Click a publication’s row for more information

### Abstract

Hot spotting in photovoltaic (PV) panels causes physical damage, power loss, reduced lifetime reliability, and increased manufacturing costs. The problem arises routinely in defect-free standard panels; any string of cells that receives uneven illumination can develop hot spots, and the temperature rise often exceeds 100°C in conventional monocrystalline-silicon panels despite on-panel bypass diodes, the standard mitigation technique. Bypass diodes limit the power dissipated in a cell subjected to reverse bias, but they do not prevent hot spots from forming. An alternative control method has been suggested by Kernahan that senses in real time the dynamic conductance $$|\mathrm{d}I/\mathrm{d}V|$$ of a string of cells and adjusts its operating current so that a partially shaded cell is never forced into reverse bias. We start by exploring the behavior of individual illuminated PV cells when externally forced into reverse bias. We observe that cells can suffer significant heating and structural damage, with desoldering of cell-tabbing and discolorations on the front cell surface. Then we test PV panels and confirm Kernahan's proposed panel-level solution that anticipates and prevents hot spots in real time. Simulations of cells and panels confirm our experimental observations and provide insights into both the operation of Kernahan's method and panel performance.

### Abstract

The snap of a finger has been used as a form of communication and music for millennia across human cultures. However, a systematic analysis of the dynamics of this rapid motion has not yet been performed. Using high-speed imaging and force sensors, we analyse the dynamics of the finger snap. We discover that the finger snap achieves peak angular accelerations of $$1.6 \times 10^{6\;\circ} \mathrm{s}^{-2}$$ in 7 ms, making it one of the fastest recorded angular accelerations the human body produces (exceeding professional baseball pitches). Our analysis reveals the central role of skin friction in mediating the snap dynamics by acting as a latch to control the resulting high velocities and accelerations. We evaluate the role of this frictional latch experimentally, by covering the thumb and middle finger with different materials to produce different friction coefficients and varying compressibility. In doing so, we reveal that the compressible, frictional latch of the finger pads likely operates in a regime optimally tuned for both friction and compression. We also develop a soft, compressible friction-based latch-mediated spring actuated model to further elucidate the key role of friction and how it interacts with a compressible latch. Our mathematical model reveals that friction plays a dual role in the finger snap, both aiding in force loading and energy storage while hindering energy release. Our work reveals how friction between surfaces can be harnessed as a tunable latch system and provides design insight towards the frictional complexity in many robotic and ultrafast energy-release structures.

### Abstract

In established theories of grain coarsening, grains disappear either by shrinking or by rotating as a rigid object to coalesce with an adjacent grain. Here we report a third mechanism for grain coarsening, in which a grain splits apart into two regions that rotate in opposite directions to match two adjacent grains’ orientations. We experimentally observe both conventional grain rotation and grain splitting in two-dimensional colloidal polycrystals. We find that grain splitting occurs via independently rotating “granules” whose shapes are determined by the underlying triangular lattices of the two merging crystal grains. These granules are so small that existing continuum theories of grain boundary energy are inapplicable, so we introduce a hard sphere model for the free energy of a colloidal polycrystal. We find that, during splitting, the system overcomes a free energy barrier before ultimately reaching a lower free energy when splitting is complete. Using simulated splitting events and a simple scaling prediction, we find that the barrier to grain splitting decreases as grain size decreases. Consequently, grain splitting is likely to play an important role in polycrystals with small grains. This discovery suggests that mesoscale models of grain coarsening may offer better predictions in the nanocrystalline regime by including grain splitting.

### Abstract

We propose a program at B-factories of inclusive, multi-track displaced vertex searches, which are expected to be low background and give excellent sensitivity to non-minimal hidden sectors. Multi-particle hidden sectors often include long-lived particles (LLPs) which result from approximate symmetries, and we classify the possible decays of GeV-scale LLPs in an effective field theory framework. Considering several LLP production modes, including dark photons and dark Higgs bosons, we study the sensitivity of LLP searches with different number of displaced vertices per event and track requirements per displaced vertex, showing that inclusive searches can have sensitivity to a large range of hidden sector models that are otherwise unconstrained by current or planned searches.

