Wave evolution in thin-film flows is highly relevant for heat and mass transfer applications, such as CO2 capture in falling film absorbers. To develop a detailed understanding of potential enhancement mechanisms associated with the evolution of three-dimensional (3D) waveforms, we perform 3D direct numerical simulations of passive scalar transport in laminar-wavy film flows, using a hybrid front-tracking/level-set method to accurately resolve interfacial features. CO2 absorption is greatly enhanced in the presence of interfacial waves with the liquid-side mass transfer coefficient increasing tenfold relative to that of a flat film for the highest film Reynolds numbers (Re) studied. This is primarily due to changes in interfacial and internal flow dynamics rather than an increase in the gas-liquid interfacial area. The recirculation region present in the leading and trailing fronts of the 3D waves intensifies mass transfer, and their effectiveness increases with Re. At low Re, there is a film region beneath the wavy interface, which remains relatively undisturbed where mass transfer is dominated by diffusion. The introduction of structured substrates to promote mass transfer under these conditions is recommended. The visco-capillary ripple region, which precedes the leading and trailing fronts for sufficiently high Re, provides a relatively high degree of spanwise advection, with the mean spanwise velocity magnitude reaching around one-quarter that in the streamwise direction. This underscores the importance of solving the fully-3D problem as these effects do not have a two-dimensional analogue.

We study the pinch-off dynamics of a fluid surrounded by a significantly more viscous one during fluid-fluid displacement in straight, cylindrical capillary tubes, where the interface evolves under the influence of a moving contact line. We investigate the influence of insoluble surfactants on the dynamics of both the contact line and pinch-off. We focus on a visco-capillary displacement regime where the imposed flow rate exceeds a critical threshold, beyond which the contact-line velocity is governed by the partial wettability of the confined geometry, becoming independent of the flow rate. Under these conditions, the fluid-fluid meniscus forms an advancing axial finger, leaving behind a thin film of the defending fluid. This unstable film retracts along the partially wetting walls, forming a dewetting rim that grows with the steady contact-line motion. Eventually, a surface-tension-driven Rayleigh-Plateau instability dominates, triggering pinch-off at the rim neck. For a surfactant-free interface, our results show that the early-stage evolution of the neck diameter follows a power law $ au <^>\alpha$ , where $ au$ is the time to the pinch-off singularity and the scaling exponent $\alpha$ depends on the contact-line velocity determined by the wettability. Over the range resolved in the present simulations, $\alpha$ decreases from values near $1/2$ at large contact-line velocity towards values close to $1/5$ as the contact-line velocity is reduced. We demonstrate that, in the presence of insoluble surfactants, the contact-line velocity, at a given wettability, scales linearly with surfactant elasticity due to Marangoni stresses along the dewetting rim interface, which affect the timing and location of the pinch-off. Despite these effects, at the early-time regime, the pinch-off dynamics exhibits the same self-similar scaling behaviour as in the surfactant-free case.

Direct numerical simulations of interfacial flows with surfactant-induced complexities involving surface viscous stresses are performed within the framework of the Level Contour Reconstruction Method (LCRM). This hybrid front-tracking/level-set approach leverages the advantages of both methods. In addition to interface-confined surfactant transport that results in surface diffusion and Marangoni stresses, the interface is endowed with shear and dilatational surface viscosities. The additional surface effects act to resist deformation arising from velocity gradients in the plane of the two-dimensional manifold of the interface, and interfacial compressibility effects. By adopting the Boussinesq-Scriven constitutive model, we provide a mathematical formulation of these effects that accurately captures the interfacial mechanics, which is then implemented within the LCRM-based code by exploiting the benefits inherent to the underlying front-tracking/level-set hybrid approach. We validate our numerical predictions against a number of benchmark cases that involve drops undergoing deformation when subjected to a flow field and when rising under the action of buoyancy. The results of these validation studies highlight the importance of adopting a rigorous approach in modelling the interfacial dynamics. We also present results that demonstrate the effects of surface viscous stresses on interfacial deformation in unsteady parametric surface waves and atomisation events.

