Active nematics are out-of-equilibrium fluids composed of rod-like subunits, which can generate large-scale, self-driven flows. We examine a microtubule-kinesin-based active nematic confined to two-dimensions, exhibiting chaotic flows with moving topological defects. Applying tools from chaos theory, we investigate self-driven advection and mixing on different length scales. Local fluid stretching is quantified by the Lyapunov exponent. Global mixing is quantified by the topological entropy, calculated from both defect braiding and curve extension rates. We find excellent agreement between these independent measures of chaos, demonstrating that the extensile stretching between microtubules directly translates into macroscopic braiding of positive defects. Remarkably, increasing extensile activity (via ATP concentration) does not increase the dimensionless topological entropy. This study represents the first application of chaotic advection to the emerging field of active nematics and the first time that the collective motion of an ensemble of defects has been quantified (via topological entropy) in a liquid crystal.

are driven by human life support systems, scientific exploration and Earth observation equipment, telecommunications, and electric propulsion systems. There is great interest in highly efficient perovskite-structured thin-film solar cells for space applications. [1,2] These are promising candidates due to their excellent optoelectronic characteristics, low-cost, high performance, [2][3][4] and their facile manufacturability [5] potentially suitable for in-space manufacturing. [6] These traits coupled with their defect tolerance, [7,8] and radiation tolerance [9] have garnered interest for aerospace applications. Prior to the widespread implementation of metal halide perovskites (MHPs) into the space environment, solar cells must pass rigorous American Institute of Aeronautics and Astronautics Standard 111 (AIAA-S111) space qualification testing. [10] Low earth orbit (LEO), 160-2000 km above the Earth's surface, is an ideal place to operate MHPs either on the International Space Station or on satellites. The harsh environment of LEO includes thermal cycling (±120 ⁰C), vacuum (10 −6 -10 −9 torr), ultra-violet radiation, exposure to atomic oxygen (flux 10 13 -10 15 AO/cm 2 with collision energy of 5 eV), plasma (10 6 cm −3 , ≤1 eV electron temperature), and ionizing radiation of electrons, protons, micrometeoroids (60 km s −1 ) and orbital debris (10 km s −1 ). [11] We must demonstrate MHP durability in relevant space environments to evidence feasibility. Implementing Metal halide perovskites (MHPs) have emerged as a prominent new photovoltaic material combining a very competitive power conversion efficiency that rivals crystalline silicon with the added benefits of tunable properties for multijunction devices fabricated from solution which can yield high specific power. Perovskites have also demonstrated some of the lowest temperature coefficients and highest defect tolerance, which make them excellent candidates for aerospace applications. However, MHPs must demonstrate durability in space which presents different challenges than terrestrial operating environments. To decisively test the viability of perovskites being used in space, a perovskite thin film is positioned in low earth orbit for 10 months on the International Space Station, which was the first long-duration study of an MHP in space. Postflight high-resolution ultrafast spectroscopic characterization and comparison with control samples reveal that the flight sample exhibits superior photo-stability, no irreversible radiation damage, and a suppressed structural phase transition temperature by nearly 65 K, broadening the photovoltaic operational range. Further, significant photo-annealing of surface defects is shown following prolonged light-soaking postflight. These results emphasize that methylammonium lead iodide can be packaged adequately for space missions, affirming that space stressors can be managed as theorized.

Aerospace In article number 2203920, Lyndsey McMillon‐Brown, Sayanatani Ghosh, and co‐workers report on the first long duration space flight of a metal halide perovskite photoactive layer on the International Space Station. Post‐flight analysis reveals that samples exhibit superior photo‐stability, no irreversible radiation damage, and a suppressed structural phase transition temperature, broadening the photovoltaic operational range. These results confirm that perovskite photovoltaics can be designed to endure the space environment.

Active fluids, composed of individual self-propelled agents, can generate complex large-scale coherent flows. A particularly important laboratory realization of such an active fluid is a system composed of microtubules, aligned in a quasi-two-dimensional (2D) nematic phase and driven by adenosine-triphosphate-fueled kinesin motor proteins. This system exhibits robust chaotic advection and gives rise to a pronounced fractal structure in the nematic contours. We characterize such experimentally derived fractals using the power spectrum and discover that the power spectrum decays as k−β for large wavenumbers k. The parameter β is measured for several experimental realizations. Though β is effectively constant in time, it does vary with experimental parameters, indicating differences in the scale-free behavior of the microtubule-based active nematic. Though the fractal patterns generated in this active system are reminiscent of passively advected dye in 2D chaotic flows, the underlying mechanism for fractal generation is more subtle. We provide a simple, physically inspired mathematical model of fractal generation in this system that relies on the material being locally compressible, though the total area of the material is conserved globally. The model also requires that large-scale density variations are injected into the material periodically. The model reproduces the power-spectrum decay k−β seen in experiments. Linearizing the model of fractal generation about the equilibrium density, we derive an analytic relationship between β and a single dimensionless quantity r, which characterizes the compressibility.

We demonstrated Raman-based detection of CO2 in hollow core photonic crystal fiber with 532nm or 785nm excitation sources at various gas concentrations and pressures. We also carried out kinetics studies and comparison with simulations.