929 research outputs found
Analytical solution of Stokes flow inside an evaporating sessile drop: Spherical and cylindrical cap shapes
Exact analytical solutions are derived for the Stokes flows within
evaporating sessile drops of spherical and cylindrical cap shapes. The results
are valid for arbitrary contact angle. Solutions are obtained for arbitrary
evaporative flux distributions along the free surface as long as the flux is
bounded at the contact line. The field equations, E^4(Psi)=0 and Del^4(Phi)=0,
are solved for the spherical and cylindrical cap cases, respectively. Specific
results and computations are presented for evaporation corresponding to uniform
flux and to purely diffusive gas phase transport into an infinite ambient.
Wetting and non-wetting contact angles are considered with the flow patterns in
each case being illustrated. For the spherical cap with evaporation controlled
by vapor phase diffusion, when the contact angle lies in the range
0<theta_c<pi, the mass flux of vapor becomes singular at the contact line. This
condition required modification when solving for the liquid phase transport.
Droplets in all of the above categories are considered for the following two
cases: the contact lines are either pinned or free to move during evaporation.
The present viscous flow behavior is compared to the inviscid flow behavior
previously reported. It is seen that the streamlines for viscous flow lie
farther from the substrate than the corresponding inviscid ones.Comment: Revised version; in review in Physics of Fluid
Which canonical algebras are derived equivalent to incidence algebras of posets?
We give a full description of all the canonical algebras over an
algebraically closed field that are derived equivalent to incidence algebras of
finite posets. These are the canonical algebras whose number of weights is
either 2 or 3.Comment: 8 pages; slight revision; to appear in Comm. Algebr
No many-scallop theorem: Collective locomotion of reciprocal swimmers
To achieve propulsion at low Reynolds number, a swimmer must deform in a way
that is not invariant under time-reversal symmetry; this result is known as the
scallop theorem. We show here that there is no many-scallop theorem. We
demonstrate that two active particles undergoing reciprocal deformations can
swim collectively; moreover, polar particles also experience effective
long-range interactions. These results are derived for a minimal dimers model,
and generalized to more complex geometries on the basis of symmetry and scaling
arguments. We explain how such cooperative locomotion can be realized
experimentally by shaking a collection of soft particles with a homogeneous
external field
Non-equilibrium hydrodynamics of a rotating filament
The nonlinear dynamics of an elastic filament that is forced to rotate at its
base is studied by hydrodynamic simulation techniques; coupling between
stretch, bend, twist elasticity and thermal fluctuations is included. The
twirling-overwhirling transition is located and found to be strongly
discontinuous. For finite bend and twist persistence length, thermal
fluctuations lower the threshold rotational frequency, for infinite persistence
length the threshold agrees with previous analytical predictions
Symmetric three-particle motion in Stokes flow: equilibrium for heavy spheres in contrast to "end-of-world" for point forces
A stationary stable solution of the Stokes equations for three identical
heavy solid spheres falling in a vertical plane is found. It has no analog in
the point-particle approximation. Three spheres aligned horizontally at equal
distances evolve towards the equilibrium relative configuration while the point
particles collapse onto a single point in a finite time.Comment: 4 pages, 7 figure
Brownian Dynamics of a Sphere Between Parallel Walls
We describe direct imaging measurements of a colloidal sphere's diffusion
between two parallel surfaces. The dynamics of this deceptively simple
hydrodynamically coupled system have proved difficult to analyze. Comparison
with approximate formulations of a confined sphere's hydrodynamic mobility
reveals good agreement with both a leading-order superposition approximation as
well as a more general all-images stokeslet analysis.Comment: 4 pages, 3 figures, REVTeX with PostScript figure
Microscale swimming: The molecular dynamics approach
The self-propelled motion of microscopic bodies immersed in a fluid medium is
studied using molecular dynamics simulation. The advantage of the atomistic
approach is that the detailed level of description allows complete freedom in
specifying the swimmer design and its coupling with the surrounding fluid. A
series of two-dimensional swimming bodies employing a variety of propulsion
mechanisms -- motivated by biological and microrobotic designs -- is
investigated, including the use of moving limbs, changing body shapes and fluid
jets. The swimming efficiency and the nature of the induced, time-dependent
flow fields are found to differ widely among body designs and propulsion
mechanisms.Comment: 5 pages, 3 figures (minor changes to text
Influence of flow confinement on the drag force on a static cylinder
The influence of confinement on the drag force on a static cylinder in a
viscous flow inside a rectangular slit of aperture has been investigated
from experimental measurements and numerical simulations. At low enough
Reynolds numbers, varies linearly with the mean velocity and the viscosity,
allowing for the precise determination of drag coefficients and
corresponding respectively to a mean flow parallel and
perpendicular to the cylinder length . In the parallel configuration, the
variation of with the normalized diameter of the
cylinder is close to that for a 2D flow invariant in the direction of the
cylinder axis and does not diverge when . The variation of
with the distance from the midplane of the model reflects the
parabolic Poiseuille profile between the plates for while it
remains almost constant for . In the perpendicular configuration,
the value of is close to that corresponding to a 2D system
only if and/or if the clearance between the ends of the cylinder
and the side walls is very small: in that latter case,
diverges as due to the blockage of the flow. In other cases, the
side flow between the ends of the cylinder and the side walls plays an
important part to reduce : a full 3D description of the flow is
needed to account for these effects
On Berenstein-Douglas-Seiberg Duality
I review the proposal of Berenstein-Douglas for a completely general
definition of Seiberg duality. To give evidence for their conjecture I present
the first example of a physical dual pair and explicitly check that it
satisfies the requirements. Then I explicitly show that a pair of toric dual
quivers is also dual according to their proposal. All these computations go
beyond tilting modules, and really work in the derived category. I introduce
all necessary mathematics where needed.Comment: 22 pages, LaTe
A dynamic density functional theory for particles in a flowing solvent
We present a dynamic density functional theory (dDFT) which takes into accou
nt the advection of the particles by a flowing solvent. For potential flows we
can use the same closure as in the absence of solvent flow. The structure of
the resulting advected dDFT suggests that it could be used for non-potential
flows as well. We apply this dDFT to Brownian particles (e.g., polymer coils)
in a solvent flowing around a spherical obstacle (e.g., a colloid) and compare
the results with direct simulations of the underlying Brownian dynamics.
Although numerical limitations do not allow for an accurate quantitative
check of the advected dDFT both show the same qualitative features. In contrast
to previous works which neglected the deformation of the flow by the obstacle,
we find that the bow-wave in the density distribution of particles in front of
the obstacle as well as the wake behind it are reduced dramatically. As a
consequence the friction force exerted by the (polymer) particles on the
colloid can be reduced drastically.Comment: 7 pages, 5 figures, 2 tables, submitte
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