Surface contamination is pervasive across various industrial systems, especially photovoltaic systems, and degrades system performance. Cleaning practices are essential to tackle this problem but remain water intensive due to low water utilization efficiency, particularly in photovoltaic fields. Here we found that cleaning efficiency depends non-monotonically on the water droplet energy and can be maximized for intermediate droplet energy values, which can be utilized to improve water utilization efficiency in surface cleaning. Our experiments and theory demonstrate that particulate removal mechanisms are governed by droplet impact velocity and particle-droplet interfacial interactions. This mechanism enables removal of contaminants with varied densities from superhydrophobic surfaces. Leveraging this mechanism, we developed the 'liquid droplet mops' method to efficiently clean superhydrophobic-coated solar panels, achieving 99.9% removal with only 10% of the water consumption of standard liquid jets. Our findings not only advance the fundamental understanding of surface cleaning but also offer a simple yet efficient water-saving strategy for surface cleaning in the water-scarcity context.

We perform three-dimensional simulations of miscible and immiscible displacements in a cylindrical pipe. For the miscible case, both laminar and turbulent displacement regimes are considered, and our numerical framework uses direct numerical simulation (DNS) and a Large Eddy Simulation (LES) approach based on a Lilly-Smagorinsky model. The dynamics of the flow are governed by the Navier-Stokes equations, coupled with a convective-diffusion equation for the concentration of the more viscous fluid when considering the miscible cases. For the immiscible laminar cases, we perform two-phase DNS considering both pinned and moving contact lines to capture the full range of immiscible dynamic behaviours. The pinned contact line reflects stationary interfaces constrained by surface heterogeneity, while the moving contact line accounts for dynamic interfacial motion influenced by viscous and capillary forces. This study shows that the viscosity contrasts between the two fluids play a significant role in determining the efficiency of 'cleaning' of a pipe containing an initially highly viscous resident fluid. When the viscosity of the displaced fluid is low, the laminar displacement flow is efficient in cleaning the pipe; however, when the viscosity increases, the laminar displacement becomes inadequate. Our numerical predictions in the turbulent regime showed that more efficient cleaning is achieved when the viscosity contrast between the two fluids is large. Lastly, our results reveal that the dynamics of a moving contact line can impact both the efficiency and the pattern of cleaning within the pipe.

Dilatational and shear surface viscosities are highly correlated parameters, making their individual contributions difficult to disentangle in Stokes flow, linearised flow models, or two-dimensional flows. We therefore investigate the three-dimensional interfacial standing waves as a means to decouple the influence of dilatational and shear surface viscosities. Two dimensionless controlling parameters are introduced: $Bq$, the total Boussinesq number, which quantifies the the relative importance of surface viscous stresses compared with bulk viscous stresses, and $\tan χ$, which quantifies the ratio of surface dilatational viscosity to surface shear viscosity. The growth rates and threshold accelerations are independent of $χ$, consistent with previous theoretical predictions. Nonlinear analyses of square and hexagonal patterns reveal that Fourier decomposition of wave-patterns can effectively decouple the intricate dynamics into axial modes, where the waves are weakly dependent on $χ$, and oblique modes, where additional damping occurs in the shear surface viscous dominant interface. These results demonstrate that Faraday wave-patterns provide a route for identifying and quantifying the distinct roles of dilatational and shear surface viscosities.

The drug release of liposomes is strongly modulated by mechanical stress, yet the interplay between carrier properties and the surrounding colloidal environment remains difficult to resolve experimentally due to the challenge of preserving shear conditions and carrier microenvironment during measurement. Here, we address this limitation using the Dispersion Releaser, a USP-referenced dialysis-based platform that enables in situ quantification of release under continuous shear. Computational fluid dynamics (CFD) simulations were used to map flow regimes across 25-100 rotations per minute (RPM), informing the selection of 25 RPM (low shear) and 75 RPM (high shear) for subsequent experiments. Two berberine-loaded liposomal formulations with distinct drug-dipalmitoylphosphatidylglycerol electrostatic interaction strengths showed significantly different release profiles under laminar flow, low-shear environment (f2 = 40.32) but converged under high shear (f2 = 68.31), and closely approached the permeation of free berberine, indicating that drug-membrane affinity governs release under mild shear but is overridden at higher mechanical stress. To investigate the role of the colloidal microenvironment, hydroxypropyl methylcellulose was introduced as an inert, non-permeable, shear-thinning matrix at concentrations of 1.5% and 2.5%. A dedicated permeation normalization separated the diffusional contribution of each matrix from carrier-mediated release. After normalization, the 2.5% matrix reduced cumulative release from ∼ 87% to ∼ 54% at 25 RPM, while the release modulation by the 1.5% was lost at 75 RPM. Notably, CFD revealed that both matrices suppressed turbulence despite wall shear stresses two to four orders of magnitude higher than in water, a finding incompatible with boundary-layer diffusion control, where a stagnant fluid layer adjacent to the vesicle surface limits mass transfer. Instead, the data support the proposed shear-shielding mechanism in which the colloidal matrix attenuates mechanical stress transmission to the vesicle. These results challenge the prevailing boundary-layer interpretation of colloid-mediated release retardation and highlight the need for shear-aware in vitro models in liposomal formulation design.

