Water Interfacial Area Research Articles

Determination of a pedotransfer function for specific air–water interfacial area in sandy soils: A pore network‐informed multigene genetic programming approach

AbstractUnderstanding specific air–water interfacial area (SAWIA) is essential for characterizing and modeling various phenomena in vadose zone hydrology, such as virus and colloid transport, contaminant dissolution, evaporation, and the hydro‐mechanical behavior of unsaturated soils. Traditional measurement methods, including X‐ray imaging and tracer techniques, often encounter challenges, leading to a scarcity of studies that provide a reliable relationship for SAWIA. Currently, no pedotransfer function in the literature links SAWIA with saturation and suction using readily measurable soil properties such as median grain size and porosity. In this study, we initially developed a pore network model capable of predicting SAWIA by calibrating it with corresponding soil‐water retention curves (SWRCs). We then used these models to compile a comprehensive database of SAWIA for six sandy soils, for which experimental SWRCs were available, covering a range of median grain sizes and porosities. Utilizing this database, we established a pedotransfer function through multigene genetic programming. The accuracy of this function was validated against experimental data not previously used in its training and testing. Our parametric study indicated that increases in either porosity or median particle size led to a decrease in the regions exhibiting higher SAWIA in terms of saturation and suction.

Unravelling the effect of droplet size on lipid oxidation in O/W emulsions by using microfluidics

Lipid oxidation in emulsions is hypothesised to increase with decreasing droplet size, as this increases the specific oil–water interfacial area, where lipid oxidation is expected to be initiated. In literature, however, contradictory results have been reported, which can be caused by confounding factors such as the oil droplet polydispersity and the distribution of components between the available phases. In this work, monodisperse surfactant-stabilised emulsions with highly controlled droplet sizes of 4.7, 9.1, and 26 µm were produced by microfluidic emulsification. We show that lipid oxidation increases with decreasing droplet size, which we ascribe to the increased contact area between lipids and continuous phase prooxidants. Besides, a significant amount of oxygen was consumed by oxidation of the surfactant itself (Tween 20), an effect that also increased with decreasing droplet size. These insights substantiate the importance of controlling droplet size for improving the oxidative stability of emulsions.

Numerical modeling of drying from saturated to unsaturated conditions with two-phase partitioning boundaries at the medium surface and interface-dependent phase change

To accurately predict the drying of a porous medium from saturated to unsaturated conditions over time, this paper proposes two-phase partitioning boundaries for liquid water evaporation and water vapor transport at the medium surface. A non-isothermal two-phase (liquid water and gas mixtures), two-component (dry air and water vapor) transport model with nonequilibrium and interface-dependent phase change is then developed. The governing equations are then numerical solved using COMSOL Multiphysics software and applied to describe a series of laboratory soil drying tests as examples. The performance of the proposed boundaries at the surface is illustrated by comparison with other three common used boundaries. The results show that a specific flux of vapor at the surface with a zero flux of liquid water cannot be adopted to describe the drying process of initially saturated media due to the lack of interconnected gas-filled pores. The merged liquid-vapor boundary applied in the Darcy–Richards model significantly prolongs the constant rate stage when the liquid water evaporation is dominant. Then, the effects of air/gas–water interfacial area and water retention parameters on the drying process are investigated. A faster reduction in the air–water relative interfacial area at the medium surface will shorten the drying process with earlier dominance of the vapor transport. An increased entry capillary pressure and a decreased pore size distribution index for the water retention curve will result in slower drying, more loss by vapor transport and a more homogeneous distribution of pore water respect to depth. The current study can promote the understanding of the evolution of drying mechanisms of a porous medium from saturated to unsaturated conditions.

Investigation of porosity variation on water retention behaviour of unsaturated granular media by using pore scale Micro-CT and lattice Boltzmann method

Most soils in nature are in unsaturated states. The key to correctly understanding its hydraulic characteristics lies in mastering the correlation between its microstructure and macro water retention characteristics. Based on an independently developed miniaturized in-situ unsaturated soil–water characteristic instrument, three-phase topological morphology and micrographic information (pore throat, liquid cluster, interface area) of unsaturated granular media were explored, supplemented by CT scanning under a high resolution (5.6 μm/pixel). In addition, three-dimensional reconstruction of CT images was performed for simulating two-phase flow in porous granular media using the lattice Boltzmann method. The experimental results indicate that the water retention capacity, air-entry value, and residual saturation of unsaturated granular specimens are negatively correlated with the porosity of specimens. As the saturation decreases, the specific air–water interfacial area first increases and then decreases, with the peak value appearing around 20% of saturation. Based on LBM simulation, the water retention capacity of the specimen is negatively correlated with the contact angle, while the liquid–solid characteristic interface area and relative permeability are positively correlated. The experimental and simulation results demonstrate good consistency with each other.

