Publications

The flow of two immiscible liquids or fluids in bounded systems where confinement geometry varies can lead to drop or bubble formation. This phenomenon has been reported in the context of oil recovery and named snap-off, or exploited for making emulsions, and then foams, by using microfluidic systems, namely, microchannel emulsification or step emulsification. We report a comprehensive experimental investigation of such an emulsification process occurring at the end of a glass rectangular tube filled with oil and immersed in a water bath. This allows us to clearly visualize the breakup event of the dispersed phase liquid finger at the capillary's end. Below a critical flow rate, the drop size varies slowly with the flow rate and it is linked to the pinching time of the dispersed phase. A semiempirical law that gives the resulting drop size as a function of fluid and geometrical properties is proposed. However, this feature is altered for an aspect ratio of the rectangular tube below 2.5 where the forming drop hinders the counterflow of the continuous phase leading to larger drops. Then, above a critical flow rate, or capillary number that weakly depends on the viscosity ratio of the two liquids, the neck adopts a quasistatic shape well accounted for by a model based on a Hele-Shaw flow. In that case, drop formation is driven by gravity and a transition from a dripping regime to a jetting regime is observed at higher flow rates. Monodisperse foam can also be formed by injecting air. While the overall dynamics of bubble formation shares similarities with an incompressible fluid, the bubble size and the critical capillary number do not follow the same scaling laws.

Convective dispersion of solutes is inherent to flow in channels because of the nonuniformity of the velocity profile. When diffusion is negligible, for large particles for example, the trajectory of particles can be solely described by a kinematic approach. Here, we investigate such a phenomenon for micrometer-size beads flowing in a circular pipe. We show that the presence of large bubbles, namely in the case of a segmented flow, either prevents the convective dispersion or leads to the accumulation of particles at the rear of the bubble moving in front. The destabilization of the initially homogeneous suspension occurs when liquid inertia comes into play. Indeed, for moderate Reynolds number of the particles, particles move away from the wall, thus exploring different flow lines that finally impact the axial dispersion features. Moreover, since the bubbles impose an axial boundary condition of the mean velocity, a net flux of particles directed along the flow direction is built up above a critical particle Reynolds number. This work is motivated by the understanding of the flow behavior of biological samples, and especially in the context of cell encapsulation.