional to the particle size. In this layer, particles have a significant (apparent) slip velocity that is reflected in the distribution of wall shear stresses. This is characterized by extreme events (both much higher and much lower than the mean). Based on these observations we provide a scaling for the particle-to-fluid apparent slip velocity as a function of the flow parameters. We also extend the scaling laws in Costa et al. (Phys. Rev. Lett., vol. 117, 2016, 134501) to second-order Eulerian statistics in the homogeneous suspension region away from the wall. The results show that finite-size effects in the bulk of the channel become important for larger particles, while negligible for lower-order statistics and smaller particles. Finally, we study the particle dynamics along the wall-normal direction. Our results suggest that single-point dispersion is dominated by particle–turbulence (and not particle–particle) interactions, while differences in two-point dispersion and collisional dynamics are consistent with a picture of shear-driven interactions.

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