Upcoming Events

Soft matter, a term not used often a few decades ago, has become a recognized branch of physics studied by researchers worldwide from multiple disciplines. My own random walk in this field started with experiments on filamentous viruses to study their behavior to form liquid crystalline phases, and then on protein filaments such as F-actin and microtubules to understand what drive their lateral aggregation, droplet formation, and dynamic patterns, and over the last decade, on a form of bacterial collective motility called swarming. I have gradually recognized that these happen to be a few good examples of soft matter systems of biological origin. Research in any of these systems must be conducted in tune with the cultures and applications of both soft matter and biological sciences.  

A prevalent form of bacterial collective motility, called swarming, occurs in a range of physiological settings, such as on human airway and intestinal epithelia. Our ongoing study focuses on a particular species of bacteria, the Enterobacter sp. SM3, which manifests strong swarming behavior. In one project, we perform experiments at the bacterial swarm front on agar, a soft substrate containing nutrients for the bacterial growth, to track the bacterial collective motion, with a focus on intercellular interactions that strongly affect the swarming behavior. In a second project, we  incorporate mucin, the major component of mucus on airway and intestinal surfaces, in an agar gel, to study how the swarming motility of SM3 on the agar surface is dramatically enhanced. Our experiments show that mucin promotes swarming by abrogating contact line pinning, rendering the surface more slippery, and enabling the bacterial swarm to expand more readily and rapidly. The results of these studies show that the fundamental fluid mechanics and interfacial physics profoundly affect the swarming behavior. In a broader picture, the new knowledge of bacterial swarming motility might offer useful tips to biomedical and environmental applications.

A variety of searches for hidden valleys/dark sectors (HV/DS) are underway at the LHC.  Many of these searches are unusual and difficult.  While QCD-like HV/DS have widely been used as a search target, many non-QCD-like HV/DS may have less familiar phenomenological signatures, and might elude existing search strategies, especially those that use highly-optimized methods.  To explore this risk, simulations of non-QCD-like theories are needed, but these do not yet exist.  To create them requires a deep dive into quantum field theory. I'll examine this complex theoretical and phenomenological issue from several perspectives.

Please come and hear about quantum gravity and cosmology from our own Professor Ben Heidenreich.

Primordial Black Holes (PBHs) could play a relevant role in several physical phenomena. They are particularly attractive as a candidate for dark matter (DM), seeds of supermassive black holes, sources of gravitational waves, etc. In addition, the observation of an evaporating black hole would provide definitive information on the elementary particles present in nature, including new degrees of freedom beyond the Standard Model. In the first part of the talk, I will revisit the constraints on evaporating PBHs from the isotropic soft γ-ray background and the diffuse soft γ-ray emission towards the inner Galaxy in the mass range 10¹⁶ − 10¹⁸ g. No PBH signal is detected, and we set the strongest limit on PBH DM for masses up to 4 × 10¹⁷ g, significantly closing in into the so-called asteroid mass range where PBH could potentially make up the totality or a large fraction of DM. In the second part of the talk, I will discuss the possibility of detecting an exploding PBH with current and projected VHE gamma-ray observatories (HAWC, LHAASO, CTA) and introduce a new scenario where a new heavy dark electron is invoked to generate a large population of exploding black holes via the formation of quasi-extremal PBHs. 

Primordial black holes (PBHs) provide an exciting prospect for accounting for dark matter. They can be produced with masses in the range required to address the present-day dark matter abundance by inflationary models that incorporate realistic features from high-energy physics, including multiple interacting scalar fields and nonminimal couplings to the spacetime Ricci scalar. Such PBHs would form well before the QCD confinement transition, and hence the black holes would form by absorbing unconfined quarks and gluons. A subpopulation of the resulting PBHs would therefore acquire a net QCD color charge; some would be extremal. Meanwhile, if PBHs do constitute a significant fraction of the present dark-matter abundance, then we may expect at least one PBH to cross through the inner Solar System per decade. Such close encounters would produce detectable perturbations to orbital trajectories of closely-tracked, visible Solar System objects such as the planet Mars. By exploiting high-precision data on the motions of various objects within the Solar System, PBHs within the (as yet) unconfined ``asteroid-mass’’ range could plausibly be detected within the next decade, or their absence used to strengthen present-day bounds. (Based on https://arxiv.org/abs/2303.02168, https://arxiv.org/abs/2310.16877, and https://arxiv.org/abs/2312.17217.)

Colloidal particles have been used either as model systems for materials science or as building blocks for making functional materials. Previous work primarily focused on assembling colloids under a single external field, where controlling particle-particle interaction is often limited. This talk will present results under a combination of electric and magnetic fields. When these two fields are orthogonally applied, we can independently tune the nature, magnitude, direction, and effective range of interparticle interactions. As a result, we can obtain well-aligned, highly dense, but individually separated linear chains at intermediate particle concentrations for isotropic microspheres or chiral colloidal clusters with tunable handedness when the building blocks are anisotropic colloidal dimers. Furthermore, we will present our recent results of dodecagonal quasicrystals assembled from one-component microspheres. We will report our experimental observations, underlying mechanisms, and tunability of the assembled structures. In addition, we have also performed Monte Carlo and Brownian Dynamics simulations in which the electric and magnetic dipolar interactions and electrohydrodynamic interactions are involved to gain a more fundamental understanding of our experimental results. Our method provides a convenient route for producing complex and hierarchical structures that are challenging to make using one field only.

Colloidal particles have been used either as model systems for materials science or as building blocks for making functional materials. Previous work primarily focused on assembling colloids under a single external field, where controlling particle-particle interaction is often limited. This talk will present results under a combination of electric and magnetic fields. When these two fields are orthogonally applied, we can independently tune the nature, magnitude, direction, and effective range of interparticle interactions. As a result, we can obtain well-aligned, highly dense, but individually separated linear chains at intermediate particle concentrations for isotropic microspheres or chiral colloidal clusters with tunable handedness when the building blocks are anisotropic colloidal dimers. Furthermore, we will present our recent results of dodecagonal quasicrystals assembled from one-component microspheres. We will report our experimental observations, underlying mechanisms, and tunability of the assembled structures. In addition, we have also performed Monte Carlo and Brownian Dynamics simulations in which the electric and magnetic dipolar interactions and electrohydrodynamic interactions are involved to gain a more fundamental understanding of our experimental results. Our method provides a convenient route for producing complex and hierarchical structures that are challenging to make using one field only.