#### From the Cover…

In this modern and distinctive textbook, Helliwell and Sahakian present classical mechanics as a thriving and contemporary field with strong connections to cutting-edge research topics in physics. Each part of the book concludes with a capstone chapter describing various key topics in quantum mechanics, general relativity, and other areas of modern physics, clearly demonstrating how they relate to advanced classical mechanics, and enabling students to appreciate the central importance of classical mechanics within contemporary fields of research. Numerous and detailed examples are interleaved with theoretical content, illustrating abstract concepts more concretely. Extensive problem sets at the end of each chapter further reinforce students' understanding of key concepts, and provide opportunities for assessment or self-testing. A detailed online solutions manual and lecture slides accompany the text for instructors. Often a flexible approach is required when teaching advanced classical mechanics, and, to facilitate this, the authors have outlined several paths instructors and students can follow through the book, depending on background knowledge and the length of their course.

### Abstract

Biological membranes are composed of lipid bilayers that are often asymmetric with regards to the lipid composition and/or aqueous solvent they separate. Studying lipid asymmetry both experimentally and computationally is challenging. Molecular dynamics simulations of lipid bilayers with asymmetry are difficult due to finite system sizes and time scales accessible to simulations. Due to the very slow flip-flop rate for phospholipids, one must first choose how many lipids are on each side of the bilayer, but the resulting bilayer may be unstable (or metastable) due to differing tensile and compressive forces between leaflets. Here we use molecular dynamics simulations to investigate a number of different asymmetric membrane systems, both with atomistic and coarse-grained models. Asymmetries studied include differences in number of lipids, lipid composition (unsaturated and saturated tails and different headgroups), and chemical gradients between the aqueous phases. Extensive analysis of the bilayers’ properties such as area per lipid, density, and lateral pressure profiles are used to characterize bilayer asymmetry. We also address how cholesterol (which flip-flops relatively quickly) influences membrane asymmetries. Our results show how each leaflet is influenced by the other and can mitigate the structural changes to the bilayer overall structure. Cholesterol can respond to changes in bilayer asymmetry to alleviate some of the effect on the bilayer structure, but that will alter its leaflet distribution, which in turn affects its chemical potential. Ionic imbalances are shown to have a modest change in bilayer structure, despite large changes in the electrostatic potential. Bilayer asymmetry can also induce a modest electrostatic potential across the membrane. Our results highlight the importance of membrane asymmetry on bilayer properties, the influence of lipid headgroups, tails and cholesterol on asymmetry, and the ability of lipids to adapt to different environments.

### Abstract

We carried out six targeted structure from motion surveys using small uninhabited aerial systems over the $$M_w$$ 6.4 and 7.1 ruptures of the Ridgecrest earthquake sequence in the first three months after the events. The surveys cover approximately 500 $$\times$$ 500 m areas just south of Highway 178 with an average ground sample distance of 1.5 cm. The first survey took place five days after the $$M_w$$ 6.4 foreshock on 9 July 2019. The final survey took place on 27 September 2019. The time between surveys increased over time, with the first five surveys taking place in the first month after the earthquake. Comparison of imagery from before and after the $$M_w$$ 7.1 earthquake shows variation in slip on the main rupture and a small amount of distributed slip across the scene. Cracks can be observed and mapped in the high-resolution imagery, which show en echelon cracking, fault splays, and a northeast-striking conjugate fault at the $$M_w$$ 7.1 rupture south of Highway 178 and near the dirt road. Initial postseismic results show little fault afterslip, but possible subsidence in the first 7–10 days after the earthquake, followed by uplift.

### Abstract

It was shown in a recent paper [J. Math. Phys. (N.Y.) 60, 102502 (2019)] that slowly lowering an electric charge into a Schwarzschild-Tangherlini (ST) black hole endows the final state with electric multipole fields, which implies that the final-state geometry is not Reissner-Nordström-Tangherlini in nature. This conclusion departs from the four-dimensional case in which the no-hair theorem (NHT) requires the final state to be a Reissner-Nordström black hole. To better understand this discrepancy clearly requires a deeper understanding of the origin of the multipole hair in the higher-dimensional case. In this paper, we advance the conjecture that charged, static, and asymptotically flat higher-dimensional black holes can acquire electric multipole hair only after they form. This supposition derives from studying the asymptotic behavior of the field of a multipole charge onto which a massive and hyperspherical shell with an exterior ST geometry is collapsing. In the mathematical limit as the shell approaches its ST radius, we find that the multipole fields (except the monopole) vanish. This implies that the only information of an arbitrary (but finite) charge distribution inside the collapsing shell that is available to an asymptotic observer is the total electric charge. Our results yield considerable insight into how higher-dimensional black holes acquire electric multipole hair, and also imply that, in four dimensions, the fadeaway of multipole moments during gravitational collapse is not strictly because of the NHT.