Representative Elementary Volume Estimation and Neural Network-Based Prediction of Change Rates of Dense Non-Aqueous Phase Liquid Saturation and Dense Non-Aqueous Phase Liquid–Water Interfacial Area in Porous Media

Investigation of the change rate for contaminant parameters is important to characterize dense non-aqueous phase liquid (DNAPL) transport and distribution in groundwater systems. In this study, four experiments of perchloroethylene (PCE) migration are conducted in two-dimensional (2D) sandboxes to characterize change rates of PCE saturation (So) and PCE–water interfacial area (AOW) under different conditions of salinity, surface active agent, and heterogeneity. Associated representative elementary volume (REV) of the change rate of So (So rate) and change rate of AOW (AOW rate) is derived over the long-term transport process through light transmission techniques. REV of So rate (SR-REV) and REV of AOW rate (AR-REV) are estimated based on the relative gradient error (εgi). Regression analysis is applied to investigate the regularity, and a model based on a back-propagation (BP) neural network is built to simulate and predict the frequencies of SR-REV and AR-REV. Experimental results indicated the salinity, surface active agent, and heterogeneity are important factors that affect the So rate, AOW rate, SR-REV, and AR-REV of the PCE plume in porous media. The first moment of the PCE plume along the vertical direction is decreased under conditions of high salinity, surface active agent, and heterogeneity, while these factors have different effects on the second moment of the PCE plume. Compared with the salinity and surface active agent, heterogeneity has the greatest effect on the GTP, the distributions of the So rate and AOW rate along the depth, and dM, dI. For SR-REV, the standard deviation is increased by the salinity, surface active agent, and heterogeneity. Simultaneously, the salinity and heterogeneity lead to lower values of the mean value of SR-REV, while the surface active agent increases the mean value of SR-REV. However, the mean and standard deviation of AR-REV have no apparent difference under different experimental conditions. These findings reveal the complexity of PCE transport and scale effect in the groundwater system, which have important significance in improving our understanding of DNAPL transport regularity and promoting associated prediction.

Pore‐Scale Modeling of PFAS Transport in Water‐Unsaturated Porous Media: Air–Water Interfacial Adsorption and Mass‐Transfer Processes in Thin Water Films

AbstractAir–water interfacial adsorption complicates per‐ and polyfluoroalkyl substance (PFAS) transport in vadose zones. Air–water interfaces can arise from pendular rings between soil grains and thin water films on grain surfaces, the latter of which account for over 90% of the total air–water interfaces for most field‐relevant conditions. However, whether all thin‐water‐film air–water interfaces are accessible by PFAS and how mass‐transfer limitations in thin water films control PFAS transport in soils remain unknown. We develop a pore‐scale model that represents both PFAS adsorption at bulk capillary and thin‐water‐film air–water interfaces and the mass‐transfer processes between bulk capillary water and thin water films (including advection, aqueous diffusion, and surface diffusion along air–water interfaces). We apply the pore‐scale model to a series of numerical experiments—constrained by experimentally determined hydraulic parameters and air–water interfacial area data sets—to examine the impact of thin‐water‐film mass‐transfer limitations in a sand medium. Our analyses suggest: (a) The mass‐transfer limitations between bulk capillary water and thin water films inside a pore are negligible due to surface diffusion. (b) However, strong mass‐transfer limitations arise in thin water films of pore clusters where pendular rings disconnect. The mass‐transfer limitations lead to early arrival and long tailing behaviors even if surface diffusion is present. (c) Despite the mass‐transfer limitations, all air–water interfaces in the thin water films were accessed by PFAS under the simulated conditions. These findings highlight the importance of incorporating the thin‐water‐film mass‐transfer limitations and surface diffusion for modeling PFAS transport in vadose zones.

