737 research outputs found
Roughness induced boundary slip in microchannel flows
Surface roughness becomes relevant if typical length scales of the system are
comparable to the scale of the variations as it is the case in microfluidic
setups. Here, an apparent boundary slip is often detected which can have its
origin in the assumption of perfectly smooth boundaries. We investigate the
problem by means of lattice Boltzmann (LB) simulations and introduce an
``effective no-slip plane'' at an intermediate position between peaks and
valleys of the surface. Our simulations show good agreement with analytical
results for sinusoidal boundaries, but can be extended to arbitrary geometries
and experimentally obtained surface data. We find that the detected apparent
slip is independent of the detailed boundary shape, but only given by the
distribution of surface heights. Further, we show that the slip diverges as the
amplitude of the roughness increases.Comment: 4 pages, 6 figure
Tensorial hydrodynamic slip
We describe a tensorial generalization of the Navier slip boundary condition
and illustrate its use in solving for flows around anisotropic textured
surfaces. Tensorial slip can be derived from molecular or microstructural
theories or simply postulated as an constitutive relation, subject to certain
general constraints on the interfacial mobility. The power of the tensor
formalism is to capture complicated effects of surface anisotropy, while
preserving a simple fluid domain. This is demonstrated by exact solutions for
laminar shear flow and pressure-driven flow between parallel plates of
arbitrary and different textures. From such solutions, the effects of rotating
a texture follow from simple matrix algebra. Our results may be useful to
extracting local slip tensors from global measurements, such as the
permeability of a textured channel or the force required to move a patterned
surface, in experiments or simulations.Comment: 10 page
Simulation of fluid flow in hydrophobic rough microchannels
Surface effects become important in microfluidic setups because the surface
to volume ratio becomes large. In such setups the surface roughness is not any
longer small compared to the length scale of the system and the wetting
properties of the wall have an important influence on the flow. However, the
knowledge about the interplay of surface roughness and hydrophobic
fluid-surface interaction is still very limited because these properties cannot
be decoupled easily in experiments.
We investigate the problem by means of lattice Boltzmann (LB) simulations of
rough microchannels with a tunable fluid-wall interaction. We introduce an
``effective no-slip plane'' at an intermediate position between peaks and
valleys of the surface and observe how the position of the wall may change due
to surface roughness and hydrophobic interactions.
We find that the position of the effective wall, in the case of a Gaussian
distributed roughness depends linearly on the width of the distribution.
Further we are able to show that roughness creates a non-linear effect on the
slip length for hydrophobic boundaries.Comment: 10 pages, 5 figure
Longitudinal and transversal flow over a cavity containing a second immiscible fluid
An analytical solution for the flow field of a shear flow over a rectangular
cavity containing a second immiscible fluid is derived. While flow of a
single-phase fluid over a cavity is a standard case investigated in fluid
dynamics, flow over a cavity which is filled with a second immiscible fluid,
has received little attention. The flow filed inside the cavity is considered
to define a boundary condition for the outer flow which takes the form of a
Navier slip condition with locally varying slip length. The slip-length
function is determined from the related problem of lid-driven cavity flow.
Based on the Stokes equations and complex analysis it is then possible to
derive a closed analytical expression for the flow field over the cavity for
both the transversal and the longitudinal case. The result is a comparatively
simple function, which displays the dependence of the flow field on the cavity
geometry and the medium filling the cavity. The analytically computed flow
field agrees well with results obtained from a numerical solution of the
Navier-Stokes equations. The studies presented in this article are of
considerable practical relevance, for example for the flow over
superhydrophobic surfaces.Comment: http://journals.cambridge.or
Slip or not slip? A methodical examination of the interface formation model using two-dimensional droplet spreading on a horizontal planar substrate as a prototype system
We consider the spreading of a thin two-dimensional droplet on a planar
substrate as a prototype system to compare the contemporary model for contact
line motion based on interface formation of Shikhmurzaev [Int. J. Multiphas.