The formation of a superlattice pattern in two-frequency-driven Faraday waves discovered and named SSS-I by Arbell Fineberg (1998, 2002) is investigated by means of Direct Numerical Simulations (DNS) of the full three-dimensional Navier–Stokes equations with a free surface. Two simulations with distinct quasi-hexagonal initial conditions run at a forcing amplitude 25% above the Faraday-wave onset followed quite different routes, but both led eventually to the same superlattice pattern after around 250 forcing periods. This regime is inaccessible to the approximations of weak nonlinearity or viscosity. The standing-wave pattern contain rows of patches, alternating in time between hills and lakes that are connected by a long skeleton resembing the backbone of DNA strands. The patches and skeleton of the pattern can be related to its spatial Fourier decomposition, which combines hexagonal modes with a spatially and temporally subharmonic mode. One of the transition routes passes through several fairly long-lived transients including different hexagonal patterns and another superlattice pattern; the other passes only through erratic and disordered states. After another 100 periods, the pattern became unstable and was succeeded by a dynamic version of SSS-I in which the superlattice is modulated and drifts in the direction of the backbone, while preserving its basic shape. Convergence to SSS-I states both experimentally in a large geometry and numerically from two different initial conditions and in a minimal geometry demonstrates the robustness of the SSS-I pattern.

The hydrodynamic characteristics of liquid-liquid flows in a micro-mixing plate with a ‘heart/spade’ geometry and a hydraulic diameter of ∼ 0.36 mm at the contraction point are studied experimentally in the Reynolds number range of 250 - 1,000 . Localised optical observations of the two-phase flow within the micro-mixing device units are performed using a high-speed camera in combination with an LED-induced fluorescence imaging technique. A qualitative interpretation of the instantaneous images enabled the development of a regime map with three stable flow patterns, each with a metastable transitional state. The formation of secondary flows and recirculation zones resulted in the breakup of interfaces and fragmentation of droplets. The droplet size distribution of an MTBE-water dispersion is studied across a broad range of the dispersed phase (water) ratios, ϕ _d = 0.091 - 0.714 . The resulting Sauter mean diameter of water droplets is used to evaluate the improvement in the specific surface area and was correlated as a function of the energy dissipation rate and the Reynolds and Weber numbers.

Predicting the release performance of subcutaneous (SC) drug formulations is challenging due to the complex interplay between physicochemical properties and the physiological microenvironment, which includes the extracellular matrix (ECM), fluid composition, and fluid availability, factors that collectively influence bioavailability and absorption rates. The ECM often acts as a bandpass filter modulated by local ion and protein content. In this study, we introduce the BioJect cell, a modern release test method based on the compendial flow-through cell, integrating a perfusion system with customizable biomatrix components. We systematically investigated the release mechanisms of four insulin formulations: regular human insulin, insulin aspart, insulin glulisine, and Neutral Protamine Hagedorn (NPH) insulin. A modified simulated subcutaneous interstitial fluid (mSSIF) comprising multiple components of the SC physiological environment was employed. It incorporates important ions and proteins (138.5 mM sodium, 10 mM potassium, 1.8 mM calcium, 0.8 mM magnesium, 111.3 mM chloride, 28 mM bicarbonate, 0.5 mM sulfate, 5 mM acetate, 4.2 mM phosphate, 30 g/L total protein added as bovine serum albumin). Our release test method discriminated the tested formulations under varying biorelevant conditions, demonstrating its biopredictive capabilities. Notably, we discovered a previously undocumented albumin binding affecting the release rate of insulin glulisine, likely occurring in the low-shear environment of SC tissue only. Additionally, the inclusion of biorelevant components like hyaluronic acid and collagen into the biomatrix of the BioJect cell provided profound insights into potential absorption and release mechanisms, supported by two in vitro-in vivo relationships (level C and level A). The BioJect cell represents a significant advancement in simulating the SC environment for drug release testing. Our findings highlight the importance of considering protein binding and ECM components in predicting drug absorption, offering a promising tool for the development and optimization of SC formulations.

