3,307 research outputs found
The creeping motion of liquid drops through a circular tube of comparable diameter
The creeping motion through a circular tube of neutrally buoyant Newtonian drops which have an undeformed radius comparable to that of the tube was studied experimentally. Both a Newtonian and a viscoelastic suspending fluid were used in order to determine the influence of viscoelasticity. The extra pressure drop owing to the presence of the suspended drops, the shape and velocity of the drops, and the streamlines of the flow are reported for various viscosity ratios, total flow rates and drop sizes
Migration of rigid spheres in a two-dimensional unidirectional shear flow of a second-order fluid
The lateral migration of a neutrally buoyant rigid sphere suspended in a second-order fluid is studied theoretically for unidirectional two-dimensional flows. The results demonstrate the existence of migration induced by normal stresses whenever there is a lateral variation of the shear rate in the undisturbed flow. The migration occurs in the direction of decreasing absolute shear rate, which is towards the centre-line for a plane Poiseuille flow and towards the outer cylinder wall for Couette flow. The direction of migration agrees with existing experimental data for a viscoelastic suspending fluid, and qualitative agreement is found between the theoretically predicted and experimentally measured sphere trajectories
The motion of a deformable drop in a second-order fluid
The cross-stream migration of a deformable drop in a unidirectional shear flow of a second-order fluid is considered. Expressions for the particle velocity due to the separate effects of deformation and viscoelastic rheology are obtained. The direction and magnitude of migration are calculated for the particular cases of Poiseuille flow and simple shear flow and compared with experimental data
Buoyancy-driven motion of a deformable drop toward a planar wall at low Reynolds number
The slow viscous motion of a deformable drop moving normal to a planar wall is studied numerically. In particular, a boundary integral technique employing the Green's function appropriate to a no-slip planar wall is used. Beginning with spherical drop shapes far from the wall, highly deformed and ‘dimpled’ drop configurations are obtained as the planar wall is approached. The initial stages of dimpling and their evolution provide information and insight into the basic assumptions of film-drainage theory
Stability of a horizontal viscous fluid layer in a vertical time periodic electric field
The stability of a horizontal interface between two viscous fluids, one of
which is conducting and the other is dielectric, acted upon by a vertical
time-periodic electric field is considered. The two fluids are bounded by
electrodes separated by a finite distance. By means of Floquet theory, the
marginal stability curves are obtained, thereby elucidating the dependency of
the critical voltage and wavenumber upon the fluid viscosities. The limit of
vanishing viscosities is shown to be in excellent agreement with the marginal
stability curves predicted by means of a Mathieu equation. The methodology to
obtain the marginal stability curves developed here is applicable to any
arbitrary but time periodic-signal, as demonstrated for the case of a signal
with two different frequencies. As a special case, the marginal stability
curves for an applied ac voltage biased by a dc voltage are depicted. It is
shown that the mode coupling caused by the normal stress at the interface due
to the electric field leads to appearance of harmonic modes and subharmonic
modes. This is in contrast to the application of a voltage with a single
frequency which always leads to a harmonic mode. Whether a harmonic or
subharmonic mode is the most unstable one depends on details of the excitation
signal. It is also shown that the electrode spacing has a distinct effect on
the stability bahavior of the system
Treatment of bimodality in proficiency test of pH in bioethanol matrix
The pH value in bioethanol is a quality control parameter related to its
acidity and to the corrosiveness of vehicle engines when it is used as fuel. In
order to verify the comparability and reliability of the measurement of pH in
bioethanol matrix among some experienced chemical laboratories, reference
material (RM) of bioethanol developed by Inmetro - the Brazilian National
Metrology Institute - was used in a proficiency testing (PT) scheme. There was
a difference of more than one unit in the value of the pH measured due to the
type of internal filling electrolytic solutions (potassium chloride, KCl or
lithium chloride, LiCl) from the commercial pH combination electrodes used by
the participant laboratories. Therefore, bimodal distribution has occurred from
the data of this PT scheme. This work aims to present the possibilities that a
PT scheme provider can use to overcome the bimodality problem. Data from the PT
of pH in bioethanol were treated by two different statistical approaches:
kernel density model and the mixture of distributions. Application of these
statistical treatments improved the initial diagnoses of PT provider, by
solving bimodality problem and contributing for a better performance evaluation
in measuring pH of bioethanol.Comment: 20 pages, 6 figures, Accepted for publication in Accreditation and
Quality Assurance (ACQUAL
Living bacteria rheology: population growth, aggregation patterns and cooperative behaviour under different shear flows
The activity of growing living bacteria was investigated using real-time and
in situ rheology -- in stationary and oscillatory shear. Two different strains
of the human pathogen Staphylococcus aureus -- strain COL and its isogenic cell
wall autolysis mutant -- were considered in this work. For low bacteria
density, strain COL forms small clusters, while the mutant, presenting
deficient cell separation, forms irregular larger aggregates. In the early
stages of growth, when subjected to a stationary shear, the viscosity of both
strains increases with the population of cells. As the bacteria reach the
exponential phase of growth, the viscosity of the two strains follow different
and rich behaviours, with no counterpart in the optical density or in the
population's colony forming units measurements. While the viscosity of strain
COL keeps increasing during the exponential phase and returns close to its
initial value for the late phase of growth, where the population stabilizes,
the viscosity of the mutant strain decreases steeply, still in the exponential
phase, remains constant for some time and increases again, reaching a constant
plateau at a maximum value for the late phase of growth. These complex
viscoelastic behaviours, which were observed to be shear stress dependent, are
a consequence of two coupled effects: the cell density continuous increase and
its changing interacting properties. The viscous and elastic moduli of strain
COL, obtained with oscillatory shear, exhibit power-law behaviours whose
exponent are dependent on the bacteria growth stage. The viscous and elastic
moduli of the mutant have complex behaviours, emerging from the different
relaxation times that are associated with the large molecules of the medium and
the self-organized structures of bacteria. These behaviours reflect
nevertheless the bacteria growth stage.Comment: 9 pages, 10 figure
Slip behavior in liquid films on surfaces of patterned wettability: Comparison between continuum and molecular dynamics simulations
We investigate the behavior of the slip length in Newtonian liquids subject
to planar shear bounded by substrates with mixed boundary conditions. The upper
wall, consisting of a homogenous surface of finite or vanishing slip, moves at
a constant speed parallel to a lower stationary wall, whose surface is
patterned with an array of stripes representing alternating regions of no-shear
and finite or no-slip. Velocity fields and effective slip lengths are computed
both from molecular dynamics (MD) simulations and solution of the Stokes
equation for flow configurations either parallel or perpendicular to the
stripes. Excellent agreement between the hydrodynamic and MD results is
obtained when the normalized width of the slip regions, , where is the (fluid) molecular diameter characterizing the
Lennard-Jones interaction. In this regime, the effective slip length increases
monotonically with to a saturation value. For and transverse flow configurations, the non-uniform interaction
potential at the lower wall constitutes a rough surface whose molecular scale
corrugations strongly reduce the effective slip length below the hydrodynamic
results. The translational symmetry for longitudinal flow eliminates the
influence of molecular scale roughness; however, the reduced molecular ordering
above the wetting regions of finite slip for small values of
increases the value of the effective slip length far above the hydrodynamic
predictions. The strong inverse correlation between the effective slip length
and the liquid structure factor representative of the first fluid layer near
the patterned wall illustrates the influence of molecular ordering effects on
slip in non-inertial flows.Comment: 12 pages, 10 figures Web reference added for animations:
http://www.egr.msu.edu/~priezjev/bubble/bubble.htm
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