46 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, âassembly modulated by particle position and shapeâ (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 cuboidâs dimensions, lattice pitch, and array angle with respect to fieldâa relationship which we capture, along with other features of the assembly process, in a âphase diagramâ. 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
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
Un substrat de micropiliers pour Ă©tudier la migration cellulaire
Les propriĂ©tĂ©s mĂ©caniques des cellules jouent un rĂŽle prĂ©pondĂ©rant dans de nombreux Ă©vĂ©nements de la vie cellulaire comme le dĂ©veloppement embryonnaire, la formation des tissus ou encore le dĂ©veloppement des mĂ©tastases. La migration cellulaire est en partie caractĂ©risĂ©e par des interactions mĂ©caniques. Ainsi, les forces de traction quâexercent les cellules sur leur environnement impliquent, en parallĂšle, une rĂ©organisation dynamique des processus dâadhĂ©rence et du cytosquelette interne de la cellule. Pour Ă©valuer ces forces, un substrat a Ă©tĂ© dĂ©veloppĂ©, constituĂ© dâun rĂ©seau forte densitĂ© de micro-piliers dĂ©formables sur lequel se dĂ©placent les cellules. Cette surface est fabriquĂ©e par des mĂ©thodes de lithographie empruntĂ©es Ă la micro-Ă©lectronique. Les piliers mesurent environ un micromĂštre et sont en caoutchouc, donc suffisamment dĂ©formables pour flĂ©chir sous lâeffet des forces exercĂ©es par les cellules. Lâanalyse au microscope des dĂ©flexions individuelles de chaque pilier a permis de quantifier en temps rĂ©el les forces locales que des cellules exercent sur leur substrat lors de leurs processus dâadhĂ©rence et de dissociation.Mechanical forces play an important role in various cellular functions, such as tumor metastasis, embryonic development or tissue formation. Cell migration involves dynamics of adhesive processes and cytoskeleton remodelling, leading to traction forces between the cells and their surrounding extracellular medium. To study these mechanical forces, a number of methods have been developed to calculate tractions at the interface between the cell and the substrate by tracking the displacements of beads or microfabricated markers embedded in continuous deformable gels. These studies have provided the first reliable estimation of the traction forces under individual migrating cells. We have developed a new force sensor made of a dense array of soft micron-size pillars microfabricated using microelectronics techniques. This approach uses elastomeric substrates that are micropatterned by using a combination of hard and soft lithography. Traction forces are determined in real time by analyzing the deflections of each micropillar with an optical microscope. Indeed, the deflection is directly proportional to the force in the linear regime of small deformations. Epithelial cells are cultured on our substrates coated with extracellular matrix protein. First, we have characterized temporal and spatial distributions of traction forces of a cellular assembly. Forces are found to depend on their relative position in the monolayer : the strongest deformations are always localized at the edge of the islands of cells in the active areas of cell protrusions. Consequently, these forces are quantified and correlated with the adhesion/scattering processes of the cells
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