Rossby waves are a pervasive feature of the large-scale motions of the Earth’s atmosphere and oceans. These waves (also known as planetary waves and r-modes) also play an important role in the large-scale dynamics of different astrophysical objects such as the solar atmosphere and interior, astrophysical discs, rapidly rotating stars, planetary and exoplanetary atmospheres. This paper provides a review of theoretical and observational aspects of Rossby waves on different spatial and temporal scales in various astrophysical settings. The physical role played by Rossby-type waves and associated instabilities is discussed in the context of solar and stellar magnetic activity, angular momentum transport in astrophysical discs, planet formation, and other astrophysical processes. Possible directions of future research in theoretical and observational aspects of astrophysical Rossby waves are outlined.

By converting energy into mechanical work, engines play a central role in most biological and technological processes. In particular, within the current trend towards the development of nanoscience and nanotechnology, microscopic engines have been attracting an ever-increasing interest. On the one hand, there has been a quest to understand how biological molecular motors work. On the other hand, several approaches have been proposed to realize artificial microscopic engines, which have been powered by the transfer of light momentum, by external magnetic fields, by in situ chemical reactions, or by the energy flow between hot and cold heat reservoirs, in scaled-down versions of macroscopic heat engines. Here, we experimentally demonstrate a microscopic engine powered by the local reversible demixing of a critical mixture. We show that, when an absorbing microsphere is optically trapped by a focused laser beam in a sub-critical mixture, it is set into rotation around the optical axis of the beam because of the emergence of diffusiophoretic propulsion; this behavior can be controlled by adjusting the optical power, the temperature, and the criticality of the mixture. Given its simplicity, this microscopic engine provides a powerful tool to power micro-and nanodevices. Furthermore, since many biological systems are tuned near criticality, this mechanism might already be at work within living organisms, for example in proteins and in cellular membranes.

By converting energy into mechanical work, engines play a central role in most biological and technological processes. In particular, within the current trend towards the development of nanoscience and nanotechnology, microscopic engines have been attracting an ever-increasing interest. On the one hand, there has been a quest to understand how biological molecular motors work. On the other hand, several approaches have been proposed to realize artificial microscopic engines, which have been powered by the transfer of light momentum, by external magnetic fields, by in situ chemical reactions, or by the energy flow between hot and cold heat reservoirs, in scaled-down versions of macroscopic heat engines. Here, we experimentally demonstrate a microscopic engine powered by the local reversible demixing of a critical mixture. We show that, when an absorbing microsphere is optically trapped by a focused laser beam in a sub-critical mixture, it is set into rotation around the optical axis of the beam because of the emergence of diffusiophoretic propulsion; this behavior can be controlled by adjusting the optical power, the temperature, and the criticality of the mixture. Given its simplicity, this microscopic engine provides a powerful tool to power micro-and nanodevices. Furthermore, since many biological systems are tuned near criticality, this mechanism might already be at work within living organisms, for example in proteins and in cellular membranes.

A counterintuitive destabilizing effect of the surface tension in planar wakes has been observed by Tammisola et al. ["Effect of surface tension on global modes of confined wake flows," Phys. Fluids 23, 014108 (2011)] and Biancofiore et al. ["Direct numerical simulations of two-phase immiscible wakes," Fluid Dyn. Res. 46, 041409 (2014)] by means of linear global analyses and direct numerical simulations, respectively.In the present study, we approximate the velocity profile of a wake flow through a piecewise broken-line profile and explain the presence of temporal unstable modes using an interfacial wave interaction perspective. With this perspective, we associate to each vorticity discontinuity an individual counterpropagating Rossby wave (RW), while the introduction of a finite amount of surface tension at the interface creates two capillary waves (CWs) which propagate with respect to the interface velocity with the same relative velocity but in opposite directions. The addition of the surface tension generates a new unstable mode, which is a Rossby-capillary mode, since it is due to the interaction between one RW and one CW. Furthermore, we capture the spatio-temporal evolution of the interacting four-waves system by means of an impulse response analysis. The spreading of the wavepacket, and consequently the absolute nature of the instability, is enhanced by a moderate surface tension, especially if the interface is located close to the faster edge of the broken-line wake profile. This can be explained by the influence of the surface tension on the group velocities of the waves, taken in isolation. C 2015 AIP Publishing LLC. [http://dx.

Self-organisation is driven by the interactions between the individual components of a system mediated by the environment, and is one of the most important strategies used by many biological systems to develop complex and functional structures. Furthermore, biologicallyinspired self-organisation offers opportunities to develop the next generation of materials and devices for electronics, photonics and nanotechnology. In this work, we demonstrate experimentally that a system of Janus particles (silica microspheres half-coated with gold) aggregates into clusters in the presence of a Gaussian optical potential and disaggregates when the optical potential is switched off. We show that the underlying mechanism is the existence of a hydrodynamic flow induced by a temperature gradient generated by the light absorption at the metallic patches on the Janus particles. We also perform simulations, which agree well with the experiments and whose results permit us to clarify the underlying mechanism. The possibility of hydrodynamic-flux-induced reversible clustering may have applications in the fields of drug delivery, cargo transport, bioremediation and biopatterning.