Reduced Accessible Air–Water Interfacial Area Accelerates PFAS Leaching in Heterogeneous Vadose Zones

AbstractPer‐ and polyfluoroalkyl substances (PFAS) are surface‐active contaminants experiencing strong retention in vadose zones due to adsorption at air–water and solid–water interfaces. Leaching of PFAS through vadose zones poses great risks of groundwater contamination. Prior PFAS transport studies have focused on homogenous or layered vadose zones that significantly underrepresented the impact of preferential flow caused by soil heterogeneities—a primary factor known to dominantly control the subsurface transport of many contaminants. We conduct numerical simulations to investigate the impact of preferential flow on PFAS leaching in stochastically generated heterogeneous vadose zones. The simulations show that while shorter‐chain PFAS experience accelerated leaching similar to non‐surfactant solutes, the accelerated leaching of more surface‐active longer‐chain PFAS is uniquely amplified by 1.1–4.5 times due to reduced accessible air–water interfacial areas along preferential flow pathways. Our study highlights the criticality of characterizing soil heterogeneities for accurately predicting the leaching of long‐chain PFAS in vadose zones.

A novel approach for wettability estimation in geological systems by fluid–solid interfacial area measurement using tracers

Wettability plays a vital role in many applications of flow in porous media and affects Darcy scale flow parameters by influencing the fluid–solid interfacial area. Therefore, quantifying the fluid–solid interfacial area can provide a way to measure wettability at the Darcy scale. Here, we experimentally explore a dual-tracer method, which can also be scaled to large geological reservoirs to quantify the fluid–solid interfacial area during the multiphase flow through a porous medium for different wetting conditions. Using our experiments, we demonstrate the influence of different saturations, wettability and flow conditions on the solid–liquid interfacial area. When oil is in the residual phase, we observe that the solid–water interfacial area increases with the increase in water saturation for the water-wet and mixed-wet cases. However, the water–solid interfacial area decreases with an increase in water saturation for the oil-wet case. We increase the water saturation by increasing the water flow rate; therefore, the anomalous behaviour seen in the oil-wet case can be attributed to the rearrangement of oil and water at higher water flow rates. When both oil and water are flowing, the solid–water interfacial area increases with water saturation for all the wettability cases and increases in water wettability as anticipated.Synopsis: Wettability measurements at Darcy-scale give a broad idea of overall subsurface wetting conditions for application in CO2 sequestration, ground-water remediation or oil recovery.

Gas hydrate nucleation in acoustically levitated water droplets

We report subcooling-dependent measurements of hydrate formation on water droplets suspended via acoustic levitation in high-pressure natural gas. Ninety independent visual induction time measurements enabled the construction of induction time distributions with nucleation rates extracted at subcoolings of (12, 13.4 and 14.5) K. Using models from classical nucleation theory, kinetic and thermodynamic nucleation parameters were determined for freely suspended droplets and subsequently compared to those measured in more traditional stirred reactors. Scaling of the observed nucleation rates by each system’s gas–water interfacial area led to improved consistency between the datasets. However, we show that while hydrate formation rates in systems with solid containing surfaces are determined by a small number of nucleation sites with low nucleation work, formation rates in levitated droplets are set by sites that have higher nucleation work but are more numerous. This leads to distinct subcooling-dependencies for nucleation rates in the two systems. These results provide insight both into hydrate promotion and avoidance strategies that are relevant to multiple applications.

Ganglia dynamics during imbibition and drainage processes in nanoporous systems

We study the gas–water transient imbibition and drainage processes in two-dimensional nanoporous media using our recently developed lattice Boltzmann model. To describe the microscopic molecular interactions, the model employs a pseudopotential that correlates the local density and interaction strength to perform simulation at a mesoscopic scale. The primary interest is ganglia dynamics in the nanoporous media affected by fluid and geometrical properties of the porous structure. We performed sensitivity analyses on the fluid and rock characteristics such as the Euler number, gas–water interfacial area, water film area, capillary pressure, pore size distribution, specific surface area, and wettability. The simulation results revealed the fingering nature of the nonwetting phase. In the imbibition process, the flow pathway of water results in isolated and trapped gas bubble clusters because of the strong attraction between water and solid surfaces. In the drainage process, the pressure difference between the gas phase and the water phase depends on both the capillary pressure and the disjoining pressure due to the presence of water film. Pore topography and specific surface area control the continuity of the fluid phases in the imbibition process. In nonwet systems, the water phase starts fingering in the nanoporous system. The present work elucidates the microscopic ganglia dynamics of gas–water two-phase flow in nanoporous media. The microscopic scale details will help establish the macroscopic flow equation to accurately predict two-phase flow in shale gas, tight oil, and caprock seals.

