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The gradients in surfactant distribution at a fluid-fluid interface can induce fluid flow known as the Marangoni flow. Fluid interfaces found in biological and environmental systems are seldom clean, where mixtures of various surfactants are present. The presence of multi-component surfactant mixtures introduces the possibility of interactions among constituents, which may impact Marangoni flows and alter flow dynamics. We employed flow visualization, surface tension and reaction kinetic measurements, and numerical simulations to quantitatively investigate the Marangoni flows induced by the reacting surfactant mixtures. Different binary surfactant mixtures were utilized for comparative analysis. The impact of surfactant interactions on Marangoni flows is confirmed through the observation of diverse complex flow patterns that result from the combination of oppositely charged surfactants in varying composition ratios and concentrations. Unique flow patterns originate from the composition-dependent interfacial phenomena upon mixing surfactants. Our findings provide vital insights that could be used to guide the development of effective oil remediation or the spreading of waterborne pathogens in contaminated regions. [ABSTRACT FROM AUTHOR]

This study explores the impact of electrostatic interactions and hydrogen bonding on tear film stability, a crucial factor for ocular surface health. While mucosal and meibomian layers have been extensively studied, the role of electrolytes in the aqueous phase remains unclear. Dry eye syndrome, characterized by insufficient tear quantity or quality, is associated with hyperosmolality, making electrolyte composition an important factor that might impact tear stability. Using a model buffer solution on a silica glass dome, we simulated physiologically relevant tear film conditions. Sodium chloride alone induced premature dewetting through salt crystal nucleation. In contrast, trace amounts of solutes with hydroxyl groups (sodium phosphate dibasic, potassium phosphate monobasic, and glucose) exhibited intriguing phenomena: quasi-stable films, solutal Marangoni-driven fluid influx increasing film thickness, and viscous fingering due to Saffman-Taylor instability. These observations are rationalized by the association of salt solutions with increased surface tension and the propensity of hydroxyl-group-containing solutes to engage in significant hydrogen bonding, altering local viscosity. This creates a viscosity contrast between the bulk buffer solution and the film region. Moreover, these solutes shield the glass dome, counteracting sodium chloride crystallization. These insights not only advance our understanding of tear film mechanics but also pave the way for predictive diagnostics in dry eye syndrome, offering a robust platform for personalized medical interventions based on individual tear film composition. [ABSTRACT FROM AUTHOR]

Poor mass transport to or from vertical gas-evolving electrodes can adversely impact energy efficiency and product purity in the production of hydrogen, chlorine, and various metals. A proper description that combines natural convection with micromixing of growing, coalescing, and departing bubbles is presently lacking. This work develops a simple, physically sound analytical model that includes the influence of bubble size, flow regime, and bubble surface coverage. By comprehensively reviewing mass transfer measurements from the water electrolysis literature, we observe that the surface coverage of oxygen bubbles increases much more strongly with increasing current density than an often-used square root scaling predicts. Strong differences are observed in the degree of micromixing of hydrogen and oxygen bubbles in alkaline and acidic electrolytes. These varied results can all be explained by a combination of electrocapillarity, and coalescence induced by either a high surface coverage or Marangoni flows. [Display omitted] • A simple formula for adding micromixing and natural convection is derived. • A single micromixing parameter between 0 and 5 describes all literature data. • Micromixing is strongest for oxygen in alkaline and hydrogen in acidic electrolytes. • Marangoni forces are attractive in this case, causing coalescence and larger bubbles. • Bubble coverage increases with the square/square root of current for oxygen/hydrogen. [ABSTRACT FROM AUTHOR]

For small droplets undergoing phase change, gravity is generally considered negligible. In the case of binary droplets evaporation, convective flows can be induced due to various mechanisms, such as continuity, buoyancy and/or selective evaporation of one of the components. Convection can also be induced by surface tension gradients resulting from concentration variations along the interface. This study presents experimental results of evaporation for binary mixture droplets. We concurrently investigate sessile and pendant droplets to assess gravity’s impact on binary droplet evaporation. We examine compositions including, pure butanol, pure methanol, pure water, and 50% per volume mixtures of water-butanol and water-methanol, evaporating in a controlled atmosphere. In the case of water-butanol mixtures, the drops contact line ‘depins’ during the evaporation process whereas the case of water-methanol mixture, the contact line of the drops remains pinned most of the lifetimes. The analysis of the evaporation dynamics reveals differences in the evaporation of these two mixtures and the effect of orientation (gravity). For water-butanol mixtures the evaporation occurs in four stages linked to preferential evaporation of the more volatile component and the ensuing surface tension gradients. In the case of water-methanol mixtures, contact lines tend to be pinned during most of the lifetimes of drops. The evaporation rate of the mixture is found to be between the ones of the pure components, i.e. water and methanol. The case of sessile drops exhibits a slight enhancement in evaporation rate in the case of the sessile configuration compared to the pendant one for pure water and mixture cases, which is explained by density differences and buoyancy driven flows. Solutal Marangoni flows in the case of water-methanol mixtures are deemed weaker compared to water-butanol ones. The use of the two mixtures allowed to have a good comparison between two cases where solutal-Marangoni effect can be strong (water-butanol) and weak (water- methanol) influence. The densities of the two organic liquids also highlighted gravitational effect due to the large difference in vapor densities. [ABSTRACT FROM AUTHOR]

