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We analyse the motion of a flagellated bacterium in a two-fluid medium using slender body theory. The two-fluid model is useful for describing a body moving through a complex fluid with a microstructure whose length scale is comparable to the characteristic scale of the body. This is true for bacterial motion in biological fluids (entangled polymer solutions), where the entanglement results in a porous microstructure with typical pore diameters comparable to or larger than the flagellar bundle diameter, but smaller than the diameter of the bacterial head. Thus, the polymer and solvent satisfy different boundary conditions on the flagellar bundle and move with different velocities close to it. This gives rise to a screening length $L_B$ within which the fluids exchange momentum and the relative velocity between the two fluids decays. In this work, both the solvent and polymer of the two-fluid medium are modelled as Newtonian fluids with different viscosities $\mu _s$ and $\mu _p$ (viscosity ratio $\lambda = \mu _p/\mu _s$), thereby capturing the effects solely introduced by the microstructure of the complex fluid. From our calculations, we observe an increased drag anisotropy for a rigid, slender flagellar bundle moving through this two-fluid medium, resulting in an enhanced swimming velocity of the organism. The results are sensitive to the interaction between the bundle and the polymer, and we discuss two physical scenarios corresponding to two types of interaction. Our model provides an explanation for the experimentally observed enhancement of swimming velocity of bacteria in entangled polymer solutions and motivates further experimental investigations.
Ever since the first exoplanets were discovered over 30 years ago, their detection has proceeded at a remarkable pace. This chapter describes the techniques for identifying these worlds, as well as characterising their atmospheres and surfaces to seek out possible signs of life. The most common methods for detecting exoplanets are reviewed: radial velocity measurements, transits, gravitational microlensing, astrometry, and direct imaging. This is followed by summarising avenues for characterising exoplanets through performing spectroscopy of three sources of radiation linked to them: (1) transmitted light passing through an exoplanetary atmosphere and reaching us; (2) thermal emission associated with the blackbody radiation of the planet; and (3) starlight reflected from that world. The chapter concludes by commenting on the bright future of exoplanetary science and future telescopes devoted to this area.
This short chapter contains basics of the mathematical formalism for thequantum measurement theory. In this book we proceed mainly withthe von Neumann measurement theory in which observables are given byHermitian operators and the state update by projections. However, we alsomention the measurement formalism based on quantum instruments, sinceit gives the general framework for quantum measurements. This formalismis used only in Chapters 10 and 18. The latter chapter is devoted to quantum-likemodeling – the applications of the mathematical formalism and methodologyof quantum mechanics (QM) to cognition, psychology, and decision making.Surprisingly, in such applications even the simplest effects can’t be described bythe von Neumann theory. One should use quantum instruments (compare withquantum physics where the main body of theory can be presented solelywithin the von Neumann measurement theory).
The precessions of the Keplerian orbital elements are calculated for several tidal-type accelerations due to the presence of a distant 3rd body: Newtonian, post-Newtonian gravitoelectric, and post-Newtonian gravitomagnetic. The calculation is made, first, in a kinematically and dynamically non-rotating frame. Then, it is repeated in a dynamically non-rotating and kinematically rotating frame accounting for the de Sitter–Fokker and Pugh–Schiff precessions of its axes.
Metastructures composed of a closely spaced plate array have been widely used in bespoke manipulation of waves in contexts of acoustics, electromagnetics, elasticity and water waves. This paper focuses on wave scattering by discrete plate array metastructures of arbitrary cross-sections, including isolated vertical metacylinders, periodic arrays and horizontal surface-piercing metacylinders. A suitable transform-based method has been applied to each problem to reduce the influence of barriers in a two-dimensional problem to a set of points in a one-dimensional wave equation wherein the solution is constructed using a corresponding Green's function. A key difference from the existing work is the use of an exact description of the plate array rather than an effective medium approximation, enabling the exploration of wave frequencies above resonance where homogenisation models fail but where the most intriguing physical findings are unravelled. The new findings are particularly notable for graded plate array metastructures that produce a dense spectrum of resonant frequencies, leading to broadband ‘rainbow reflection’ effects. This study provides new ideas for the design of structures for the bespoke control of waves with the potential for innovative solutions to coastal protection schemes or wave energy converters.
The physics and chemistry underpinning the origins of the Universe, stars, elements, and molecules is described in this chapter. It begins with outlining our understanding of the Big Bang, and how gravity subsequently facilitated the emergence of order and complexity in the Universe. This is followed by a brief exposition of star formation, stellar evolution of low- and high-mass stars, and the multiple pathways responsible for the production of elements in stars (i.e., stellar nucleosynthesis) such as the triple alpha process. The chapter concludes with an introduction to the broad subject of astrochemistry. The emphasis is on delineating the sites of molecule formation (e.g., molecular clouds), as well as the processes involved in gas-phase chemistry and grain-surface chemistry that drive the synthesis of molecules.
