Hydrodynamics: a theoretical physicist’s perspective

• Pavel Kovtun

Room: Albano 3: 4205 - SU Conference Room (40 seats)

Non-invertible symmetries, chiral magnetohydrodynamics and lattice QCD

• Napat Poovuttikul

Room: Albano 3: 4205 - SU Conference Room (40 seats)

Hydrodynamics of active matter: from motile organisms to non-reciprocal magnets

• Sriram Ramaswamy

Room: Albano 3: 4205 - SU Conference Room (40 seats) I will present a theoretical framework for building symmetry-based coarse-grained descriptions of systems whose constituents receive a sustained free-energy supply that they convert to work. I'll summarise progress in the field and offer a prospective outlook on its future. I hope to build bridges to other parts of the large enterprise of hydrodynamic approaches to nonequilibrium physics.

Unflappable flocks at fluid interfaces

• Ananyo Maitra

Room: Albano 3: 4205 - SU Conference Room (40 seats) More than two decades ago it was shown [PRL 89, 058101 (2002)] that flocks in fluids are unstable at least in the viscous limit. This instability of uniaxial orientational order in bulk active fluids is an inescapable consequence of the conservation of total mass and momentum. In this talk, I will use the fluctuating, broken-symmetry hydrodynamic framework for internally driven (i.e., active) systems to show that the very activity that conspires with conservation laws to destroy bulk nematic or polar ordering can instead promote it, with radically suppressed fluctuations, in a layer of active fluid in contact with a solid or fluid medium. These escapes from the bulk instability and active stabilisation of uniaxial order via dynamical, nonequilibrium analogues of the Anderson-Higgs mechanism, would be impossible in equilibrium systems in which the existence of order and all equal-time correlators are independent of dynamics and the presence or absence of conservation laws.

Geometrical and topological mysteries of intracellular membranes

• Gregory Huber

Room: Albano 3: 4205 - SU Conference Room (40 seats)

Elastic membrane hydrodynamics

• Varghese Mathai

Room: Albano 3: 4205 - SU Conference Room (40 seats)

Beyond Drude transport in hydrodynamic metals

• Ashish Shukla

Room: Albano 3: 4205 - SU Conference Room (40 seats) Hydrodynamics provides an effective macroscopic description for systems near thermal equilibrium. In hydrodynamic metals, due to the weak explicit breaking of translation invariance from impurity and/or phonon scattering, momentum relaxes slowly with a certain rate, which formally appears on the right-hand side of the momentum dynamical equation and causes a Drude-like peak in the thermoelectric conductivities. In this talk, I will discuss a systematic approach for constructing a hydrodynamic theory which incorporates momentum-relaxing gradient corrections beyond Drude i.e. arising at subleading order in the gradient expansion. The subleading terms arise at the same order as the shear and bulk viscosities. They effectively renormalize the weight of the Drude pole in the thermoelectric conductivities, and contribute to the dc conductivities at the same order as the incoherent conductivities. Our model is able to perfectly capture the transport properties of strongly-coupled holographic metallic phases with broken translation invariance.

Surprises in Fermi liquid theory: Anomalously long lifetimes in 2D systems

• Johannes Hoffman

Room: Albano 3: 4205 - SU Conference Room (40 seats) I will give an introduction to transport phenomena and transport calculations in non-relativistic Fermi liquids. Such systems encompass interacting electron gases in the hydrodynamic regime, which has recently been accessed experimentally, but also atomic quantum gases. In particular, I will present calculations of the shear viscosity within kinetic theory at finite temperature. I will then discuss in more detail the structure of quasiparticle scattering in the presence of a Fermi surface in two-dimensional systems, which reveals a parity-dependence of relaxation rates. Such an odd-even effect gives rise to a new transport regime with long-lived odd-parity modes in addition to hydrodynamic ones, and the talk will conclude by discussing signatures of this regime in the collective mode spectrum.

Hydrodynamics of pseudo-Goldstones and singular Goldstones

• Ruben Lier

Room: Albano 3: 4205 - SU Conference Room (40 seats) At all scales, phases of matter can be found which can be understood from the point of view of spontaneous symmetry breaking. For such phases of matter, it is possible to formulate an adequate hydrodynamic description by including Goldstone fields. In my seminar, I will consider ways in which damped hydrodynamics can arise when the spontaneous symmetry breaking is undermined weakly. This undermining can happen either through explicit symmetry breaking, which gives rise to pseudo-Goldstones, or through topological defects, which give rise to singular Goldstones. I will then discuss how to systematically construct the most general hydrodynamic theory of pseudo-Goldstones and singular Goldstones and what are the distinct effects on transport.

