389 research outputs found
A new conjecture extends the GM law for percolation thresholds to dynamical situations
The universal law for percolation thresholds proposed by Galam and Mauger
(GM) is found to apply also to dynamical situations. This law depends solely on
two variables, the space dimension d and a coordinance numberq. For regular
lattices, q reduces to the usual coordination number while for anisotropic
lattices it is an effective coordination number. For dynamical percolation we
conjecture that the law is still valid if we use the number q_2 of second
nearest neighbors instead of q. This conjecture is checked for the dynamic
epidemic model which considers the percolation phenomenon in a mobile
disordered system. The agreement is good.Comment: 8 pages, latex, 3 figures include
Dense bubble flow in a silo: an unusual flow of a dispersed medium
The dense flow of air bubbles in a two-dimensional silo (through an aperture
D) filled with a liquid is studied experimentally. A particle tracking
technique has been used to bring out the main properties of the flow:
displacements of the bubbles, transverse and axial velocities. The behavior of
the air bubbles is observed to present similarities with non-deformable solid
grains in a granular flow. Nevertheless, a correlation between the bubble
velocities and their deformations has been evidenced. Moreover, a new discharge
law (Beverloo-like) must be considered for such a system, where the flow rate
is observed to vary as D^{1/2} and depends on the deformability of the
particles.Comment: 5 pages, 6 figure
Magnetocapillary self-assemblies: locomotion and micromanipulation along a liquid interface
This paper presents an overview and discussion of magnetocapillary
self-assemblies. New results are presented, in particular concerning the
possible development of future applications. These self-organizing structures
possess the notable ability to move along an interface when powered by an
oscillatory, uniform magnetic field. The system is constructed as follows. Soft
magnetic particles are placed on a liquid interface, and submitted to a
magnetic induction field. An attractive force due to the curvature of the
interface around the particles competes with an interaction between magnetic
dipoles. Ordered structures can spontaneously emerge from these conditions.
Furthermore, time-dependent magnetic fields can produce a wide range of dynamic
behaviours, including non-time-reversible deformation sequences that produce
translational motion at low Reynolds number. In other words, due to a
spontaneous breaking of time-reversal symmetry, the assembly can turn into a
surface microswimmer. Trajectories have been shown to be precisely
controllable. As a consequence, this system offers a way to produce microrobots
able to perform different tasks. This is illustrated in this paper by the
capture, transport and release of a floating cargo, and the controlled mixing
of fluids at low Reynolds number.Comment: 10 pages, 8 figures review pape
Compound droplet manipulations on fiber arrays
Recent works demonstrated that fiber arrays may constitue the basis of an
open digital microfluidics. Various processes, such as droplet motion,
fragmentation, trapping, release, mixing and encapsulation, may be achieved on
fiber arrays. However, handling a large number of tiny droplets resulting from
the mixing of several liquid components is still a challenge for developing
microreactors, smart sensors or microemulsifying drugs. Here, we show that the
manipulation of tiny droplets onto fiber networks allows for creating compound
droplets with a high complexity level. Moreover, this cost-effective and
flexible method may also be implemented with optical fibers in order to develop
fluorescence-based biosensor
Waveguides for walking droplets
When gently placing a droplet onto a vertically vibrated bath, a drop can
bounce without coalescing. Upon increasing the forcing acceleration, the
droplet is propelled by the wave it generates and becomes a walker with a well
defined speed. We investigate the confinement of a walker in different
rectangular cavities, used as waveguides for the Faraday waves emitted by
successive droplet bounces. By studying the walker velocities, we discover that
1d confinement is optimal for narrow channels of width of . We also propose an analogy with waveguide models based on the
observation of the Faraday instability within the channels.Comment: 8 pages, 6 figure
Statics and dynamics of magnetocapillary bonds
When ferromagnetic particles are suspended at an interface under magnetic
fields, dipole-dipole interactions compete with capillary attraction. This
combination of forces has recently given promising results towards controllable
self-assemblies, as well as low Reynolds swimming systems. The elementary unit
of these assemblies is a pair of particles. Although equilibrium properties of
this interaction are well described, dynamics remain unclear. In this letter,
the properties of magnetocapillary bonds are determined by probing them with
magnetic perturbations. Two deformation modes are evidenced and discussed.
These modes exhibit resonances whose frequencies can be detuned to generate
non-reciprocal motion. A model is proposed which can become the basis for
elaborate collective behaviours
Remote control of self-assembled microswimmers
Physics governing the locomotion of microorganisms and other microsystems is
dominated by viscous damping. An effective swimming strategy involves the
non-reciprocal and periodic deformations of the considered body. Here, we show
that a magnetocapillary-driven self-assembly, composed of three soft
ferromagnetic beads, is able to swim along a liquid-air interface when powered
by an external magnetic field. More importantly, we demonstrate that
trajectories can be fully controlled, opening ways to explore low Reynolds
number swimming. This magnetocapillary system spontaneously forms by
self-assembly, allowing miniaturization and other possible applications such as
cargo transport or solvent flows.Comment: 5 pages, 5 figures articl
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