The theme of how life and its environment have coevolved together for about four billion years on Earth is explored in this chapter. The major evolutionary events that unfolded in the Archean eon (4 to 2.5 billion years ago), Proterozoic eon (2.5 to 0.539 billion years ago), and the Phanerozoic eon (0.539 billion years ago to present) are outlined, such as the origin(s) of multicellularity, eukaryotes, complex multicellular organisms, and humans. By drawing on this evolutionary timeline, theoretical paradigms for understanding and grouping the notable evolutionary events are sketched (e.g., major transitions in evolution). The next part of the chapter illustrates the intricate interplay between life and its environment by chronicling the rise in molecular oxygen levels, its possible causes and profound consequences, and its potential connections with key geological changes like the putative Snowball Earth episodes. Lastly, the ‘Big Five’ mass extinctions that transpired in the Phanerozoic, along with their triggers and ramifications, are described.
Life-as-we-know-it harnesses carbon for the scaffolding in biomolecules and liquid water as the solvent. This chapter delineates the beneficial properties of carbon and water, and then investigates whether viable alternatives to this duo exist (i.e., ‘exotic’ life). With regard to the latter, the likes of ammonia, sulfuric acid, and liquid hydrocarbons are expected to have some physical and/or chemical advantages relative to water, while also exhibiting certain downsides. In contrast, it is suggested that few options appear feasible aside from carbon, with silicon representing a partial exception. The chapter subsequently delves into the habitability of the clouds of Venus and the lakes of Titan, because the alternative solvents sulfuric acid and liquid hydrocarbons (methane and ethane) are, respectively, documented therein. Both these environments might be conducive to hosting exotic life, but it is cautioned that they are likely subjected to severe challenges.
The impact of the 1pN gravitomagnetic Lense–Thirring acceleration, generalized also to the case of two massive spinning bodies of comparable masses and angular momenta, is calculated for different types of observation-related quantities (Keplerian orbital elements, anomalistic, draconitic, and sidereal orbital periods, two-body range and range rate, radial velocity curve and radial velocity semiamplitude of spectroscopic binaries, astrometric angles RA and dec., times of arrival of binary pulsars, characteristic timescales of transiting exoplanets, and their sky-projected spin-orbit angle). The results are applied to a test particle orbiting a primary, a Sun–Jupiter exoplanet system, and an S star in Sgr A*.
The combined effects of heater position, mean flow parameters and flame models on thermoacoustic instability in a one-dimensional Rijke tube are studied systematically by classic linear stability analysis (LSA) and lattice Boltzmann method (LBM) simulation. In the former, the stability range of the linear flame model under low Mach number assumption is solved analytically, while in the more general case, it is obtained by numerically solving the dispersion relation. Both the linear and nonlinear flame model cases are studied using the LBM with a spectral multiple-relaxation-time collision model and a newly developed heat source term. With the linear flame model, the LBM is in good agreement with LSA in predicting the transition point and growth rates, while with the nonlinear flame model, LBM simulations are consistent with solutions of limit cycle theory in the fully developed state. These results demonstrate the applicability of the LBM in solving complex thermoacoustic problems.
A gas bubble sitting at a liquid–gas interface can burst following the rupture of the thin liquid film separating it from the ambient, owing to the large surface energy of the resultant cavity. This bursting bubble forms capillary waves, a Worthington jet and subsequent droplets for a Newtonian liquid medium. However, rheological properties of the liquid medium like elastoviscoplasticity can greatly affect these dynamics. Using direct numerical simulations, this study exemplifies how the complex interplay between elasticity (in terms of elastic stress relaxation) and yield stress influences the transient interfacial phenomenon of bursting bubbles. We investigate how bursting dynamics depends on capillary, elastic and yield stresses by exploring the parameter space of the Deborah number ${{\textit {De}}}$ (dimensionless relaxation time of elastic stresses) and the plastocapillary number $\mathcal {J}$ (dimensionless yield-stress of the medium), delineating four distinct characteristic behaviours. Overall, we observe a non-monotonic effect of elastic stress relaxation on the jet development while plasticity of the elastoviscoplastic (EVP) medium is shown to affect primarily the jet evolution only at faster relaxation times (low ${{\textit {De}}}$). The role of elastic stresses on jet development is elucidated with the support of energy budgets identifying different modes of energy transfer within the EVP medium. The effects of elasticity on the initial progression of capillary waves and droplet formation are also studied. In passing, we study the effects of solvent–polymer viscosity ratio on bursting dynamics and show that polymer viscosity can increase the jet thickness apart from reducing the maximum height of the jet.