Effects of liquid–liquid interfaces on flow of oil-in-water emulsions in a capillary tube

We examine the flow of oil-in-water emulsions in a capillary tube with various sizes and volume fractions of the oil droplets. The front position of flowing emulsions can be accurately described by a power law formula of the form x0(t/t0)n, in which x0 is a constant and (t/t0)n is a dimensionless time to the nth power. Both x0 and n depend on the sample and on the diameter of the capillary tube. We reveal that the ratios of the oil–water interfacial area to water volume (So-w/Vw) and water–wall interfacial area to water volume (Sw-w/Vw) are important factors in describing x0. For the various samples, the values of x0 are largely determined by So-w/Vw and Sw-w/Vw in combination with x0_0 vol, which is the value of x0 in a sample with no oil droplets. In contrast, n is primarily determined by So-w/Vw. Finally, the role of the liquid–liquid interfacial area is discussed.

Model validation and analyses of parameter sensitivity and uncertainty for modeling long-term retention and leaching of PFAS in the vadose zone

PFAS are emerging contaminants widespread in the environment. As surfactants, PFAS tend to accumulate at solid–water and air–water interfaces in the vadose zone, which may pose long-term threats to groundwater. The primary factors that control the long-term retention of PFAS in the vadose zone remain poorly understood. To address this knowledge gap, we first use multiple datasets from transport experiments to validate a state-of-the-art mathematical model that incorporates transient variably saturated flow, surfactant-induced flow, and rate-limited and nonlinear solid-phase and air–water interfacial adsorption. We then employ the validated model to simulate and analyze the primary processes and parameters controlling the retention and leaching of PFAS in the vadose zone at a model fire-training-area site. Our simulations show that adsorption at solid–water and air–water interfaces leads to strong retention of PFAS in the vadose zone. The strength of retention increases with PFAS chain length and porewater ionic strength, while it decreases at greater PFAS concentrations due to nonlinear adsorption. Comprehensive parameter sensitivity analyses reveal that model predictions are most sensitive to parameters related to the air–water interfacial area and PFAS interfacial properties when air–water interfacial adsorption (AWIA) is more important than solid-phase adsorption (SPA). Predicted PFAS leaching rates vary by a wide range resulting from uncertainties in the input parameters, but the uncertainty range is much greater for longer-chain PFAS than that of their shorter-chain counterparts. The simulated arrival times to groundwater were found to follow log-normal distributions. Finally, model complexity analysis reveals that nonlinearity in AWIA and kinetic SPA and kinetic AWIA have a minimal impact on the long-term retention of PFAS under the wide range of field conditions examined in the present study.

Effects of emulsifier concentration in a high-internal-phase, W/O emulsion binder on particle agglomeration

The kinetics of hydrophobic particle recovery by flotation declines with particle sizes below 20 µm. This work investigated a much-faster novel process, agglomeration, which utilises a high-internal-phase, water-in-oil emulsion as the binder. The effects of changes in the binder composition were studied by varying the emulsifier:oil ratio from 0.2:1 to 100:1, while maximizing the concentration of the dispersed aqueous phase within the binder. The oil consumption was assessed as an inferred film thickness surrounding the individual hydrophobic particles captured by the agglomeration. This film thickness decreased from 156 nm to as little as 4.5 nm by increasing the emulsifier concentration. This result is consistent with an increase in the emulsifier concentration causing an increase in the internal oil–water interfacial area within the binder. Water permeation into the binder, which reduces the viscous resistance to particle capture, decreased to an asymptotic level beyond a 5 wt% emulsifier concentration.