Parametric oscillations of an interface separating two fluid phases create nonlinear surface waves, called Faraday waves, which organise into simple patterns, such as squares and hexagons, as well as complex structures, such as double hexagonal and superlattice patterns. In this work, we study the influence of surfactant-induced Marangoni stresses on the formation and transition of Faraday-wave patterns. We use a control parameter, $B$ , that assesses the relative importance of Marangoni stresses as compared with the surface-wave dynamics. Our results show that the threshold acceleration required to destabilise a surfactant-covered interface through vibration increases with increasing $B$ . For a surfactant-free interface, a square-wave pattern is observed. As $B$ is incremented, we report transitions from squares to asymmetric squares, weakly wavy stripes and ultimately to ridges and hills. These hills are a consequence of the bidirectional Marangoni stresses at the neck of the ridges. The mechanisms underlying the pattern transitions and the formation of exotic ridges and hills are discussed.

We use microparticle image velocimetry and numerical simulations to study the vortex evolution in liquid-liquid plug flows in a 400 mu m T-junction microchannel. Experiments are conducted using the [C4mim][T2fN] ionic liquid as the continuous phase and a glycerolwater blend as the dispersed phase, giving a dispersed to continuous phase viscosity ratio, lambda, of 0.117. The range of mixture velocities studied is 0.002 - 0.03 m/s, which corresponds to a capillary number range of Cap similar to 0.0075 - 0.16. We characterize the vortical structures based on geometry, vorticity, and circulation times within the plugs. Good agreement is found between the experimental data and the numerical predictions in terms of plug length, film thickness, and circulation patterns. The plug length is found to decrease while the film thickness increases with Cap. Three pairs of vortices are formed within the plug in a reference frame moving with the steady plug speed. The pairs comprise a main pair that occupies the central region of the plug and two secondary pairs located at the plug front and rear. The secondary vortices diminishment with Cap is quantified based on their geometry and vorticity, subsequently correlated to the critical film thickness for vortex loss based on the predictions of Balestra et al. [Microfluid. Nanofluid.22, 67 (2018)]. It is found that the front and rear secondary vortex pairs diminish at different rates depending on Cap. With increasing Cap, based on their size, the rear vortex pair is lost before the front one, while, based on vorticity, only the front vortex pair is lost.

This study investigates the transport of particles in density-stratified fluids, a prevalent natural phenomenon. In the ocean, particles and marine snow descend through fluids with significant density variations due to salinity and temperature gradients. Such heterogeneity in the background fluid affects the settling or rising rates of particles, often leading to accumulation at transitional density layers. Previous research has primarily focused on spherical particles, examining their isolated motion, pairwise interactions, and collective transport in stratified fluids. This work, however, extends the investigation to the interaction between two spheroidal particles settling in-line in a linearly stratified fluid. This study employs an immersed-boundary technique to perform particle-resolved numerical simulations in a three-dimensional Cartesian domain. The results showcase the effects of varying the stratification strength through the Froude number, the particles’ aspect ratios, and the initial separation distance between the particles on the interaction dynamics between the settling spheroids.

We demonstrate the application of a recurrent neural network (RNN) to perform multistep and multivariate time-series performance predictions for stirred and static mixers as exemplars of complex multiphase systems. We employ two network architectures in this study, fitted with either long short-term memory and gated recurrent unit cells, which are trained on high-fidelity, three-dimensional, computational fluid dynamics simulations of the mixer performance, in the presence and absence of surfactants, in terms of drop size distributions and interfacial areas as a function of system parameters; these include physicochemical properties, mixer geometry, and operating conditions. Our results demonstrate that while it is possible to train RNNs with a single fully connected layer more efficiently than with an encoder-decoder structure, the latter is shown to be more capable of learning long-term dynamics underlying dispersion metrics. Details of the methodology are presented, which include data preprocessing, RNN model exploration, and methods for model performance visualization; an ensemble-based procedure is also introduced to provide a measure of the model uncertainty. The workflow is designed to be generic and can be deployed to make predictions in other industrial applications with similar time-series data.

