Abstract

The year 2020 has seen the emergence of a global pandemic as a result of the disease COVID-19. This report reviews knowledge of the transmission of COVID-19 indoors, examines the evidence for mitigating measures, and considers the implications for wintertime with a focus on ventilation.

Boussinesq salt-water laboratory experiments of Rayleigh-Taylor instability (RTI) can achieve mixing efficiencies greater than 0.75 when the unstable interface is confined between two stable stratifications. This is much greater than that found when RTI occurs between two homogeneous layers when the mixing efficiency has been found to approach 0.5. Here, the mixing efficiency is defined as the ratio of energy used in mixing compared with the energy available for mixing. If the initial and final states are quiescent then the mixing efficiency can be calculated from experiments by comparison of the corresponding density profiles. Varying the functional form of the confining stratifications has a strong effect on the mixing efficiency. We derive a buoyancy-diffusion model for the rate of growth of the turbulent mixing region, h = 2 √ αAgh (where A = A(h) is the Atwood number across the mixing region when it extends a height h, g is acceleration due to gravity and α is a constant). This model shows good agreement with experiments when the value of the constant α is set to 0.07, the value found in experiments of RTI between two homogeneous layers (where the height of the turbulent mixing region increases as h = αAgt 2 , an expression which is equivalent to that derived forḣ).

Biological systems often involve the self-assembly of basic components into complex and functioning structures. Artificial systems that mimic such processes can provide a well-controlled setting to explore the principles involved and also synthesize useful micromachines. Our experiments show that immotile, but active, components self-assemble into two types of structure that exhibit the fundamental forms of motility: translation and rotation. Specifically, micron-scale metallic rods are designed to induce extensile surface flows in the presence of a chemical fuel; these rods interact with each other and pair up to form either a swimmer or a rotor. Such pairs can transition reversibly between these two configurations, leading to kinetics reminiscent of bacterial run-and-tumble motion.Self-assembly is a hallmark of biological systems as organisms construct functional complex materials and structures from simpler components. The use of selfassembly in the fabrication of new materials is attractive owing to its inherent versatility and potential for mass production [1]. Research has focused primarily on systems where the formation of macrostructures does not require the continuous input of energy [2-5], a process known as equilibrium self-assembly. Another route to self-assembly is dynamic: where structures persist only while energy is being supplied to the system [6]. Artificial micron-scale motors, immersed in a fuel laden fluid, have been shown to spontaneously self-organize into crystal structures [7], and form into asters of two or more particles [8]. These studies have active components, producing a local fluid flow which leads to motility. A striking aspect of these active systems is their potential for emergent dynamics, whereby individuals and groups have qualitatively different behaviour [9], such as flocking [10], formation of flow structures on larger scales than the individual components [11], or collective flows which are faster than those induced by individual active particles [12].Here we describe how two of the simplest machines can be formed by dynamic self-assembly from a single type of building block. Active, but immotile, rods spontaneously self-assemble into structures that exhibit the two fundamental types of motion: rotation and translation ( Fig. 1d-g). This occurs without imposing any external field. Assemblies can transition between rotor and swimmer, leading to behaviour reminiscent of the run-and-tumble motion of E. coli [13]. The type and direction of motion is determined by the configuration of rods in an assembly, thereby linking the self-assembly to the emergent motility. This is a rare example of artificial dynamic self-assembly and therefore represents an important step towards making more complex micron-scale machines.