52 research outputs found
Odd viscosity in chiral active fluids
Chiral active fluids are materials composed of self-spinning rotors that
continuously inject energy and angular momentum at the microscale.
Out-of-equilibrium fluids with active-rotor constituents have been
experimentally realized using nanoscale biomolecular motors, microscale active
colloids, or macroscale driven chiral grains. Here, we show how such chiral
active fluids break both parity and time-reversal symmetries in their steady
states, giving rise to a dissipationless linear-response coefficient called odd
viscosity in their constitutive relations. Odd viscosity couples pressure and
vorticity leading, for example, to density modulations within a vortex profile.
Moreover, chiral active fluids flow in the direction transverse to applied
compression as in shock propagation experiments. We envision that this
collective transverse response may be exploited to design self-assembled
hydraulic cranks that convert between linear and rotational motion in
microscopic machines powered by active-rotors fluids
Optimal power and efficiency of odd engines
Odd materials feature antisymmetric response to perturbations. This anomalous
property can stem from the nonequilibrium activity of their components, which
is sustained by an external energy supply. These materials open the door to
designing innovative engines which extract work by applying cyclic
deformations, without any equivalent in equilibrium. Here, we reveal that the
efficiency of such energy conversion, from local activity to macroscopic work,
can be arbitrarily close to unity when the cycles of deformation are properly
designed. We illustrate these principles in some canonical viscoelastic
materials, which leads us to identify strategies for optimizing power and
efficiency according to material properties, and to delineate guidelines for
the design of more complex odd engines.Comment: 6 pages, 2 figure
Hydrodynamic correlation functions of chiral active fluids
The success of spectroscopy to characterize equilibrium fluids, for example the heat capacity ratio, suggests a parallel approach for active fluids. Here, we start from a hydrodynamic description of chiral active fluids composed of spinning constituents and derive their low-frequency, long-wavelength response functions using the Kadanoff-Martin formalism. We find that the presence of odd (equivalently, Hall) viscosity leads to mixed density-vorticity response even at linear order. Such response, prohibited in time-reversal invariant fluids, is a large-scale manifestation of the microscopic breaking of time-reversal symmetry. Our work suggests possible experimental probes that can measure anomalous transport coefficients in active fluids through dynamic light scattering
Anisotropic odd viscosity via a time-modulated drive
At equilibrium, the structure and response of ordered phases are typically
determined by the spontaneous breaking of spatial symmetries. Out of
equilibrium, spatial order itself can become a dynamically emergent concept. In
this article, we show that spatially anisotropic viscous coefficients and
stresses can be designed in a far-from-equilibrium fluid by applying to its
constituents a time-modulated drive. If the drive induces a rotation whose rate
is slowed down when the constituents point along specific directions,
anisotropic structures and mechanical responses arise at long timescales. We
demonstrate that the viscous response of such anisotropic driven fluids can
acquire a tensorial, dissipationless component called anisotropic odd (or Hall)
viscosity. Classical fluids with internal torques can display additional
components of the odd viscosity neglected in previous studies of quantum Hall
fluids that assumed angular momentum conservation. We show that these
anisotropic and angular momentum-violating odd-viscosity coefficients can
change even the bulk flow of an incompressible fluid by acting as a source of
vorticity. In addition, shear distortions in the shape of an inclusion result
in torques.Comment: 12 pages, 2 figure
Topological edge states in equidistant arrays of Lithium Niobate nano-waveguides
We report that equidistant 1D arrays of thin-film Lithium Niobate
nano-waveguides generically support topological edge states. Unlike
conventional coupled-waveguide topological systems, the topological properties
of these arrays are dictated by the interplay between intra- and inter-modal
couplings of two families of guided modes with different parities. Exploiting
two modes within the same waveguide to design a topological invariant allows us
to decrease the system size by a factor of two and substantially simplify the
structure. We present two example geometries where topological edge states of
different types (based on either quasi-TE or quasi-TM modes) can be observed
within a wide range of wavelengths and array spacings
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