Upcoming Events

Colloidal Active Carpets for Spatiotemporal control over Microscopic Matter Transport

Speaker: Prasanna More

Authors: Prasanna T More, William Seavey, and Manasa Kandula

Active carpets, collection of active components on the surface, are ubiquitous in nature. Examples range from ciliary beds to sessile suspension feeders, generating cooperative hydrodynamic flows for nutrient capture. Even extracellular matrices dynamically regulate adhesion and sensing in real time. Emulating such behavior in synthetic systems offers transformative potential for lab-on-chip diagnostics, biosensing, and microparticle sorting. In this talk, I will present our work on synthetic active carpets, built using programmable dielectric matter. We show that dielectric colloids patterned on electrodes act as reconfigurable flow actuators under AC electric fields. By optically tuning the conductivity of the colloidal posts and designing their spatial arrangement on the electrode, we achieve spatiotemporally tunable electrokinetics for directed fluid transport. Our colloidal active carpets enable site-specific capture or release of diverse microparticles. Leveraging the universal electrokinetic response of microscopic particulate matter, we expect our platform to be broadly applicable for transporting microparticles like microplastics, bacteria, and vesicles.
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Learning Measurement-induced Phase Transitions Using Attention

Speaker: Abhishek Kumar

Authors: Hyejin Kim, Abhishek Kumar, Yiqing Zhou, Yichen Xu, Romain Vasseur, Eun-Ah Kim

In the monitored dynamics with entangling evolution subjected to mid-circuit measurements, a measurement-induced phase transition (MIPT) can be characterized by the learnability of quantum information extracted from the trajectories. The probabilistic nature of measurement makes it challenging to observe MIPT without relying on a non-scalable protocol, such as post-selecting measurement trajectories. We propose a post-selection-free approach that utilizes Quantum Attention Networks (QuAN) [1] to detect MIPT under generic Haar random unitaries and weak measurements. QuAN, which leverages the attention mechanism's power to drive large language models, is an efficient classical machinery to process measurement trajectories. QuAN is designed to access high-order moments of bit-string distribution while maintaining permutation invariance. We demonstrate that QuAN can witness MIPT, predicting the phase boundary that aligns with exact results. A sample complexity study highlights the potential for QuAN to learn MIPT from experimental data without post-selection, as it requires only a small number of samples readily accessible with current experimental platforms.
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Edge Superfluidity of He-4 Droplet on Graphite 

Speaker: Thomas Pétuaud-Létang

Authors: Thomas Pétuaud-Létang, Nikolay Prokof’ev, and Boris Svistunov

Based on the first-principles simulations by worm algorithm, we obtain compelling evidence of edge superfluidity in a droplet (as well as in an incomplete layer) of He-4 adsorbed on graphite. In the light of our results, the He-4 droplet on graphite and, possibly, on other substrates emerges as the experimental system of choice for a full-scale study of rich physics of edge superfluidity, including the regime of transverse quantum fluid.

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The Physics Colloquium covers a wide range of topics and should be accessible to advanced Physics Majors.  Tea and coffee will be served from 3:45pm with the presentation beginning at 4pm.

Shape Instabilities and Curvature Localization in Photo-isomerizing Lipid GUVs Controlled by Excitation Rate and Tension

Speaker: Chris Oville

Authors: Chris Oville, Haim Diamant, and Tony Dinsmore

We drive compression and shape instabilities in lipid membranes composed of photo-isomerizing, azobenzene-based lipids and develop quantitative design principles. Under blue or UV exposure, giant unilamellar vesicles (GUVs) undergo rapid, reversible changes in area and stress. By modulating the rate of increase of UV intensity, we tune the rate of isomerization from straight to bent lipid tails and thereby reduce tension – and even drive compression. Slow UV-ramp rates produce nearly isotropic expansion with fluctuation amplitudes up to 1,000x thermal. Ramping rates above 1/(15s) produce transient "extended modes"  - lobed excitations with dominant mode numbers ranging from n = 3 to n = 7, extracted from the vesicle's equatorial contour. An increase in n with UV ramping rate is evident in multiple same-vesicle experiments as well as across vesicles. From real-space fluctuations, we measure the tension during photo-excitation and find periods of compression lasting upwards of 60s. We find quantitative agreement of the peak mode numbers and growth rates with a continuum theory of extended mode growth as a function of relative perimeter change, radius, bending modulus, initial tension, and a novel measurement of an effective dilational viscosity. In some GUVs, a single proboscis-like protrusion or "localized mode" is observed. We examine the energetics of this state theoretically. In cell membranes, active proteins give rise to control of membrane tension, shape, topology, and permeability. Here, we show that we can achieve similar responses in a robust, photo-isomerizing, synthetic lipid system.

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Extreme Clusters and Particle Memory in Decaying Hydrodynamic Turbulence

Speaker: Zhiyu Yang

Authors: Zhiyu Yang, Enrico Calzavarini, Rodolfo Monico, and Varghese Mathai

Particles of different densities are known to show variations in acceleration statistics and preferential concentrations in statistically stationary turbulence. Yet, there is a limited understanding of whether a universal clustering response is possible in decaying isotropic turbulence. We show using experiments and simulations that decay can drive an increase in the clustering, exceeding even the upper bound of clustering possible in stationary turbulence. Standard deviation of cluster volumes show non-monotonic trends depending on the particle inertia and density, although the mean cluster size does grow in decaying turbulence. We show that the fractal dimension of clusters for particles of wide-ranging densities may be collapsed using a revised effective evolving Stokes built on an intermediate sized eddy.
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Low-temperature Depletion of Superfluid Density in the Absence of Galilean Symmetry

Speaker: Viktor Berger

Authors: Viktor Berger, Nikolay Prokof'ev, and Boris Svistunov

Landau theory of superfluidity associates low-temperature flow of the normal component with the phonon wind. This picture does not apply to superfluids in which Galilean invariance is broken either by disorder, porous media, or lattice potential, and the phonon wind is no longer solely responsible for depletion of the superfluid component. Based on Popov's hydrodynamic action with anharmonic terms, we present a general theory for low-temperature (T) dependence of the superfluid stiffness, which reproduces Landau result as a special case when several parameters of the hydrodynamic action are fixed by Galilean invariance, and validate it with numerical simulations of interacting lattice bosons.  In a broader context, our approach reveals universal low-temperature thermodynamics of superfluids with an intrinsic connection between finite-T and finite-size (L) effects implying universal scaling, T^d+1 and 1/L^d+1, respectively, for a large class of thermodynamic quantities. We discuss the experimental detection of this law and compare our prediction to the existing literature.
 

The Physics Colloquium covers a wide range of topics and should be accessible to advanced Physics Majors.  Tea and coffee will be served from 3:45pm with the presentation beginning at 4pm.

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