170 research outputs found
Buckling instability causes inertial thrust for spherical swimmers at all scales
Microswimmers, and among them aspirant microrobots, generally have to cope
with flows where viscous forces are dominant, characterized by a low Reynolds
number (). This implies constraints on the possible sequences of body
motion, which have to be nonreciprocal. Furthermore, the presence of a strong
drag limits the range of resulting velocities. Here, we propose a swimming
mechanism, which uses the buckling instability triggered by pressure waves to
propel a spherical, hollow shell. With a macroscopic experimental model, we
show that a net displacement is produced at all regimes. An optimal
displacement caused by non-trivial history effects is reached at intermediate
. We show that, due to the fast activation induced by the instability, this
regime is reachable by microscopic shells. The rapid dynamics would also allow
high frequency excitation with standard traveling ultrasonic waves. Scale
considerations predict a swimming velocity of order 1 cm/s for a
remote-controlled microrobot, a suitable value for biological applications such
as drug delivery.Comment: To appear in Phys. Rev. Lett See demonstration movie on
https://www.youtube.com/watch?v=cEXMsFwEqs
Birth and growth of cavitation bubbles within water under tension confined in a simple synthetic tree
Water under tension, as can be found in several systems including tree
vessels, is metastable. Cavitation can spontaneously occur, nucleating a
bubble. We investigate the dynamics of spon- taneous or triggered cavitation
inside water filled microcavities of a hydrogel. Results show that a stable
bubble is created in only a microsecond timescale, after transient
oscillations. Then, a diffusion driven expansion leads to filling of the
cavity. Analysis reveals that the nucleation of a bubble releases a tension of
several tens of MPa, and a simple model captures the different time scales of
the expansion process
Propulsion of bubble-based acoustic microswimmers
Acoustic microswimmers present a great potential for microfluidic applications and targeted drug delivery. Here, we introduce armored microbubbles (size range, 10–20 μm) made by three-dimensional microfabrication, which allows the bubbles to last for hours even under forced oscillations. The acoustic resonance of the armored microbubbles is found to be dictated by capillary forces and not by gas volume, and its measurements agree with a theoretical calculation. We further measure experimentally and predict theoretically the net propulsive flow generated by the bubble vibration. This flow, due to steady streaming in the fluid, can reach 100 mm/s, and is affected by the presence of nearby walls. Finally, microswimmers in motion are shown, either as spinning devices or free swimmers.P. M. acknowledges financial support from the European Community’s Seventh Framework Programme (FP7/2007-2013) ERC Grant Agreement Bubbleboost No. 614655. This work has been performed with the help of the “Plateforme Technologique Amont” de Grenoble, with the financial support of the “Nanosciences aux limites de la Nanoélectronique” Foundation. Support from the EPSRC (T. A. S.) and from a Marie Curie Grant (E. L.) is also gratefully acknowledged.This is the author accepted manuscript. The final version is available from American Physical Society via http://dx.doi.org/10.1103/PhysRevApplied.4.06401
An elastic, plastic, viscous model for slow shear of a liquid foam
We suggest a scalar model for deformation and flow of an amorphous material
such as a foam or an emulsion. To describe elastic, plastic and viscous
behaviours, we use three scalar variables: elastic deformation, plastic
deformation rate and total deformation rate; and three material specific
parameters: shear modulus, yield deformation and viscosity. We obtain equations
valid for different types of deformations and flows slower than the relaxation
rate towards mechanical equilibrium. In particular, they are valid both in
transient or steady flow regimes, even at large elastic deformation. We discuss
why viscosity can be relevant even in this slow shear (often called
"quasi-static") limit. Predictions of the storage and loss moduli agree with
the experimental literature, and explain with simple arguments the non-linear
large amplitude trends
Generic flow profiles induced by a beating cilium
We describe a multipole expansion for the low Reynolds number fluid flows
generated by a localized source embedded in a plane with a no-slip boundary
condition. It contains 3 independent terms that fall quadratically with the
distance and 6 terms that fall with the third power. Within this framework we
discuss the flows induced by a beating cilium described in different ways: a
small particle circling on an elliptical trajectory, a thin rod and a general
ciliary beating pattern. We identify the flow modes present based on the
symmetry properties of the ciliary beat.Comment: 12 pages, 6 figures, to appear in EPJ
Discrete element modelling of the packing of spheres and its application to the structure of porous metals made by infiltration of packed beds of NaCl beads
A numerical model, using the discrete element method, has been developed to quantify specific parameters that are pertinent to the packing behaviour of relatively large, spherical NaCl beads and mixtures of beads of different sizes. These parameters have been compared with porosity and connectivity measurements made on porous aluminium castings made by molten metal infiltration into packed beds of such beads, after removal of the NaCl by dissolution.
DEM has been found to accurately predict the packing fraction for salt beads with both mono and binary size distributions and from this the pore fractions in castings made by infiltration into packed beds of beads could be predicted. Through simple development of the condition for contacting of neighbouring beads, the number of windows linking neighbouring pores, and their size, could also be predicted across a wide range of small bead additions. The model also enables an insight into the mixing quality and changes in connectivity introduced through the addition of small beads. This work presents significant progress towards the delivery of a simulation based approach to designing preform architectures in order to tailor the resulting porous structures to best suit specific applications
Mechanical model of the ultra-fast underwater trap of Utricularia
The underwater traps of the carnivorous plants of the Utricularia species
catch their preys through the repetition of an "active slow deflation / passive
fast suction" sequence. In this paper, we propose a mechanical model that
describes both phases and strongly supports the hypothesis that the trap door
acts as a flexible valve that buckles under the combined effects of pressure
forces and the mechanical stimulation of trigger hairs, and not as a panel
articulated on hinges. This model combines two different approaches, namely (i)
the description of thin membranes as triangle meshes with strain and curvature
energy, and (ii) the molecular dynamics approach, which consists in computing
the time evolution of the position of each vertex of the mesh according to
Langevin equations. The only free parameter in the expression of the elastic
energy is the Young's modulus E of the membranes. The values for this parameter
are unequivocally obtained by requiring that the trap model fires, like real
traps, when the pressure difference between the outside and the inside of the
trap reaches about 15 kPa. Among other results, our simulations show that, for
a pressure difference slightly larger than the critical one, the door buckles,
slides on the threshold and finally swings wide open, in excellent agreement
with the sequence observed in high-speed videos.Comment: Accepted for publication in Physical Review
Spontaneous Firings of Carnivorous Aquatic Utricularia Traps: Temporal Patterns and Mechanical Oscillations
Aquatic species of Utricularia are carnivorous plants living in environments poor in nutrients. Their trapping mechanism has fascinated generations of scientists and is still debated today. It was reported recently that Utricularia traps can fire spontaneously. We show here that these spontaneous firings follow an unexpected diversity of temporal patterns, from “metronomic” traps which fire at fixed time intervals to “random” patterns, displaying more scattered firing times. Some “bursting” traps even combine both aspects, with groups of fast regular firings separated by a variable amount of time. We propose a physical model to understand these very particular behaviors, showing that a trap of Utricularia accomplishes mechanical oscillations, based on continuous pumping and sudden opening of the trap door (buckling). We isolate the key parameters governing these oscillations and discuss the effect of their fluctuations
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