Hydrodynamics of a relativistic charged fluid in the presence of a periodically modulated chemical potential

• Nicolas Chagnet

Room: Albano 3: 4205 - SU Conference Room (40 seats) We study charged relativistic hydrodynamics driven by an external chemical potential made periodic in order to mimick the charge modulation generated by an ionic lattice. The background periodicity fluctuations are mixing Bloch waves rather than independent single momentum Fourier waves. The modes at momenta separated by integer multiples of the modulation wavevector interact and lead to novel physical effects which bear resemblance to the physics of cuprates. We connect our results to explicit realizations in AdS/CFT models with an outlook towards condensed matter applications.

Bernhard Mehlig - Statistical models for turbulent aerosols Room: Albano 3: 4205 - SU Conference Room (40 seats) When very small particles are suspended in a fluid in motion, they tend to follow the flow. How such tracer particles are mixed, transported, and dispersed by turbulent flow has been successfully described by statistical models. Heavy particles, with mass densities larger than that of the carrying fluid, can detach from the flow. This results in preferential sampling, small-scale fractal clustering, and large collision velocities. To describe these effects of particle inertia, it is necessary to consider both particle positions and velocities in phase space. In recent years, statistical phase-space models have significantly contributed to our understanding of inertial-particle dynamics in turbulence. These models help to identify the key mechanisms and non-dimensional parameters governing the particle dynamics, and have made qualitative, and in some cases quantitative predictions. This article reviews statistical phase-space models for the dynamics of small, yet heavy, spherical particles in turbulence. We evaluate their effectiveness by comparing their predictions with results from numerical simulations and laboratory experiments, and summarise their successes and failures.

Sebastian Waeber - Turbulent flow from a stochastic gravitational potential Room: Albano 3: 4205 - SU Conference Room (40 seats) The fluid/gravity correspondence, the duality between the dynamics of black holes in Anti-de Sitter space and fluid behaviour of field theories, may be used to geometrize turbulent flow and translate statistical properties of turbulent fluids into geometric observables. Likewise turbulent solutions to Navier-Stokes equations can be obtained from black hole dynamics. We study the fluid phase of conformal matter driven by a randomly fluctuating gravitational potential, by numerically solving the evolution of a black hole in Anti-de Sitter space with a fluctuating, stochastic boundary metric. We discuss subtleties regarding the fluid's compressibility and the non-relativistic limit in this set up, and identify observables in the dual gravity theory which correspond to correlators of the fluid velocity and their higher moments with the goal to translate anomalous scaling exponents into geometry.

Piotr Surówka - Elasticty, hydrodynamics and fractons Room: Albano 3: 4205 - SU Conference Room (40 seats) This seminar explores the evolving field of fractons, quasiparticles with limited mobility, and their relationship with elasticity and fluid mechanics. We review key developments in fracton-elasticity dualities, using examples from Cauchy, Cosserat, and curved elasticity theories to demonstrate their significance. The talk will also introduce a hydrodynamic theory for dipole moment-conserving excitations and discuss its generalization to dipole-conserving superfluids.

Akash Jain - Aspects of Schwinger-Keldysh hydrodynamics Room: Albano 3: 4205 - SU Conference Room (40 seats) Schwinger-Keldysh effective field theory (SK-EFTs) is a framework for systematically including stochastic thermal fluctuations into the hydrodynamic framework. In this talk, we will review some aspects of SK hydrodynamics and how it goes past the regime of applicability of the conventional Martin-Siggia-Rose (MSR) formulation of stochastic hydrodynamics. We will pay particular attention to the dynamical Kubo-Martin-Schwinger (KMS) symmetry, which is responsible for implementing n-point fluctuation-dissipation theorems (FDTs) in SK-EFTs. Time permitting, we will also discuss the extensions of these ideas to non-conserved degrees of freedom, in particular to the Muller-Israel-Stewart model of relativistic hydrodynamics.

