An experimental study on stratified particle-laden plumes is presented and five steady-state flow regimes have been identified. The steady-state behaviour of the plume is directly related to the magnitude of the convective velocity associated with particle-induced instabilities, $U_c$ , in relation to the terminal settling velocity of each individual particle, $u_{st}$ . When $u_{st}>U_c$ , the ratio of particle to fluid buoyancy flux at the source, $P$ , becomes important. For $P<0.2$ , the plume dynamics appears very similar to a single-phase plume as particle recycling has minimal impact on the steady-state plume height. When $P>0.2$ , the plume height decreases significantly, creating an anvil-shaped intrusion similar to those associated with explosive volcanic eruptions. Importantly, the measured steady-state heights of plumes within this settling regime validate the collapse model of Apsley & Lane-Serff (J. Fluid Mech., vol. 865, 2019, pp. 904–927). When $u_{st}\leqslant U_c$ , particle re-entrainment behaviour changes significantly and the plume dynamics becomes independent of $P$ . When $u_{st}\approx U_c$ , a trough of fluid becomes present in the sedimenting veil due to a significant flux of descending particles at the edge of the plume. Once $u_{st}< U_c$ , the particles spreading in the intrusion become confined to a defined radius around the plume due to the significant ambient convection occurring beneath the current. For $u_{st}\ll U_c$ , or in the case of these experiments, when $U_c\geqslant 1\ \text{cm s}^{-1}$ , ambient convection becomes so strong that intrusion fluid is pulled down to the plume source, creating a flow reminiscent of a stratified fountain with secondary intrusions developing between the original current and the tank floor. Through an extension of the work of Cardoso & Zarrebini (Chem. Engng Sci., vol. 56, issue 11, 2001a, pp. 3365–3375), an analytical expression is developed to determine the onset of convection in the environment beyond the edge of the plume, which for a known particle settling velocity, can be used to characterise a plume's expected settling regime. In all plume regimes, the intrusion fluid is observed to rise in the environment following the sedimentation of particles and a simple model for the change in intrusion fluid height has been developed using the steady-state particle concentration at the spreading level.

Weak bubble plumes carry liquid from the environment upwards and release it at multiple intermediate levels in the form of radial intrusive currents. In this study, laboratory experiments are performed to explore the spreading of turbulent axisymmetric bubble plumes in a liquid with linear density stratification. The thickness, volumetric flowrate and spreading rates of multiple radial intrusions of plume fluid were measured by tracking the movement of dye injected at the source of bubbles. The experimental results are compared with scaling predictions. Our findings suggest that the presence of multiple intrusions reduces their spreading rate, compared to that of a single intrusion. This work is of relevance to the spreading of methane plumes issuing from the seabed in the Arctic Ocean, above methane-hydrate deposits. The slower, multiple spreading favours the presence of methane-rich seawater close to the plume, which may reduce the dissolution of methane in the bubbles, and thus promote the direct transport of methane to the atmosphere. This article is part of the theme issue ‘Stokes at 200 (part 2)’.

Nitride-based quantum dots (QDs) show promise as sources for single photon emission, enabling comparably high temperature emission and access to the blue and green spectral region. Some droplets forming during modified droplet epitaxy on non-polar (11-20) surfaces of InGaN epilayers on GaN are associated with underlying ring-like structures. The work by Springbett et al. (pp. 840–844) discusses droplet etching as a possible mechanism for ring formation, whereby In and Ga atoms are incorporated into the droplet and diffuse to the edges. Thereafter, recrystallization occurs, leading to the formation of a ring. It is hypothesised that the droplet then creeps in a direction determined by the crystallographic anisotropy and the surface energetics. Once this movement halts, etching continues. The resulting structure consists of a recession enclosed by a double ring. Transmission electron microscopy (TEM) analysis shows the droplets move along the ⟨0001⟩ c-axis, and energy dispersive X-ray spectroscopy (EDXS) indicates that they have a very high In content. These studies may help reveal the underlying QD formation mechanism during modified droplet epitaxy.

Droplets grown by modified droplet epitaxy on non‐polar (11‐20) surfaces of InGaN epilayers on GaN have been seen to be associated with underlying ring‐like structures. This work discusses droplet etching as a possible mechanism for ring formation, and droplet creeping as a possible explanation for the droplets sitting askew of the ring centre. Transmission electron microscopy (TEM) analysis shows the droplets to move along the c‐axis, and indicates that they have a very high indium content.The image shows atomic force microscopy (AFM) data of a double‐ring structure, rendered in 3D.

The detailed description is given in the paper and references therein. The spreadsheet contains seven separate sheets with data and plots associated with each of the paper figures. See the uploaded .docx file for a detailed description.