The coalescence time of bubbles in a liquid depends on the nature of the liquid, which determines both its surface thermodynamics and the molecular interactions between the gas/liquid interfaces, and on the geometry, prescribed by the curvature of the bubbles. Coalescence is well described in pure liquids that have the same composition in bulk and at interfaces and in which the interactions are attractive. In contrast, the mechanisms are poorly understood in more complex liquids in which coalescence times are orders of magnitudes larger than in pure liquids and are unpredictable. To provide insight on these mechanisms, we use model systems: binary mixtures of miscible oils. In these liquids, interfaces have purely attractive molecular interactions and the surface thermodynamics can simply be described using a well-determined Gibbs elastic modulus, which is controlled by the composition of the mixture. We measure the coalescence rate by forming periodic trains of bubbles in millifluidic tubes whose radius varies over 1.5 decade. We report coalescence times spanning more than three decades and, for a given composition, varying according to a power law with curvature, with an exponent larger than that reported in pure liquids and independent of Gibbs elasticity. The experimental behavior is in excellent agreement with a numerical solution of the coupled thermodynamical and hydrodynamical equations, performed in the simple geometry of a suspended liquid film. Our results clearly reveal how geometry and surface thermodynamics modify the coalescence process of bubbles in the limit of small Gibbs elasticity.

Foams may form in oil mixtures, such as lubricants, as a result of air entrainment. The long lifetimes of those foams significantly impair the thermal properties of lubricants and increase power losses by engines [Zhan et al. (2022)]. In order to improve the efficiency of lubricants, we offer here to gain insights in the stability of bubbles in binary mixtures of miscible oils as a function of bubble size and liquid composition. To do so, using a micro-millifluidic set-up, we control the formation of bubbles in oil mixtures and study variations in their coalescence time. The set-up allows to easily vary the curvature of the bubbles over one decade, perform statistics over a large number of coalescence events and measure coalescence times that span more than three orders of magnitude.

Emulsion separation remains a persistent challenge in chemical and process industries due to the metastable nature of dispersed droplets. In gravity separators, the overall separation rate is governed by the formation of a densely packed zone (DPZ) of deforming and coalescing droplets that mediates between the dispersed and continuous phases. Conventional bottle tests fail to capture the dynamic behavior of such systems under continuous-flow operation. In this work, a minimal millifluidic platform is developed to emulate gravity-driven separation under steady inflow, enabling direct observation of DPZ evolution. By independently controlling the emulsion feed rate and coalescence rate through chemical additives, two distinct operating regimes are identified: one yielding a stationary DPZ height and another exhibiting unbounded growth. A stochastic coalescence model captures this transition and quantitatively predicts the critical feed rate and steady-state DPZ height through dimensionless parameters. This framework advances predictive understanding of emulsion separation in continuous-flow processes.

We investigate the behavior of droplets flowing through flexible rectangular channels and uncover a fascinating phenomenon: pancake-shaped confined droplets migrate transversely toward the channel center. Our findings reveal that this migration is driven by a gradient in interfacial energy, created by the deformation of the channel under the flow of the carrier phase. To explain this behavior, we introduce a simple hydrodynamic model that accurately predicts the trajectories of individual droplets and their dependence on key experimental parameters, such as droplet volume and total flow rate. In concentrated emulsions, this effect results in a striking segregation phenomenon, where all droplets converge and concentrate in the central region of the channel. These insights pave the way for deeper understanding and control of droplet dynamics in confined environments.

Using confocal fluorescence microscopy in both hydrophilic and hydrophobic glass capillaries, this study explores (i) the influence of wall wettability on spontaneous emulsification at the oil/water interface and (ii) the mechanisms governing the interaction of water/oil microdroplets with solid surfaces, ultimately enhancing oil recovery during low-salinity water injection. Our findings reveal that microdroplet formation predominantly occurs on solid surfaces when they are hydrophilic, whereas in hydrophobic environments, droplets emerge at the water/oil interface, specifically within the thin film between the meniscus and the solid wall. The growth dynamics of these droplets follows distinct scaling laws depending on the wettability of the surface. Upon injection of low-salinity water, the apparent contact angle increases on hydrophilic walls but remains unchanged on hydrophobic walls. In both cases, however, the advancing meniscus leaves behind a residual oil film containing trapped water microdroplets. The mobility of these droplets whether sessile or not, depends on surface properties. Over time, they merge with the surrounding water, forming water patches within the residual oil film. As these patches expand and coalesce, they create a structure reminiscent of a two-dimensional foam. Ultimately, these patches continue to grow until the solid wall is fully exposed to the free water phase.

We study foam production and destabilization through a flow-focusing geometry, namely a single pore of rectangular cross-section, by coinjecting gas and liquid at constant pressure, Pg, and constant flow rate, Qw. We observe that bubble production results from a Rayleigh-Plateau destabilization of the internal gas thread that occurs at the pore neck when its width becomes comparable to the height of the rectangular-section channel. Using a simple model and numerical approach, we (i) predict the shape of the gas jet and its stability range as a function of flow parameters and device geometry, which we successfully compare with our experimental results, and (ii) demonstrate the existence of a critical local pressure drop at the pore neck that determines whether or not a stable gas flow can form. We thus show that bubble foam generation exhibits hysteretic behavior due to hydrodynamic feedback and demonstrate that there is a maximum bubble volume fraction that the generated foam cannot exceed, the value of which is fixed by the geometry. Our results suggest that the foam collapse observed in porous media when the fractional gas flow becomes too large may result from hydrodynamic feedback inhibiting foam generation and not necessarily from coalescence between bubbles, as is usually claimed.

