Marine snow is central to the marine carbon cycle, and quantifying its small-scale settling dynamics in different physical environments is essential to understanding its role in biogeochemical cycles. Previous field observations of marine aggregate thin layers associated with sharp density gradients have led to the hypothesis that these layers may be caused by a decrease in aggregate settling speed at density interfaces. Here, we present experimental data on aggregate settling behavior, showing that these particles can dramatically decrease their settling velocity when passing through sharp density transitions. This delayed settling can be caused by 2 potential mechanisms: (1) entrainment of lighter fluid from above as the particle passes through the density gradient, and (2) retention at the transition driven by changes in the density of the particle due to its porosity. The aggregates observed in this study exhibited 2 distinct settling behaviors when passing through the density transition. Quantitatively comparing these different behaviors with predictions from 2 models allow us to infer that the delayed settling of the first group of aggregates was primarily driven by diffusion-limited retention, whereas entrainment of lighter fluid was the dominant mechanism for the second group. Coupled with theory, our experimental results demonstrate that both entrainment and diffusion-limited retention can play an important role in determining particle settling dynamics through density transitions. This study thus provides insight into ways that delayed settling can lead to the formation of aggregate thin layers, important biological hotspots that affect trophic dynamics, and biogeochemical cycling in the ocean.

Recent experiments of thin films flowing down a vertical fiber with varying nozzle diameters present a wealth of new dynamics that illustrate the need for more advanced theory. We present a detailed analysis using a full lubrication model that includes slip boundary conditions, nonlinear curvature terms, and a film stabilization term. This study brings to focus the presence of a stable liquid layer playing an important role in the full dynamics. We propose a combination of these physical effects to explain the observed velocity and stability of traveling droplets in the experiments and their transition to isolated droplets. This is also supported by stability analysis of the traveling wave solution of the model.

A sphere exhibits a prolonged residence time when settling through a stable stratification of miscible fluids due to the deformation of the fluid-density field. Using a Green's function formulation, a first-principles numerically assisted theoretical model for the sphere-fluid coupled dynamics at low Reynolds number is derived. Predictions of the model, which uses no adjustable parameters, are compared with data from an experimental investigation with spheres of varying sizes and densities settling in stratified corn syrup. The velocity of the sphere as well as the deformation of the density field are tracked using time-lapse images, then compared with the theoretical predictions. A settling rate comparison with spheres in dense homogeneous fluid additionally quantifies the effect of the enhanced residence time. Analysis of our theory identifies parametric trends, which are also partially explored in the experiments, further confirming the predictive capability of the theoretical model. The limit of infinite fluid domain is considered, showing evidence that the Stokes paradox of infinite fluid volume dragged by a moving sphere can be regularized by density stratifications. Comparisons with other possible models under a hierarchy of additional simplifying assumptions are also presented.

The behavior of settling particles in stratified fluid is important in a variety of applications, from environmental to medical. We document a phenomenon in which a sphere, when crossing density transitions, slows down substantially in comparison to its settling speed in the bottom denser layer, due to entrainment of buoyant fluid. We present results from an experimental study of the effects of the fluid interface on flight times as well as a theoretical model derived from first principles in the low Reynolds number regimes for stratified miscible fluids. Our work provides a new predictive tool and gives insight into the role of strong stratification in particle settling.The accumulation of particulate matter in the vicinity of strong density transitions is an important observation in gravitational settling of particles through haloclines or thermoclines in the environment. 1-5 Surprisingly, there are very few investigations within controlled laboratory experiments that elucidate the basic underlying physical mechanisms for settling through stable, miscible stratifications. We stress that the natural stratification for these environmental applications involve miscible fluids, in contrast to the more commonly studied situation in which surface tension between immiscible fluids plays a dominant role. 6,7 For the miscible case, in a regime where inertial effects are important, Srdić-Mitrović et al. 8 studied falling particles in stratified fluids experimentally and observed a prolonged residence time in regions of high stratification. For yet stronger inertial effects, Abaid et al. 9 found cases in which a sphere would levitate, even reverse direction, as it passes through a density transition. In this paper, we focus on the case of settling at low Reynolds number through stratified layers, a relevant regime for a wide range of bio-and geophysical applications.A body translating in a solution will entrain ambient fluid, regardless of stratification ͑or even viscosity 10,11 ͒. This is exhibited in Fig. 1, where a sphere can be seen dragging fluid ͑dyed purple and appearing dark gray in the black and white photos͒ from the lighter top layer into the clear, denser bottom fluid. The entrained fluid has interesting dynamical effects in the presence of stratification. To compare the homogeneous with the stratified case, first note, as well known, that a sphere released from rest inside a homogeneous fluid will accelerate to terminal velocity and fall at this constant speed due to the balance of viscous and buoyant forces. Suppose we now replace the upper half of this medium with less dense fluid and release the sphere from rest inside the upper fluid, allowing it to pass though the density transition. If the depth of the top layer is sufficient, the sphere would accelerate and approach its terminal velocity there. Similarly, if the depth of the bottom layer is sufficient, the sphere must approach closely the ͑lower͒ terminal velocity of the denser fluid. As the sphere crosses into the bottom layer, we observe a nonmonoton...

We revisit the tears of wine problem for thin films in water-ethanol mixtures and present a new model for the climbing dynamics. The new formulation includes a Marangoni stress balanced by both the normal and tangential components of gravity as well as surface tension which lead to distinctly different behavior. The prior literature did not address the wine tears but rather the behavior of the film at earlier stages and the behavior of the meniscus. In the lubrication limit we obtain an equation that is already well-known for rising films in the presence of thermal gradients. Such models can exhibit non-classical shocks that are undercompressive. We present basic theory that allows one to identify the signature of an undercompressive (UC) wave. We observe both compressive and undercompressive waves in new experiments and we argue that, in the case of a pre-coated glass, the famous "wine tears" emerge from a reverse undercompressive shock originating at the meniscus.