### Abstract

We propose a simple model in which the baryon asymmetry and dark matter are created via the decays and inverse decays of QCD-triplet scalars, at least one of which must be in the TeV mass range. Singlet fermions produced in these decays constitute the dark matter. The singlets never reach equilibrium, and their coherent production, propagation, and annihilation generates a baryon asymmetry. We find that the out-of-equilibrium condition and the dark matter density constraint typically require the lightest scalar to be long-lived, giving good prospects for detection or exclusion in current and upcoming colliders. In generalizing the leptogenesis mechanism of Akhmedov, Rubakov and Smirnov, our model expands the phenomenological possibilities for low-scale baryogenesis.

# Recent Publications

Student authorFaculty author

1. William Lamb, Dallon E. Asnes, Jonathan G. Kupfer, Emma Grace Lickey, Jeremy Kenechukwu Bakken, Richard Campbell Haskell, Peter N. Saeta, and Qimin Yang Real-Time Anticipation and Prevention of Hot Spots by Monitoring the Dynamic Conductance of Photovoltaic Panels IEEE Journal of Photovoltaics (2022) 1-10. Raghav Acharya, Elio J Challita, Mark Ilton, and M Saad Bhamla The Ultrafast Snap of a Finger Is Mediated by Skin Friction Journal of the Royal Society Interface 18 (2021) 0672. Anna Rose Barth, Maya Helena Martinez, Cora Emerson Payne, Christopher Gray Couto, Izabela Joy Quintas, Thorthong Soncharoen, Nina M. Brown, Eli Joseph Weissler, and Sharon Gerbode Grain Splitting Is a Mechanism for Grain Coarsening in Colloidal Polycrystals Physical Review E 104 (2021) L052601. Mason Andrew Acevedo, Albany Eve Blackburn, Nikita Blinov, Brian Shuve, and Mavis Voilac Stone Multi-Track Displaced Vertices At B-Factories Journal of High Energy Physics (2021) 154. Thomas McCaffree Helliwell and Vatche Sahakian Modern Classical Mechanics Cambridge University Press, 2021. Madison Rae Blumer, Sophia Laurice Harris, Mengzhe Li, Luis Angel Martinez, Michael Untereiner, Peter N. Saeta, Timothy S. Carpenter, Helgi I. Ingólfsson, and W. F. Drew Bennett Simulations of Asymmetric Membranes Illustrate Cooperative Leaflet Coupling and Lipid Adaptability Frontiers in Cell and Developmental Biology 8 (2020) 575. Andrea Donnellan, Gregory A. Lyzenga, Adnan Ansar, Christine A Goulet, Jun Wang, and Marlon Pierce Targeted High-Resolution Structure From Motion Observations Over the $$M_W$$ 6.4 and 7.1 Ruptures of the Ridgecrest Earthquake Sequence Seismological Research Letters (2020) . Matthew Stephen Fox Electric Multipole Fields of Higher-Dimensional Massive Bodies Physical Review D 102 (2020) 044008. Scott J Brandenberg, Jonathan P Stewart, Pengfei Wang, Chukwuebuka C Nweke, Kenneth Hudson, Christine A Goulet, Xiaofeng Meng, Craig A Davis, Sean K Ahdi, Martin B Hudson, Andrea Donnellan, Gregory A. Lyzenga, Marlon Pierce, Jun Wang, Maria A Winters, Marie-Pierre Delisle, Joseph Lucey, Yeulwoo Kim, Timu W Gallien, Andrew Lyda, J Sean Yeung, Omar Issa, Tristan Buckreis, and Zhengxiang Yi Ground Deformation Data From Geer Investigations of Ridgecrest Earthquake Sequence Seismological Research Letters (2020) 1–11. Brian Shuve and David Tucker-Smith Baryogenesis and Dark Matter From Freeze-In Physical Review D 101 (2020) 115023.