Laurence Yaffe - Relativistic hydro for quark-gluon plasma: applicability, initial conditions & limits of validity. Room: Albano 3: 4205 - SU Conference Room (40 seats)

Michael Landry - Macroscopic quantum effects: a systematic formulation of chiral anomalous magnetohydrodynamics Room: Albano 3: 4205 - SU Conference Room (40 seats) We introduce a novel approach for understanding how electromagnetic fields interact with quantum matter, giving rise to macroscopically observable quantum mechanical effects. This method is useful for systematically studying complex phenomena involving strong magnetic fields, with applications to a wide variety of physics subfields including condensed matter, nuclear, and astrophysics. Specific applications include the study of Dirac and Weyl semimetals, electroweak plasmas, heavy ion collisions, and neutron stars. We focus specifically on magnetohydrodynamics (MHD) with an Adler-Bell-Jackiw (ABJ) anomaly (also known as the triangle anomaly or chiral anomaly). This new framework allows us to consolidate various effects, like the chiral magnetic effect, electric separation effect, and much more, into a single comprehensive model. As a testable prediction, we explore a new type of chiral wave that we term the chiral magnetic electric separation wave. This prediction is emphatically distinct from the previously-predicted chiral magnetic wave which we show fails to exist in the presence of dynamical electromagnetic fields. Finally, we introduce a simplified model to investigate the outcome of the well-known chiral instability and comment on a potential application to neutron stars.

Alan Dorsey - Hydrodynamics of supersolids Room: Albano 3: 4205 - SU Conference Room (40 seats) In this talk I will provide an overview of the search for supersolids, an elusive state of matter in which superfluid and solid behaviors coexist. Theoretical speculations on the coexistence of off-diagonal long-range order and crystalline order (with the accompanying shear rigidity) date from the 1960s; after a long dormant period, there was great excitement in 2004 when several experimental groups announced the observation of nonclassical rotational inertia, a hallmark of the putative supersolid phase, in samples of solid 4He. Alas, careful experiments from 2004-2012 determined that the signals of a supersolid phase were most likely a result of defects in the solid that affected the shear modulus, with little to do with ODRLO. However, the experiments stimulated interest in a hydrodynamic theory of supersolids, with the identification of new collective modes. I will review this hydrodynamic theory, in which point defects in the solid play a starring role. While not immediately relevant to solid 4He, variants of this work may be relevant to supersolid phases observed in dipolar atomic gases over the past five years.

Axel Brandenburg - Hydrodynamics with magnetic fields: new qualities Room: Albano 3: 4205 - SU Conference Room (40 seats) The presence of magnetic fields brings entirely new qualities into the game. Magnetic fields imply the presence of an additional energy reservoir, and energy can flow between magnetic and kinetic energies. Unlike hydrodynamics, where energy gets thermalized through viscous heating, it can now also be thermalized through Joule heating. The macroscopic dynamics, and the relative importance of both heating processes depend on ill-known microphysical processes and transport coefficients. Magnetic fields also facilitate the emergence of spatio-temporal coherence associated with inverse cascading. Spatio-temporal coherence can also emerge in ordinary hydrodynamics, but such effects are mostly due to boundaries. When magnetic fields are strong, there can be inverse cascading of two different types: with and without net magnetic helicity. The helical case is relevant in all types of large-scale dynamos in much of astrophysics ranging from planetary and stellar scales all the way to galactic scales. Even the entire Universe could have magnetic fields with magnetic helicity of one sign, for example if it is generated by the chiral magnetic effect, or during inflation from an axion-like field. A major qualitative difference to hydrodynamics is the conservation of magnetic helicity in conducting media. But even in the absence of net magnetic helicity, this conservation plays an important role due to the conservation of what is now known as the Hosking integral. This is a very new subject and we are just beginning to appreciate the full extent of the phenomenon of inverse cascading in cases when the net magnetic helicity vanishes. In addition to numerical simulations and astrophysical observations, there are laboratory experiments that allow us to study important processes such as dynamos and reconnection. The Pencil Code is an example of a community effort to develop a continuously growing toolbox to address many of those questions. Laboratory experiments, on the other hand, focus often on liquid sodium experiments, but many of the processes can also be studied in plasma experiments.

