353 research outputs found
Assembly Modulated by Particle Position and Shape: A New Concept in Self-Assembly
In this communication we outline how the bespoke arrangements and design of micron-sized superparamagnetic shapes provide levers to modulate their assembly under homogeneous magnetic fields. We label this new approach, ‘assembly modulated by particle position and shape’ (APPS). Specifically, using rectangular lattices of superparamagnetic micron-sized cuboids, we construct distinct microstructures by adjusting lattice pitch and angle of array with respect to a magnetic field. Broadly, we find two modes of assembly: (1) immediate 2D jamming of the cuboids as they rotate to align with the applied field (rotation-induced jamming) and (2) aggregation via translation after their full alignment (dipole-dipole assembly). The boundary between these two assembly pathways is independent on field strength being solely a function of the cuboid’s dimensions, lattice pitch, and array angle with respect to field—a relationship which we capture, along with other features of the assembly process, in a ‘phase diagram’. In doing so, we set out initial design rules to build custom made assemblies. Moreover, these assemblies can be made flexible thanks to the hinged contacts of their particle building blocks. This flexibility, combined with the superparamagnetic nature of the architectures, renders our assembly method particularly appropriate for the construction of complex actuators at a scale hitherto not possible
Microfluidic In Situ Measurement of Poisson's Ratio of Hydrogels
Being able to precisely characterize the mechanical properties of soft
microparticles is essential for numerous situations from the understanding of
the flow of biological fluids to the development of soft micro-robots. Here we
present a simple measurement technique for the Poisson's ratio of soft
micron-sized hydrogels in the presence of a surrounding liquid. This methods
relies on the measurement of the deformation in two orthogonal directions of a
rectangular hydrogel slab compressed uni-axially inside a microfluidic channel.
Due to the in situ character of the method, the sample does not need to be
dried, allowing for the measurement of the mechanical properties of swollen
hydrogels. Using this method we determine the Poisson's ratio of hydrogel
particles composed of polyethylene glycol (PEG) and varying solvents fabricated
using a lithography technique. The results demonstrate with high precision the
dependence of the hydrogel compressibility on the solvent fraction and
character. The method, easy to implement, can be adapted for the measurement of
a variety of soft and biological materials
Dynamics of flexible fibers in viscous flows and fluids
International audienceThe dynamics and deformations of immersed flexible fibers are at the heart of important industrial and biological processes, induce peculiar mechanical and transport properties in the fluids that contain them, and are the basis for novel methods of flow control. Here we focus on the low Reynolds number regime where advances in studying these fiber-fluid systems have been especially rapid. On the experimental side this is due to new methods of fiber synthesis, microfluidic flow control, and of microscope based tracking measurement techniques. Likewise, there have been continuous improvements in the specialized mathematical modeling and numerical methods needed to capture the interactions of slender flexible fibers with flows, boundaries, and each other
Collective beating of artificial microcilia
We combine technical, experimental and theoretical efforts to investigate the
collective dynamics of artificial microcilia in a viscous fluid. We take
advantage of soft-lithography and colloidal self-assembly to devise microcapets
made of hundreds of slender magnetic rods. This novel experimental setup is
used to investigate the dynamics of extended cilia arrays driven by a
precessing magnetic field. Whereas the dynamics of an isolated cilium is a
rigid body rotation, collective beating results in a symmetry breaking of the
precession patterns. The trajectories of the cilia are anisotropic and
experience a significant structural evolution as the actuation frequency
increases. We present a minimal model to account for our experimental findings
and demonstrate how the global geometry of the array imposes the shape of the
trajectories via long range hydrodynamic interactions.Comment: 5 pages, 3 figure
Morphological transitions of elastic filaments in shear flow
International audienceThe morphological dynamics, instabilities and transitions of elastic filaments in viscous flows underlie a wealth of biophysical processes from flagellar propulsion to intracellular streaming, and are also key to deciphering the rheological behavior of many complex fluids and soft materials. Here, we combine experiments and computational modeling to elucidate the dynamical regimes and morphological transitions of elastic Brownian filaments in a simple shear flow. Actin filaments are employed as an experimental model system and their conformations are investigated through fluorescence microscopy in microfluidic channels. Simulations matching the experimental conditions are also performed using inextensible Euler-Bernoulli beam theory and non-local slender-body hydrodynamics in the presence of thermal fluctuations, and agree quantitatively with observations. We demonstrate that filament dynamics in this system is primarily governed by a dimensionless elasto-viscous number comparing viscous drag forces to elastic bending forces, with thermal fluctuations only playing a secondary role. While short and rigid filaments perform quasi-periodic tumbling motions, a buckling instability arises above a critical flow strength. A second transition to strongly-deformed shapes occurs at a yet larger value of the elasto-viscous number and is characterized by the appearance of localized high-curvature bends that propagate along the filaments in apparent "snaking" motions. A theoretical model for the so far unexplored onset of snaking accurately predicts the transition and explains the observed dynamics. We present a complete characterization of filament morphologies and transitions as a function of elasto-viscous number and scaled persistence length and demonstrate excellent agreement between theory, experiments and simulations
Morphological transitions of elastic filaments in shear flow
The morphological dynamics, instabilities and transitions of elastic
filaments in viscous flows underlie a wealth of biophysical processes from
flagellar propulsion to intracellular streaming, and are also key to
deciphering the rheological behavior of many complex fluids and soft materials.
Here, we combine experiments and computational modeling to elucidate the
dynamical regimes and morphological transitions of elastic Brownian filaments
in a simple shear flow. Actin filaments are employed as an experimental model
system and their conformations are investigated through fluorescence microscopy
in microfluidic channels. Simulations matching the experimental conditions are
also performed using inextensible Euler-Bernoulli beam theory and non-local
slender-body hydrodynamics in the presence of thermal fluctuations, and agree
quantitatively with observations. We demonstrate that filament dynamics in this
system is primarily governed by a dimensionless elasto-viscous number comparing
viscous drag forces to elastic bending forces, with thermal fluctuations only
playing a secondary role. While short and rigid filaments perform
quasi-periodic tumbling motions, a buckling instability arises above a critical
flow strength. A second transition to strongly-deformed shapes occurs at a yet
larger value of the elasto-viscous number and is characterized by the
appearance of localized high-curvature bends that propagate along the filaments
in apparent "snaking" motions. A theoretical model for the so far unexplored
onset of snaking accurately predicts the transition and explains the observed
dynamics. For the first time, we present a complete characterization of
filament morphologies and transitions as a function of elasto-viscous number
and scaled persistence length and demonstrate excellent agreement between
theory, experiments and simulations.Comment: 17 pages, 12 figure
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