From swarms of swimming bacteria to the moving contents of cells, biology is replete with active systems whose microscopic constituents interact by performing mechanical work on a surrounding fluidic medium. This can lead to large-scale, sometimes functional, self-organized structures and complex dynamics. I'll overview the modeling of such systems, focusing first on continuum kinetic theories that couple the micro and macroscopic scales to describe how suspensions of active particles, such as swimming microorganisms, evolve in time. While high-dimensional (5+1) these models have been used to understand observations of novel instabilities, turbulent-like dynamics, and strange rheology, and have been incorporated into more complex models of biological systems. I'll then pivot to describe the emergence of large scale, spontaneously appearing transport flows in developing egg cells. Building on a conception of molecular motors carrying payloads on a flexible polymer assembly, I'll develop an active porous medium model whose instabilities naturally drive the system towards large-scale "twister" flows consistent with experiments.

Bio:  Dr. Michael J. Shelley is an applied mathematician who works on the modeling and simulation  of complex systems arising in physics and biology. He is the Lyttle Professor of Applied Mathematics at the Courant Institute, co-founder of the Courant Institute's Applied Mathematics Lab, and is the Director of the Center for Computational Biology at the Flatiron Institute.  He holds a B.A. in mathematics from the University of Colorado and a Ph.D. in applied mathematics from the University of Arizona. He was a postdoctoral researcher at Princeton University and a member of the mathematics faculty at the University of Chicago before joining NYU. Shelley has received the François Frenkiel Award from the American Physical Society and the Julian Cole Lectureship from the Society for Industrial and Applied Mathematics, and he is a Fellow of both societies. He is also a Fellow of the American Academy of Arts and Sciences and a member of the National Academy of Sciences.
PSB = Physical Science Building
 

I will present some introductory notions of integrability and discuss their application to some important models in condensed matter physics such as the Hubbard model, the Heisenberg spin chain and the Kondo model.

Thomas Gomez ^1,2*
1University of Colorado
²National Solar Observatory
^*Hale Fellow

The equation of state (EOS) is one of the outstanding challenges in neutron star (NS) astrophysics. Accurate determination of their masses and radii will constrain the EOS of nuclear matter. There are multiple efforts underway to determine mass and radius, such as using observations of hot spot radiation being gravitationally bent around the NS. This goal can also be accomplished by measuring the spectrum of the NS directly. In dense plasmas, the widths of spectral lines are dominated by pressure broadening and can therefore be used to determine a star’s gravity. However, the high magnetic field in a NS atmosphere complicates the physics of line broadening creating competing broadening mechanisms. Theoretical developments of line broadening in high magnetic fields indicate that collisions with plasma particles exceed the broadening from the motional Stark effect from the magnetic field. Contrary to past results, this means that spectral lines from neutron star atmospheres can be used to directly determine mass and radius. Recent measurements from the Chandra X-ray telescope give us a clue as to the feasibility of this method.

After a brief introduction to soft condensed matter physics, I will describe work from my lab with colloids and liquid crystals. The colloid experiments explore novel phases, phase transition mechanisms, and relaxation in crystal films and glasses that are relevant to a broad range of hard and soft materials. In a different vein, experiments with liquid crystal drops reveal remarkable “almost biological” shape transformation behaviors.

Band theory, one of the enduring successes of the quantum revolution of the past century, has given us a purview into the physics of electrons in solids and has been extensively probed through linear response, and particularly, charge current response.

Here, I will show that nonlinear responses such as optical rectification and second harmonic generation offer a unique window into quantum observables that encode the basic elements of “quantum geometry”, a feature of multiband solids with broken symmetries. Such signals are only measurable going beyond linear response.

I will demonstrate that familiar analogues from the world of differential geometry -- curvatures and metrics – naturally appear in the nonlinear response regime and give rise to currents forbidden in linear response. As direct applications, I will present the mechanism for rectified current generation in twisted Moire superlattices, the Hall current in topological antiferromagnets, the role of quantum geometry in sliding ferroelectricity and time-reversal odd rectification in crystals.

I will conclude with an overview of the many prospective uses of the quantum geometric viewpoint on light-matter interaction, touching on how nonlinear signals can diagnose the order parameter of a topological superconductor, in a manner accessible only beyond the linear regime.

Space telescopes like the NASA Kepler and TESS missions as well as the forthcoming PLATO mission are driving a data revolution in astrophysics. The ultra-precise observations provided by these missions are challenging our best models of how stars evolve, and are in turn granting insights into the formation and evolution of planetary systems and the Galaxy as a whole. They furthermore present novel opportunities to probe far-ranging physics, such as dark matter and theories of gravity beyond general relativity. In this talk, I will give an overview of the data, models, challenges, and opportunities in astronomy, and highlight the role that machine learning is playing in advancing our knowledge across astrophysics.

View abstract at https://physics.qc.cuny.edu/uploads/colloquia/Jianbo_Liu_F2025.pdf

Using an established simulation technique [1] and motivated by recent work [2] showing the rapid onset of system-spanning rigid structures (identified by a pebble game algorithm) with increase of volume fraction in a highly-concentrated (or dense) suspension, we show that the number of contacts in the network is an increasing function of imposed shear stress, but it also fluctuates during flow.  The microscopic interactions between particles and the general behavior of this shear-thickening suspension will be outlined briefly to provide insight to the network development leading to the changes in rheological properties. 
 
We will then explore, or two-dimensional suspensions (monolayers) the development of minimally rigid structures in the shear-thickened suspension as it approaches jamming at high stress, and show that the onset of large rigid clusters exhibits critical behavior.  The extension to 3D and the possible relationship of this behavior to other work showing critical scaling at the onset of shear thickening and jamming will be discussed.
 