High‐resolution temperature modeling of stream reconstruction alternatives

AbstractStreams are complex where biology, hydrology, and atmospheric processes are all important. Because quantifying and modeling of these systems can be challenging, many teams go directly to prescribed restoration treatments and principles. Restoration on the Middle Fork of the John Day River in Oregon, USA, shows how a project that was designed according to widely accepted restoration principles may lead to outcomes contrary to one of the project's stated goals: reducing peak temperatures for endangered salmonids on the site. This study employed the most sophisticated equipment available for stream temperature monitoring, including approximately 1 million independent hourly measurements in the 2‐week period considered. These data were collected along the river channel with fiber optic–distributed temperature sensing and were used to quantify thermal dynamics. These observations were paired with a physically based stream temperature model which was then employed to predict temperature change from design alternatives. Restored‐reach impact on peak temperature was directly correlated with the air–water interfacial area and the percentage of effective shade (R2 > 0.99). The increase in air–water area of the proposed design was predicted to increase daytime stream temperature by as much as 0.5°C upon completion of the work. Shade from riparian vegetation was found to potentially mitigate stream temperature increases, though only after decades of growth. A moderately dense canopy of 5 m tall trees blocking 17% of daily shortwave solar radiation is predicted to mitigate predicted temperature increases over the 1,800 m reach but also increases nighttime temperatures due to blocking of long‐wave radiation. These outcomes may not be intuitive to restoration practitioners and show how quantitative analysis can benefit the design of a project. This is significant in an area where riparian vegetation has been difficult to reestablish. Without quantitative analysis, restoration efforts can lead to outcomes opposite to stated goals and may be costly and disruptive interventions to fragile stream systems.

Nano-enhanced Bioremediation for Oil Spills: A Review

Oil seepage and spills have lasting detrimental effects on the environment, especially on nearby marine ecosystems. Despite quick and safe removal of contaminating oil being of utmost importance, remediation methods have remained largely unchanged over the last 20+ years. Removal methods such as skimmers, in situ burning, and sorbents can be difficult to implement in inclement weather. Marine microorganisms consume spilled oil as a primary energy source in a method known as bioremediation. Recently the introduction of nanoparticles to areas contaminated by oil has emerged as a potential method to enhance the efficacy of oil degradation by marine microorganisms. Nanoparticles have been primarily utilized as magnetic sorbents but can also act as emulsifiers, increasing the bioavailability of the oil by giving microbes a surface (droplets) to which they can attach and facilitate proliferation. Emulsification increases the oil–water interfacial area and often leads to an increase in the amount of oil degraded by microorganisms. Nanoenhanced bioremediation is effective on a variety of oil compositions and is minimally impacted by weather. It offers a sustainable alternative to traditional remediation methods, i.e., chemical dispersants, sorbents, and in situ burning, which are often toxic and ineffective.

A new criterion for determining the representative elementary volume of translucent porous media and inner contaminant

Abstract. Representative elementary volume (REV) is essential for measuring and quantifying the effective parameters of a complex heterogeneous medium. To overcome the limitations of the existing REV estimation criteria, a new REV estimation criterion (χi) based on dimensionless range and gradient calculation is proposed in this study to estimate the REV of a translucent material based on light transmission techniques. Three sandbox experiments are performed to estimate REVs of porosity, density, tortuosity, and perchloroethylene (PCE) plume using multiple REV estimation criteria. In comparison with χi, previous REV estimation criteria based on the coefficient of variation (CVi), the entropy dimension (DIi) and the relative gradient error (εgi) are tested in REV quantification of translucent silica and the inner PCE plume to achieve their corresponding effects. Results suggest that the new criterion (χi) can effectively identify the REV in the materials, whereas the coefficient of variation and entropy dimension (F=-2.01×10-12+12π×1.50e-(REV-4.35)22⋅1.502) are not effective. The relative gradient error can make the REV plateau obvious, while random fluctuations make the REV plateau difficult to identify accurately. Therefore, the new criterion is appropriate for REV estimation of the translucent materials and inner contaminant. Models are built based on a Gaussian equation to simulate the distribution of REVs for media properties, whose frequency of REV is dense in the middle and sparse on both sides. REV estimation of the PCE plume indicates that a high level of porosity leads to a large value of mean and standard deviation for REVs of PCE saturation (So) and PCE–water interfacial area (AOW). Fitted equations are derived from distribution of REVs for the PCE plume related to dm (distances from mass center to considered point) and dI (distances from injection position to considered point). Moreover, relationships between REVs of the PCE plume and So are fitted using regression analysis. Results suggest a decreasing trend appears for So-REV when So increases, while AOW-REV increases with increasing So.