Predictive performance assays are crucial for the development and approval of nanomedicines and their bioequivalent successors. At present, there are no established compendial methods that provide a reliable standard for comparing and selecting these formulation prototypes, and our understanding of the in vivo release remains still incomplete. Consequently, extensive animal studies, with enhanced analytical resolution for both, released and encapsulated drug, are necessary to assess bioequivalence. This significantly raises the cost and duration of nanomedicine development. This work presents the development of a discriminatory and biopredictive release test method for liposomal prednisolone phosphate. Using model-informed deconvolution, we identified an in vivo target release. The experimental design employed a discrete L-optimal configuration to refine the analytical method and determine the impact of in vitro parameters on the dosage form. A three-point specification evaluated the key phases of in vivo release: early (T-5%), intermediate (T-20%), and late release behavior (T-40%), compared to the in vivo release profile of the reference product, NanoCort®. Various levels of shear responses and the influence of clinically relevant release media compositions were tested. This enabled an assessment of the effect of shear on the release, an essential aspect of their in vivo deformation and release behavior. The type and concentration of proteins in the medium influence liposome release. Fetal bovine serum strongly impacted the discriminatory performance at intermediate shear conditions. The method provided deep insights into the release response of liposomes and offers an interesting workflow for in vitro bioequivalence evaluation.

This study investigates the interaction between a freely rising, deformable bubble and a freely settling particle of the same size due to gravity. Initially, an in-line configuration is considered while varying the Bond, Galilei and Archimedes numbers. The study shows that as the bubble and particle approach each other, a liquid film forms between them that undergoes drainage. The formation of the liquid film leads to dissipation of kinetic energy, and for sufficiently large bubble velocities, particle flotation takes place. Increasing the Bond number causes the bubble to deform more severely, which may allow the particle to pass through the bubble as it ruptures. This work also considers an offset configuration, which shows that the bubble slides away from the particle, affecting its settling trajectory.

We present a parametric study of the unsteady phenomena associated with the flow of elongated gas bubbles travelling through liquid-filled square capillaries under high Weber number conditions. These conditions consistently induce the formation of a re-entrant jet at the back of the bubble that commonly gives way to a deep liquid cavity. Subsequent steps include pinch-off events in the cavity to generate one or multiple encapsulated drops which may coalesce, in conjunction with the bursting of the bubble-liquid interface by either the cavity or the drops. Some of these interfacial instabilities have previously been reported experimentally (Olbricht 1996) and numerically (Izbassarov & Muradoglu 2016) for liquid-liquid flow in microchannels. We carry out three-dimensional direct numerical simulations based on a hybrid interface-tracking/level-set method capable of accounting for the presence and dynamic exchange of surfactants between the liquid bulk phase and the liquid-gas interface. Our results indicate that the delicate interplay amongst inertia, capillarity, viscosity, surfactant adsorption/desorption kinetics, and Marangoni stresses has a dramatic influence over the non-axisymmetric morphological structures of the encapsulated drops-elongated bubble. This strong coupling also influences the pinch-off time, penetration depth of the cavity, and number, size, and velocity of the encapsulated drops across the bubble. The observed phenomena are summarised in three main morphological regimes based on surfactant-related parameters and dimensionless groups. A discussion of the flow regime maps is also provided.

We present a numerical study of the main sub-stages preceding aerosol formation via bursting bubbles: capillary wave propagation along the bubble, convergence at the bubble's apex, the ascent of a Worthington jet and its break-up to release liquid drops. We focus on two crucial yet overlooked aspects of the system: the presence of surface-active agents and dynamics driven by non-negligible gravitational effects, quantified by the Bond number. Our results propose, for the first time, a mechanism explaining capillary wave retardation in the presence of surfactants, involving the transition from bi- to uni-directional Marangoni stresses, which pull the interface upwards, countering the motion of the waves. We also quantitatively elucidate the variable nature of the waves' velocity with various surfactant parameters, including surfactant solubility and elasticity, a departure from the constant behaviour well-documented in clean interfaces.