Ármann Gylfason - Non-intrusive Lagrangian temperature measurements in convective turbulent flows Room: Albano 3: 4205 - SU Conference Room (40 seats) Turbulent convection, and thermal flows, remain interesting areas of study due to the presence of complex structures and multiscale coupling. Advances in experimental methods have rapidly improved measurable quantities of convective flows systems, and expanded the range of parameters that are accessible. Lagrangian Particle Tracking is among other optical measurement techniques that have gained considerable momentum in the last few decades with rapid development of imaging systems and computational capabilities and replaced point-based measurement of flow quantities to some extent. In this presentation I will present our recent attempts to expand the traditional Lagrangian Particle Tracking methodology to simultaneously detect the Lagrangian temperature. In this work, we have found that one such promising experimental method is a temperature measurement based on the application of thermochromic liquid crystals.

Umut Gursoy - Hydrodynamics of quarks Room: Albano 3: 4205 - SU Conference Room (40 seats) I will review recent developments, state of the arts and some of the open problems in relativistic hydrodynamics applied to the deconfined matter of quarks and gluons. I will focus on how the data obtained in heavy ion collisions is matched onto results of hydrodynamic simulations and how the various transport properties of the quark-gluon plasma can be obtained through Bayesian analysis. Among the many open problems, I will discuss i) inclusion of electromagnetic properties of the quark-gluon matter, and, ii) effects of vorticity e.g. explanation of the recently observed global spin polarization of hadrons through spin transport. These problems urge us to develop a full-fledged relativistic theory of spin-magneto-hydrodynamics. I will discuss challenges associated to this theory, and, if time permits also how holography could help solving these challenges.

Natalia Pinzani Fokeeva - Horizon symmetries, hydrodynamics, and chaos Room: Albano 3: 4205 - SU Conference Room (40 seats) To write fluid dynamics from a modern effective field theory point of view, an infinite reparameterization symmetry is required. Curiously, black hole horizons possess the same redundancy. In this talk, I will show how horizon symmetries can be interpreted as symmetries of a dual hydrodynamic theory. In addition, I will show how the horizon structure leads to additional symmetries that are responsible for the chaotic behavior of the dual theory.

Marzena Szymanska - Driven-dissipative non-equilibrium quantum fluids of light Room: Albano 3: 4205 - SU Conference Room (40 seats)

Sriram Ganeshan - Dynamics of Fractional Quantum Hall Effect: Lessons from geophysics Room: Albano 3: 4205 - SU Conference Room (40 seats) In this talk, I will explore the similarities and differences between the fluid dynamical equations that govern ocean waves and the "hydrodynamics" of Fractional Quantum Hall (FQH) fluids. The linearized edge dynamics of the FQH hydro reveals two chiral edge modes propagating in the same direction: a non-dispersing Kelvin mode (observed on coasts) and a dispersing chiral boson mode. The presence of two modes at the edge of a Laughlin state is somewhat perplexing because only one chiral mode is expected for these states.Contrary to what is discussed in literature, I will explain that the Kelvin mode is incompatible with the gauge anomaly and, thus, cannot be associated with the charge transport at the edge. However, the chiral boson mode is consistent with the anomaly-induced chiral edge dynamics. By invoking a fluid dynamical perspective, we can gain further insights into the non-linear dynamics of FQH edge.

John Toner - Birth, Death, and Flocking: The Hydrodynamics of Dry Active matter Room: Albano 3: 4205 - SU Conference Room (40 seats) In creatures ranging from birds to fish to wildebeest, we observe the collective and coherent motion of large numbers of organisms, known as ‘flocking’. In this talk, I'll use the hydrodynamic theory of flocking to explain why a crowd of people can all walk, but not point, in the same direction. Along the way I'll illustrate how one goes about formulating a hydrodynamic theory for heretofore unconsidered states and system, using powerful techniques from theoretical condensed matter physics such as hydrodynamic theories, the gradient expansion, and the renormalization group, and using concepts from fluid mechanics.

Enkeleida Lushi - Micro-swimmers in complex confinement Room: Albano 3: 4205 - SU Conference Room (40 seats) Interactions between micro-swimmers and solid boundaries play an important role in many biological and technological processes. I will discuss our ongoing work in modeling and simulations that aim to understand the motion of micro-swimmers such as bacteria, micro-algae, spermatozoa or active colloids in various confinements or in structured environments. Our results highlight a complex interplay of the hydrodynamic and contact interactions of the individuals with each-other and the boundaries to give rise to non-trivial individual and collective behavior.