171 research outputs found
Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces
Superhydrophobic surfaces (SHSs) have the potential to achieve large drag
reduction for internal and external flow applications. However, experiments
have shown inconsistent results, with many studies reporting significantly
reduced performance. Recently, it has been proposed that surfactants,
ubiquitous in flow applications, could be responsible, by creating adverse
Marangoni stresses. Yet, testing this hypothesis is challenging. Careful
experiments with purified water show large interfacial stresses and,
paradoxically, adding surfactants yields barely measurable drag increases. This
suggests that other physical processes, such as thermal Marangoni stresses or
interface deflection, could explain the lower performance. To test the
surfactant hypothesis, we perform the first numerical simulations of flows over
a SHS inclusive of surfactant kinetics. These simulations reveal that
surfactant-induced stresses are significant at extremely low concentrations,
potentially yielding a no-slip boundary condition on the air--water interface
(the "plastron") for surfactant amounts below typical environmental values.
These stresses decrease as the streamwise distance between plastron stagnation
points increases. We perform microchannel experiments with thermally-controlled
SHSs consisting of streamwise parallel gratings, which confirm this numerical
prediction. We introduce a new, unsteady test of surfactant effects. When we
rapidly remove the driving pressure following a loading phase, a backflow
develops at the plastron, which can only be explained by surfactant gradients
formed in the loading phase. This demonstrates the significance of surfactants
in deteriorating drag reduction, and thus the importance of including
surfactant stresses in SHS models. Our time-dependent protocol can assess the
impact of surfactants in SHS testing and guide future mitigating designs.Comment: 25 pages including supplemental information, 7 figures; videos
available on reques
Real-time quantum feedback prepares and stabilizes photon number states
Feedback loops are at the heart of most classical control procedures. A
controller compares the signal measured by a sensor with the target value. It
adjusts then an actuator in order to stabilize the signal towards its target.
Generalizing this scheme to stabilize a micro-system's quantum state relies on
quantum feedback, which must overcome a fundamental difficulty: the
measurements by the sensor have a random back-action on the system. An optimal
compromise employs weak measurements providing partial information with minimal
perturbation. The controller should include the effect of this perturbation in
the computation of the actuator's unitary operation bringing the incrementally
perturbed state closer to the target. While some aspects of this scenario have
been experimentally demonstrated for the control of quantum or classical
micro-system variables, continuous feedback loop operations permanently
stabilizing quantum systems around a target state have not yet been realized.
We have implemented such a real-time stabilizing quantum feedback scheme. It
prepares on demand photon number states (Fock states) of a microwave field in a
superconducting cavity and subsequently reverses the effects of
decoherence-induced field quantum jumps. The sensor is a beam of atoms crossing
the cavity which repeatedly performs weak quantum non-demolition measurements
of the photon number. The controller is implemented in a real-time computer
commanding the injection, between measurements, of adjusted small classical
fields in the cavity. The microwave field is a quantum oscillator usable as a
quantum memory or as a quantum bus swapping information between atoms. By
demonstrating that active control can generate non-classical states of this
oscillator and combat their decoherence, this experiment is a significant step
towards the implementation of complex quantum information operations.Comment: 12 pages, 4 figure
A regular Hamiltonian halting ratchet for matter wave transport
We report on the design of a Hamiltonian ratchet exploiting periodically at
rest integrable trajectories in the phase space of a modulated periodic
potential, leading to the linear non-diffusive transport of particles. Using
Bose-Einstein condensates in a modulated one-dimensional optical lattice, we
make the first observations of this new spatial ratchet transport. In the
semiclassical regime, the quantum transport strongly depends on the effective
Planck constant due to Floquet state mixing. We also demonstrate the interest
of quantum optimal control for efficient initial state preparation into the
transporting Floquet states to enhance the transport periodicity.Comment: 5 pages + supplementary materia
A theory for the slip and drag of superhydrophobic surfaces with surfactant.
