ALBERT

Description: Rectilinear collisions of three wetted spheres are considered under conditions of high capillary numbers, for which viscous lubrication forces dominate over capillary forces. The viscous forces resist the relative motion, as characterized by the Stokes number (a dimensionless ratio of particle inertia and viscous forces). At high Stokes numbers, the particles penetrate the fluid layers between them with sufficient inertia that they collide and rebound. Both simultaneous and sequential collisions are simulated, and various outcomes are demonstrated: full agglomeration of the three spheres at low Stokes numbers, full separation or Newton's cradle at large Stokes numbers and even reverse Newton's cradle at intermediate Stokes numbers when there is a thicker combined fluid layer between the two target spheres than between the striker sphere and the first target sphere. When there is an initial air gap between the two target spheres, even more exotic outcomes are predicted, such as full separation after the initial collisions followed by full agglomeration or reverse Newton's cradle (intermediate Stokes numbers) or Newton's cradle (large Stokes numbers) after the subsequent collisions when the striker sphere catches back up to the target spheres. The approach and findings of this work are expected to provide input and guidance to future work on discrete-element modelling of collisions of many wet particles. © 2019 Cambridge University Press.

Description: The current paper presents an experimental study of the energy budget of a two-dimensional internal wave attractor in a trapezoidal domain filled with uniformly stratified fluid. The injected energy flux and the dissipation rate are simultaneously measured from a two-dimensional, two-component, experimental velocity field. The pressure perturbation field needed to quantify the injected energy is determined from the linear inviscid theory. The dissipation rate in the bulk of the domain is directly computed from the measurements, while the energy sink occurring in the boundary layers is estimated using the theoretical expression for the velocity field in the boundary layers, derived recently by Beckebanze et al. (J. Fluid Mech., vol. 841, 2018, pp. 614-635). In the linear regime, we show that the energy budget is closed, in the steady state and also in the transient regime, by taking into account the bulk dissipation and, more importantly, the dissipation in the boundary layers, without any adjustable parameters. The dependence of the different sources on the thickness of the experimental set-up is also discussed. In the nonlinear regime, the analysis is extended by estimating the dissipation due to the secondary waves generated by triadic resonant instabilities, showing the importance of the energy transfer from large scales to small scales. The method tested here on internal wave attractors can be generalized straightforwardly to any quasi-two-dimensional stratified flow. © 2019 Cambridge University Press.

Description: The interfacial behaviour of surfactant-laden drops squeezing through tight constrictions in a uniform far-field flow is modelled with respect to capillary number, drop-to-medium viscosity ratio and surfactant contamination. The surfactant is treated as insoluble and non-diffusive, and drop surface tension is related to surfactant concentration by a linear equation of state. The constriction is formed by three solid spheres held rigidly in space. A characteristic aspect of this confined and contaminated multiphase system is the rapid development of steep surfactant-concentration gradients during the onset of drop squeezing. The interplay between two physical effects of surfactant, namely the greater interface deformability due to decreased surface tension and interface immobilization due to Marangoni stresses, results in particularly rich drop-squeezing dynamics. A three-dimensional boundary-integral algorithm is used to describe drop hydrodynamics, and accurate treatment of close squeezing and trapped states is enabled by advanced singularity subtraction techniques. Surfactant transport and hydrodynamics are coupled via the surface convection equation (or convection-diffusion equation, if artificial diffusion is included), the interfacial stress balance and a solid-particle contribution based on the Hebeker representation. For extreme conditions, such as drop-to-medium viscosity ratios significantly less than unity, it is found that upwind-biased methods are the only stable approaches for modelling surfactant transport. Two distinct schemes, upwind finite volume and flow-biased least squares, are found to provide results in close agreement, indicating negligible numerical diffusion. Surfactant transport is enhanced by low drop-to-medium viscosity ratios, at which extremely sharp concentration gradients form during various stages of the squeezing process. The presence of surfactant, even at low degrees of contamination, significantly decreases the critical capillary number for droplet trapping, due to the accumulation of surfactant at the downwind pole of the drop and its subsequent elongation. Increasing the degree of contamination significantly affects surface mobility and further decreases the critical capillary number as well as drop squeezing times, up to a threshold above which the addition of surfactant negligibly affects squeezing dynamics. © 2019 Cambridge University Press.

Description: A small fauna of vertebrates is recorded from the Insect Limestone, Bembridge Marls Member, Bouldnor Formation, late Priabonian, latest Eocene, of the Isle of Wight, UK. The taxa represented are unidentified teleost fishes, lizards including a scincoid, unidentified birds and the theridomyid rodent Isoptychus. The scincoid represents the youngest record of the group in the UK. Of particular note is the taphonomic interpretation based on the preservation of anatomical parts of land-based tetrapods that would have been most likely transported to the site of deposition by wind, namely bird feathers and pieces of shed lizard skin. These comprise the majority of the specimens and suggest that the dominant transport mechanism was wind.

Description: We examine the environmental impacts of a cash transfer program in rural Zambia and investigate whether variation in market access is associated with heterogeneous impacts on natural resource use. We consider households’ use of firewood, charcoal, bushmeat and land for farming, as well as their ownership of non-farm businesses. We find that cash transfers increase the likelihood of charcoal consumption as well as the amount consumed for those living close to paved roads. The transfers also enable households to increase the size of their farms and establish non-farm businesses. These impacts are most pronounced for those living far from paved roads. While remoteness is associated with farm expansion in response to the cash transfer, more education causes those receiving the transfer to decrease the size of their farms. This impact heterogeneity has important implications for sustainable development.