80,801 research outputs found
Optimized cross-slot flow geometry for microfluidic extension rheometry
A precision-machined cross-slot flow geometry with a shape that has been optimized by numerical simulation of the fluid kinematics is fabricated and used to measure the extensional viscosity of a dilute polymer solution. Full-field birefringence microscopy is used to monitor the evolution and growth of macromolecular anisotropy along the stagnation point streamline, and we observe the formation of a strong and uniform birefringent strand when the dimensionless flow strength exceeds a critical Weissenberg number Wicrit 0:5. Birefringence and bulk pressure drop measurements provide self consistent estimates of the planar extensional viscosity of the fluid over a wide range of deformation rates (26 s1 "_ 435 s1) and are also in close agreement with numerical simulations performed by using a finitely extensible nonlinear elastic dumbbell model
Epitaxial Transition from Gyroid to Cylinder in a Diblock Copolymer Melt
An epitaxial transition from a bicontinious double gyroid to a hexagonally
packed cylinder structure induced by an external flow is simulated using
real-space dynamical self-consistent field technique. In order to simulate the
structural change correctly, we introduce a system size optimization technique
by which emergence of artificial intermediate structures are suppressed. When a
shear flow in [111] direction of the gyroid unit cell is imposed, a nucleation
and growth of the cylinder domains is observed. We confirm that the generated
cylindrical domains grow epitaxially to the original gyroid domains as gyroid
cylinder . In a steady state under the shear
flow, the gyroid shows different reconnection processes depending on the
direction of the velocity gradient of the shear flow. A kinetic pathway
previously predicted using the self-consistent field theory where three fold
junctions transform into five fold junctions as an intermediate state is not
observed.Comment: 24 pages, 14 figures, submitted to Macromolecule
Optimal sensing for fish school identification
Fish schooling implies an awareness of the swimmers for their companions. In
flow mediated environments, in addition to visual cues, pressure and shear
sensors on the fish body are critical for providing quantitative information
that assists the quantification of proximity to other swimmers. Here we examine
the distribution of sensors on the surface of an artificial swimmer so that it
can optimally identify a leading group of swimmers. We employ Bayesian
experimental design coupled with two-dimensional Navier Stokes equations for
multiple self-propelled swimmers. The follower tracks the school using
information from its own surface pressure and shear stress. We demonstrate that
the optimal sensor distribution of the follower is qualitatively similar to the
distribution of neuromasts on fish. Our results show that it is possible to
identify accurately the center of mass and even the number of the leading
swimmers using surface only information
Efficiency optimization and symmetry-breaking in a model of ciliary locomotion
A variety of swimming microorganisms, called ciliates, exploit the bending of
a large number of small and densely-packed organelles, termed cilia, in order
to propel themselves in a viscous fluid. We consider a spherical envelope model
for such ciliary locomotion where the dynamics of the individual cilia are
replaced by that of a continuous overlaying surface allowed to deform
tangentially to itself. Employing a variational approach, we determine
numerically the time-periodic deformation of such surface which leads to
low-Reynolds locomotion with minimum rate of energy dissipation (maximum
efficiency). Employing both Lagrangian and Eulerian points of views, we show
that in the optimal swimming stroke, individual cilia display weak asymmetric
beating, but that a significant symmetry-breaking occurs at the organism level,
with the whole surface deforming in a wave-like fashion reminiscent of
metachronal waves of biological cilia. This wave motion is analyzed using a
formal modal decomposition, is found to occur in the same direction as the
swimming direction, and is interpreted as due to a spatial distribution of
phase-differences in the kinematics of individual cilia. Using additional
constrained optimizations, as well as a constructed analytical ansatz, we
derive a complete optimization diagram where all swimming efficiencies,
swimming speeds, and amplitude of surface deformation can be reached, with the
mathematically optimal swimmer, of efficiency one half, being a singular limit.
Biologically, our work suggests therefore that metachronal waves may allow
cilia to propel cells forward while reducing the energy dissipated in the
surrounding fluid.Comment: 29 pages, 20 figure
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