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Colloquium Schedule

Fall 2021

Coordinating Adhesion with Repulsion: How Cells Use Polymer Brushes to Orchestrate Life

Jennifer E. Curtis, Associate Professor, Georgia Tech Physics, College of Sciences
Thu., Nov 18, 2021, 4:30 pm

Multi-cellular organisms rely on reversible adhesions to orchestrate the motion and organization of cells. To date, the physics of tissue has primarily focused on the molecular adhesions between cells and the balance of forces throughout the tissue.  In this talk, I will introduce an important but neglected physical mechanism that cells may use to break or weaken cellular adhesions in a controllable, dynamic fashion. At the heart of this control is cells’ ability to rapidly extrude giant sugar polymers to form a polymer brush-like structure at cell interfaces. I will present experiments confirming that the repulsive forces generated by this compressed brush (glycocalyx) substantially modify the adhesive state of cells. Then I will show how we hijacked the cell’s glycocalyx enzymatic machinery to generate a novel class of ultra-thick polymer brush and in turn, how we have used these tunable brushes as a model system to systematically explore the forces exerted by glycocalyx on adherent cells. Adding to the evidence from our other biophysical assays, these experiments clearly demonstrate that sugar enzymes can push polymer into tight confined spaces and mechanically drive cell deformation. In light of the observed upregulation of glycocalyx synthesis in biological events that require adhesion modulation, ranging from embryogenesis to synaptogenesis, I will conclude with the hypothesis that controlled expression of sweet but repulsive polymers at the cell interface may play a serious role in tissue organization and regulation in multicellular organisms.

Searching for Another Earth

Suvrath Mahedevan, Penn State
Fri., Oct 1, 2021, 12:45 PM

Modern astronomical spectrometers are approaching the exquisite sensitivity to detect the signature of an Earth-mass planet around a Sun-like star, and can already do so for such planets around M dwarf stars. In this talk I shall discuss the challenges involved in making these difficult measurements with the Doppler radial velocity technique, and the evolution of the design of these instruments as they seek ever-tighter control of environmental parameters, and increased measurement precision. A suite of new technologies like frequency stabilized laser combs, low drift etalons, and deeper understanding of the detectors is enabling a new level of precision in radial velocity measurements - as well as illustrating new challenges.  I will use two such instruments we have built (HPF and NEID), to illustrate the underlying physics and measurement challenges.  I will then discuss how the stars themselves are the remaining challenge, and how magnetically driven processes create ‘stellar activity’ noise that can masquerade as planets and obfuscate their detection. I shall highlight a few paths the community is exploring to mitigate this – using our star, the Sun, as a guide.

Time permitting, I will also describe how beam-shaping diffusers are now enabling space-quality photometry from the ground to aid in photometric follow-up and confirmation of transiting exoplanets, and how the combination of precision spectroscopy and photometry is necessary to unveil planet properties.


Multi-Wavelength Studies of the Not So Perfect Clocks in the Sky

Natalia Lewandowska, Swarthmore College
Fri., Sept. 17, 2021, 12:45 PM

Pulsars are highly magnetized and fast-rotating neutron stars. They are the stellar remnants of a supernova explosion which takes place when a massive star uses up all ingredients for nuclear fusion in its interior and collapses under the influence of its gravity. A pulsar has a diameter of about 7 miles and about twice the mass of the Sun. It spins several times per second and has a magnetic field that can be a trillion times stronger than the one on Earth. As a pulsar ages it emits electromagnetic radiation ranging from radio waves, visible light possibly up to gamma-rays. Its rotation causes the emission from such a star to be received in the form of pulses on Earth (similar to rays from a lighthouse). Their regularity is the reason why some pulsars are often referred to as clocks in the sky. A small group of pulsars even competes with atomic clocks in terms of regularity.

In the present talk, I will give an overview of multi-wavelength observations of pulsars that also show an irregular form of emission known as radio giant pulses. Currently only observed in a handful of pulsars, giant pulses represent an enigmatic form of radio emission. I will give an overview of how we study them also at other wavelengths like optical, X-rays (with the Neutron Star Interior Composition Explorer, or NICER) and gamma-rays (with Imaging Air Cherenkov Telescopes and satellites like Fermi-LAT) and what such studies can unravel in terms of their potential emission mechanisms.

Natalia Lewandowska

Upcoming talks

October 29: Ben Good ('10), Stanford

November 18: Jennifer Curtis, Georgia Institute of Technology