1. R. Mari, R. Seto, J. F. Morris & M. M. Denn 2014 Shear thickening, frictionless and frictional rheologies. J. Rheol. 58, 1693.
2. M. van der Naald, A. Singh, T.T. Eid, K. Tang, J. J. de Pablo & H. M. Jaeger, H.M. 2024. Minimally rigid clusters in dense suspension flow. Nature Physics 20, 653–659.

At Home with QC presents:

QC astronomer Keaton Bell uses video recordings from space telescopes to measure vibrations of dead stars called white dwarfs. White dwarf stars are the glowing hot embers left over when most stars run out of nuclear fuel. Some white dwarfs vibrate spontaneously, revealing resonant frequencies of the stars that can be used to map their interior structures. This presentation will describe the physics of stellar vibrations by analogy with musical instruments. We will review how the QC White Dwarf Research Group interprets video recordings of vibrating stars to study their structures and discuss the importance of studying white dwarf stars. This talk will premiere an exciting new discovery that has never been seen by a public audience.

For more information about the presentation and Keaton Bell, click here.

RSVP: bit.ly/AHWQC-KeatonBell

Cavity optomechanics typically relies on mechanical modulation of optical frequencies to couple optical and mechanical degrees of freedom. Here we study a different mechanism, based on mechanically induced symmetry breaking that couples otherwise independent optical modes. Systems with this type of interaction exhibit non-trivial Hamiltonian dynamics even in the absence of external drive and dissipation, in contrast to standard models where the absence of pumping leads only to a trivial equilibrium shift. This dynamics is marked by a bifurcation: above a critical photon number the trivial equilibrium becomes unstable. In the stable regime, optical and mechanical degrees of freedom share a common spectrum and undergo amplitude-modulated oscillations. Beyond the bifurcation, their behavior diverges: the mechanical oscillator settles into periodic motion at its natural frequency, while the optical modes oscillate at much higher frequencies determined by the mechanical amplitude, with adiabatic modulation. Such adiabatic behavior is interrupted by sudden jumps reminiscent of Landau–Zener transitions in a quantum two-level system. This symmetry-breaking-mediated interaction provides an alternative route for controlling energy exchange between optical and mechanical subsystems, and establishes symmetry breaking as a principle for engineering optomechanical interactions in degenerate cavities.

Light is an electromagnetic wave defined by several degrees of freedom (DoF), including frequency, momentum, amplitude, phase, and polarization. Controlling these properties is crucial across scientific disciplines, and is a key goal of a wide array of technologies. Nanophotonic devices called “metasurfaces” fill this need by structuring common materials (such as silicon, glass, and metals) at subwavelength scales (micrometer and smaller). The result is tailored light-matter interactions determined by the details of the structure, no longer limited by the materials we are given by nature. These interactions can be customized at will—a sandbox for invention in a platform that is readily manufacturable.
My research aims to both (1) invent, design and develop new devices using novel and emerging physical phenomena and (2) apply these new tools to exciting applications. In particular, starting from an array of uniform subwavelength structures, we have found that introducing small geometric perturbations that break specific symmetries can impart remarkable control to light point-by-point across the device—in some cases, “complete” control over the physically relevant DoF. Applications include new methods for optical combiners in augmented reality systems, custom couplers in integrated photonics for communications systems, and controlling the directionality and polarization of thermal emission in surprising ways.

Adam Overvig is an Assistant Professor in the Department of Physics at Stevens Institute of Technology, since Fall 2023, where he was awarded the AFOSR YIP. He received his PhD in applied physics from Columbia University in 2020, where he was an NSF IGERT fellow. He was then a postdoctoral researcher at the Advanced Science Research Center at the City University of New York until 2023, where he was recognized as a Finalist for the Blavatnik Regional Awards for Young Scientists. His research interests include metasurfaces, symmetries in open systems, holography, thermal photonics, and applications of photonics to emerging quantum technologies.

Optical metasurfaces and planar optics provide wavefront manipulation within a sub-wavelength distance, which hold great promise for miniaturizing optical systems with a reduced footprint and improved functionality. My lab aims to create ultra-compact meta-optics for imaging/sensing with diagnostic applications, and light delivery with therapeutic applications. The mismatch of dimensions and mechanical properties between bulky, rigid substrates and soft biological tissues is a general challenge for in vivo applications. Using computational inverse design strategies, we create ultrathin resonant metasurfaces to facilitate the transfer and heterogeneous integration with various electronic and photonic devices. For example, the integration of metasurfaces on optical fiber tips will improve the compactness and precision of endoscopic optical probes. Furthermore, we leverage resonant metasurfaces that are sensitive to their dielectric environments. We create dynamic tunable metasurfaces, where the resonances of meta-atoms are tuned by the surrounding medium, in order to modulate the far-field optical functions with spatiotemporal control for dynamic light delivery. As a proof of concept, we demonstrate medium-switchable meta-holograms, which also provide direct visual reporting for refractometric sensing. Eliminating plasmonic ohmic loss and heating issues, dielectric metasurfaces enable broad biosensing and diagnostic applications with higher Q factors, better repeatability, reliability, and biocompatibility.

Accretion disks around supermassive black holes (SMBHs) power active galactic nuclei (AGN), yet their small angular sizes make direct observation challenging. In this talk, I will explore how reverberation mapping and gravitational microlensing provide complementary insights into accretion disk structure, size. Furthermore, I will describe how combining reverberation mapping and microlensing simulations, together with machine learning techniques, enables more efficient and accurate constraints on accretion disk properties. I will also discuss the search for lensed quasars, which serve as ideal laboratories for these studies, and highlight upcoming opportunities from the Rubin Observatory.