Prediction of capillary suction in porous media based on micro-CT technology and B–C model

Abstract Moisture variation in porous media depends mainly on the pore characteristics. This article used the micro-computed tomography (micro-CT) (a non-destructive imaging technique to generate a three-dimensional virtual model) and the Brooks–Corey model to deduce the moisture migration in sand. Relationship between capillary rise height and time (h–t) was achieved by numerical simulation in the capillary suction process, where the parameters fractal dimension, porosity, and air–water interfacial area were obtained by the micro-CT scanning. Meanwhile, experiments of capillary rise in sand column were performed using four different sizes washed sand, and the capillary heights at different times were recorded. Results show that the capillary suction is decided by the aperture size and phase morphology simultaneously, and particle size has obvious effect on capillarity, and the wetting front lowers with the increase in grain size and the decrease in rising rate. Parameters air entry pressure and pore-size distribution index obtained by micro-CT scanning technology and empirical formula are accurate. Method of combing micro-CT images and Brooks–Corey model can predict well the capillary suction of porous media. It is also proved that the capillary suction is decided by the aperture size and phase morphology simultaneously.

Effect of capillary number on morphological characterizations of trapped gas bubbles: Study by using micro-tomography

A series of capillary trapping experiments were conducted in unconsolidated porous media using high-resolution micro-tomography. Morphological characterizations of trapped gas bubbles, including residual gas saturation, bubble size distribution and interfacial area, were determined from reconstructed three-dimensional images. Different flooding flow rates, corresponding to capillary number ranges from 1.0 × 10−7 to 3.8 × 10−4, were performed to investigate the impact of flow rate on these characterizations during imbibition processes. At low capillary number, where capillary forces are dominant, the residual gas saturation is independent of capillary number and the bubble size distribution shows a universal power-law behavior predicted from percolation theory. When the capillary number increases above a critical value, the residual saturation decreases sharply, and the bubble size distribution no longer exhibits power-law behavior. As capillary number increases, the volume fraction of single-pore bubbles increases from about 16% to more than 80%. Furthermore, we explored the interfacial area of trapped bubbles, including gas–solid and gas–water (meniscus) interfacial areas, which directly affect the mass transfer process inside porous media. The specific interfacial area exhibits a linear relation with residual saturation, and the effect of particle size can be normalized by multiplying the median particle diameter. Moreover, we observed that the meniscus/total interfacial area ratio increases from 0.61 to 0.78 with an increase in capillary number, because the smaller singlet has a higher interfacial contact with the wetting phase.

Molecular Dynamics Simulation of Decomposition of Methane Hydrate and Interfacial Characteristics in Nanostructure Region

Methane hydrate decomposition is crucial to its safe and efficient exploitation and utilization. The water-methane hydrate–water system in the nanostructure region during decomposition process was studied by molecular dynamics simulation. Taking the decomposition of 3 × 3 × 6 SI hydrate at an initial temperature of 310 K as an example, the decomposition processes of different layers at different moments were studied. The density distribution of methane molecules, radial distribution function of water molecules in each layer and F3 order parameter of each layer during the decomposition process were analyzed. The locations of hydrate–water interfaces at different temperatures and different moments were estimated, and furthermore the decomposition rate of hydrate was obtained. In addition, this work is also concerned with the influence of hydrate–water interfacial contact area on decomposition. It is noticed that hydrate decomposes row by row from outside to inside. The formation of methane nano-scale bubbles is observed during decomposition. Due to the increase of the heat and mass transfer resistance, the decomposition rate becomes slower with time. However, higher initial temperature is helpful to promote the decomposition rate. Besides, expanding hydrate–water interfacial contact area can enhance the hydrate decomposition efficiently.

Formation of compound droplets during fragmentation of turbulent buoyant oil jet in water

Fragmentation of a vertical buoyant silicone oil jet injected into sugar water is elucidated by refractive index matching and planar laser-induced fluorescence. Compound droplets containing multiple water droplets, some with smaller oil droplets, form regularly at jet Reynolds numbers of $Re=1358$ and 2122 and persist for at least up to 30 nozzle diameters. In contrast, they rarely appear at $Re=594$. The origin of some of the encapsulated water droplets can be traced back to the entrained water ligaments during the initial roll-up of Kelvin–Helmholtz vortices. Analysis using random forest-based procedures shows that the fraction of compound droplets does not vary significantly with $Re$, but increases rapidly with droplet diameter, reaching 78 % for 2 mm droplets. Consequently, the size distributions of compound droplets have peaks that increase in magnitude and shift to a lower diameter with increasing $Re$. On average, the interior pockets raise the oil–water interfacial area by 15 %, increasing with diameter and axial location. Also, while the oil droplets are deformed by the jet’s shear field, the interior interfaces remain nearly spherical, consistent with prior studies of the deformation of isolated compound droplets for relevant capillary numbers and viscosity ratio.