We present a new experimental approach to further understand the injectivity impairment due to reinjection of produced water in an oilfield, containing residual oil and solids. A unique microfluidic setup with imposed flowrate is characterized by excellent reproducibility and allows one to determine the kinetics of external cake formation and the propagation of the damage inside the porous medium, similar to what happens at the injection wellbore. The growth rates of the external cake and that of the propagation of the internal damage exhibit discontinuities, likely related to a pressure buildup up to a threshold Laplace pressure above which the O/W Pickering droplets are pushed through, and which sets a limit to the cake growth. Finally, the external cake reaches a quasi-stationary thickness whose mechanisms are discussed below. Direct visualization readily achieved in microfluidic experiments, coupled with spatiotemporal image analysis, enables better spatial resolution than core flooding experiments and shows that the damage occurs in a small region close to the entry to the porous medium. These developments lead to the derivation of an analytical model of the damage formation. It appears that although very localized, this damage strongly decreases the global permeability of the whole porous medium. Finally, controlled temperature experiments permit to identify the variation of the viscosity of the oil droplets (or the viscosity ratio), as the primary mechanism by which temperature influences clogging. Clogging is slowed at high temperatures, but the final state is characterized by particle clogging and is thus irreversible.

In microchannels, the stability of a fluid jet injected into another immiscible fluid strongly depends on its degree of geometric confinement. When the width of the jet, w, is larger than the channel height, H, the surface tension driven Rayleigh–Plateau instability is suppressed so that the 2D (bidimensional)-confined jet is absolutely stable and never collapses into bubbles (or drops) in contrast to what occurs when w ≤ H [Dollet et al., “Role of the channel geometry on the bubble pinch-off in flow-focusing,” Phys. Rev. Lett. 100(3), 034504 (2008); Guillot et al., “Stability of a jet in confined pressure-driven biphasic flows at low Reynolds number in various geometries,” Phys. Rev. E 78(1), 016307 (2008)]. We here demonstrate both experimentally and theoretically that this picture is, indeed, no longer valid when Marangoni effects are considered. We experimentally show that the addition of small length alcohol molecules into the liquid phase destabilizes a 2D-confined gas–water microfluidic stream ( w > H), leading to the generation of steady non-linear waves and further to the production of bubbles. Using a simple hydrodynamic model, we show through a linear analysis that the destabilization of the gas stream may result from a Marangoni instability due to the fast adsorption of the alcohol molecules, which occurs on a timescale comparable to that of the microfluidic flow.

We investigate the spontaneous motion of a soap film in a conical geometry connected to a long tube and show how it can be used to measure the dynamic viscosity of air. In contrast to other techniques that are complicated to implement and require expensive and sophisticated equipment, this measurement method relies only on soapy water and three everyday life objects: a smartphone, a funnel, and a hose. More precisely, to determine the viscosity of air, we use a smartphone to record the spontaneous motion of a soap film placed in a funnel when the motion of the film is quasistatic and the flow of air escaping the geometry is viscously dominated. This simple experiment should be of value to undergraduate physics students in learning about effects of both fluid viscosity and surface tension (another fluid property which they could also measure with a smartphone; Goy et al., Phys. Teach. 55, 498–499 (2017)), and the usefulness of reasonable approximations in physics.

By conducting both a bottle test and isolate drop-drop experiments, we determine the coalescence rates of water droplets within water-in-oil emulsions stabilized by a large amount of Span 80 in the presence of Tween 20, a surfactant that acts as a demulsifier. Using a microscopic model based on a theory of hole nucleation, we establish an analytical formula that quantitatively predicts the coalescence frequency per unit area of droplets whose interfaces are fully covered by surfactant molecules. Despite its simplicity and the strong assumptions made for its derivation, this formula captures our experimental findings on Span 80-stabilized emulsions as well as other results, found in the literature, remarkably well on a wide range of water-in-crude oil systems.

A specific organization of optically active nanoscale objects can greatly affect the optical response of a system. Here, we report the controlled modification of the fluorescent emission by the assembly of water-soluble quantum rods (QRs). Our study combines optical, electron microcopy, and X-ray scattering characterizations to reveal a correlation between the self-assembly behavior of QRs into ordered 3D-arrays and the optical properties (luminescence) of formed assemblies, where the observed optical response is highly dependent on the QR aspect ratio. Specifically, shorter, 18 nm long QRs (QR(18)), exhibiting a well-defined smectic packing, demonstrate an enhancement of the emission intensity accompanied by a red shift and a lifetime reduction. In contrast, 40 nm long QRs (QR(40)), forming a columnar phase, does not show these optical properties.