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...