Flow 19, 589 (1993)], to the more commonly used continuum fluid dynamical
equations augmented with the Navier-slip condition. Considering quasistatic
droplet evolution and using the method of matched asymptotics, we find that the
evolution of the droplet radius using the interface formation model reduces to
an equivalent expression for a slip model, where the prescribed microscopic
dynamic contact angle has a velocity dependent correction to its static value.
This result is found for both the original interface formation model
formulation and for a more recent version, where mass transfer from bulk to
surface layers is accounted for through the boundary conditions. Various
features of the model, such as the pressure behaviour and rolling motion at the
contact line, and their relevance, are also considered in the prototype system
we adopt.Comment: 45 pages, 18 figure
Cyclic motion and inversion of surface flow direction in a dense polymer brush under shear
Using molecular simulations, we study the properties of a polymer brush in
contact with an explicit solvent under Couette and Poiseuille flow. The solvent
is comprised of chemically identical chains. We present evidence that
individual, unentangled chains in the dense brush exhibit cyclic, tumbling
motion and non-Gaussian fluctuations of the molecular orientations similar to
the behaviour of isolated tethered chains in shear flow. The collective
molecular motion gives rise to an inversion of hydrodynamic flow direction in
the vicinity of the brush-coated surface. Utilising Couette and Poiseuille
flow, we investigate to what extend the effect of a brush-coated surface can be
described by a Navier slip condition.Comment: 6 pages, 6 figures, submitted for publicatio
Measurement of Newtonian fluid slip using a torsional ultrasonic oscillator
The composite torsional ultrasonic oscillator, a versatile experimental
system, can be used to investigate slip of Newtonian fluid at a smooth surface.
A rigorous analysis of slip-dependent damping for the oscillator is presented.
Initially, the phenomenon of finite surface slip and the slip length are
considered for a half-space of Newtonian fluid in contact with a smooth,
oscillating solid surface. Definitions are revisited and clarified in light of
inconsistencies in the literature. We point out that, in general oscillating
flows, Navier's slip length b is a complex number. An intuitive velocity
discontinuity parameter of unrestricted phase is used to describe the effect of
slip on measurement of viscous shear damping. The analysis is applied to the
composite oscillator and preliminary experimental work for a 40 kHz oscillator
is presented. The Non-Slip Boundary Condition (NSBC) has been verified for a
hydrophobic surface in water to within ~60 nm of |b|=0 nm. Experiments were
carried out at shear rate amplitudes between 230 and 6800 /s, corresponding to
linear displacement amplitudes between 3.2 and 96 nm.Comment: Revised with minor edits for revie
Hydraulic engineering legends Listed on the Eiffel Tower
While the Eiffel Tower has become a landmark of Paris and France, few know about the names of 72 scientists engraved around the first floor. Herein, the names of 14 hydraulic engineers and scholars are reviewed and their selection is discussed. It is shown that most were leading engineers and lecturers during the French Revolution and early 19th century, and Gustave Eiffel's selection highlighted the influence of leading engineers on the French Society
Statics and dynamics of a cylindrical droplet under an external body force
We study the rolling and sliding motion of droplets on a corrugated substrate
by Molecular Dynamics simulations. Droplets are driven by an external body
force (gravity) and we investigate the velocity profile and dissipation
mechanisms in the steady state. The cylindrical geometry allows us to consider
a large range of droplet sizes. The velocity of small droplets with a large
contact angle is dominated by the friction at the substrate and the velocity of
the center of mass scales like the square root of the droplet size. For large
droplets or small contact angles, however, viscous dissipation of the flow
inside the volume of the droplet dictates the center of mass velocity that
scales linearly with the size. We derive a simple analytical description
predicting the dependence of the center of mass velocity on droplet size and
the slip length at the substrate. In the limit of vanishing droplet velocity we
quantitatively compare our simulation results to the predictions and good
agreement without adjustable parameters is found.Comment: Submitted to the Journal of Chemical Physic
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