Superhydrophobic surfaces (SHSs) have the potential to reduce drag at solid boundaries. However, multiple independent studies have recently shown that small amounts of surfactant, naturally present in the environment, can induce Marangoni forces that increase drag, at least in the laminar regime. To obtain accurate drag predictions, one must solve the mass, momentum, bulk surfactant and interfacial surfactant conservation equations. This requires expensive simulations, thus preventing surfactant from being widely considered in SHS studies. To address this issue, we propose a theory for steady, pressure-driven, laminar, two-dimensional flow in a periodic SHS channel with soluble surfactant. We linearise the coupling between flow and surfactant, under the assumption of small concentration, finding a scaling prediction for the local slip length. To obtain the drag reduction and interfacial shear, we find a series solution for the velocity field by assuming Stokes flow in the bulk and uniform interfacial shear. We find how the slip and drag depend on the nine dimensionless groups that together characterize the surfactant transport near SHSs, the gas fraction and the normalized interface length. Our model agrees with numerical simulations spanning orders of magnitude in each dimensionless group. The simulations also provide the constants in the scaling theory. Our model significantly improves predictions relative to a surfactant-free one, which can otherwise overestimate slip and underestimate drag by several orders of magnitude. Our slip length model can provide the boundary condition in other simulations, thereby accounting for surfactant effects without having to solve the full problem.Raymond and Beverly Sackler Foundation, the European Research Council Grant 247333, Mines ParisTech, the Schlumberger Chair Fund, the California NanoSystems Institute through a Challenge Grant, ARO MURI W911NF-17- 1-0306 and ONR MURI N00014-17-1-267
Observation and control of quantized scattering halos
We investigate the production of s-wave scattering halos from collisions
between the momentum components of a Bose-Einstein condensate released from an
optical lattice. The lattice periodicity translates in a momentum comb
responsible for the quantization of the halos' radii. We report on the
engineering of those halos through the precise control of the atom dynamics in
the lattice: we are able to specifically enhance collision processes with given
center-of-mass and relative momenta. In particular, we observe quantized
collision halos between opposite momenta components of increasing magnitude, up
to 6 times the characteristic momentum scale of the lattice.Comment: 11 pages, 7 figure
Quantum Zeno dynamics of a field in a cavity
We analyze the quantum Zeno dynamics that takes place when a field stored in
a cavity undergoes frequent interactions with atoms. We show that repeated
measurements or unitary operations performed on the atoms probing the field
state confine the evolution to tailored subspaces of the total Hilbert space.
This confinement leads to non-trivial field evolutions and to the generation of
interesting non-classical states, including mesoscopic field state
superpositions. We elucidate the main features of the quantum Zeno mechanism in
the context of a state-of-the-art cavity quantum electrodynamics experiment. A
plethora of effects is investigated, from state manipulations by phase space
tweezers to nearly arbitrary state synthesis. We analyze in details the
practical implementation of this dynamics and assess its robustness by
numerical simulations including realistic experimental imperfections. We
comment on the various perspectives opened by this proposal
Slip on three-dimensional surfactant-contaminated superhydrophobic gratings
Trace amounts of surfactants have been shown to critically prevent the drag
reduction of superhydrophobic surfaces (SHSs), yet predictive models including
their effects in realistic geometries are still lacking. We derive theoretical
predictions for the velocity and resulting slip of a laminar fluid flow over
three-dimensional SHS gratings contaminated with surfactant, which allow for
the first direct comparison with experiments. The results are in good agreement
with our numerical simulations and with measurements of the slip in
microfluidic channels lined with SHSs, which we obtain via confocal microscopy
and micro-particle image velocimetry. Our model enables the estimation of a
priori unknown parameters of surfactants naturally present in applications,
highlighting its relevance for microfluidic technologies.Comment: 6 pages, 3 figures, 11 supplemental pages, 2 supplemental figure
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