We probe the temporal evolution of internal thermo-hydrodynamic features of evaporating sessile droplets under external forced convection. A transient numerical model based on Arbitrary Lagrangian-Eulerian (ALE) framework is adopted. The governing equations corresponding to the transport phenomena (mass, momentum and energy) are solved in a fully coupled manner in both the droplet (liquid) and ambient (gaseous) domains while accurately tracing the droplet interface evolution. The role of initial wetting state, external convection velocity and convective heating/cooling effects due to the air-flow are explored in details, and good agreements are noted with previous reports from literature. Results show that although the evaporation rate is augmented significantly in forced convection conditions, the same does not increase in proportion at higher Reynolds numbers. This is primarily due to the enhanced cooling effects that reduce saturated vapor concentration at droplet surface, and thereby supress the diffusion mechanism. Also, depending upon the contact angle, the evaporative cooling effects are compensated partially or completely due to convective heating. This altered thermal profile substantially influences internal hydrodynamic features (Marangoni flow and heat advection) during the transient and stable regimes of droplet evaporation. At higher degrees of convective heating, the droplet interface temperature becomes higher than its base temperature. This condition results in a completely opposite internal Marangoni vortex at stable state compared to that induced due to evaporative cooling effects. We also provide a regime map to show the role of the gas-phase Stefan number on the droplet-phase hydrodynamic regimes. [ABSTRACT FROM AUTHOR]

Microscopic active droplets are able to swim autonomously in viscous flows. This puzzling feature stems from solute exchanges with the surrounding fluid via surface reactions or their spontaneous solubilization and from the interfacial flows resulting from these solutes' gradients. Contrary to asymmetric active colloids, these isotropic droplets swim spontaneously by exploiting the nonlinear coupling of solute transport with self-generated Marangoni flows; such coupling is also responsible for secondary transitions to more complex individual and collective dynamics. Thanks to their simple design and their sensitivity to physico-chemical signals, these droplets are fascinating to physicists, chemists, biologists, and fluid dynamicists alike in analyzing viscous self-propulsion and collective dynamics in active-matter systems, developing synthetic cellular models, or performing targeted biomedical or engineering applications. I review here the most recent and significant developments of this rapidly growing field, focusing on the mathematical and physical modeling of these intriguing droplets, together with their experimental design and characterization. [ABSTRACT FROM AUTHOR]

Trajectories of free-floating droplets burned in experiments on the International Space Station (ISS) are evaluated from digital images. n-Heptane droplets are observed to move in irregular paths after cool flame extinction with acceleration levels of tens or hundreds of μg and frequencies of 0.2 - 0.3 Hz. Flame oscillations for burning methanol and n-heptane droplets can affect droplet acceleration components. During flame oscillation, droplets exhibit oscillatory acceleration patterns with characteristic frequencies of 0.2 - 0.3 Hz and accelerations of the order of 50 μg. The droplet acceleration magnitudes are significantly larger than measured ISS acceleration levels (g-jitter). It is concluded that motions of free droplets are initially a result of the deployment and ignition processes, while motions later in droplet lifetimes are a result of interactions between the droplets and the gas phase including influences of thermal and solutal Marangoni stresses at the liquid-gas interface. [ABSTRACT FROM AUTHOR]

Microcopper particles are introduced to trigger and trace persistent surface convection of eutectic gallium indium (eGaIn) when the liquid-metal is partially immersed in shallow alkaline solution. It is found that the surface convection, which further induces large-scale shape transformation of the liquid-metal body, is driven by the surface-tension imbalance originated from the numerous tiny copper-eGaIn galvanic couples through the electrocapillary mechanism. Meanwhile, the particular configuration of particles binding to the liquid-metal surface and surfing with the flow also enables one to capture the flow patterns and visualize the flow field with the assistance of particle imaging velocimetry technology. This overcomes the difficulties in directly observing and quantifying the dynamic surface-flow behaviors of such nontransparent yet highly reflective liquid-metals. Characteristic flow regimes of the liquid-metal surface convection are revealed and their indications regarding the flow beneath the surface are also made. [ABSTRACT FROM AUTHOR]

This review explores the intersection between two important fields of colloid and interface science – that of active colloidal particles and of (passive) particles at fluid-fluid interfaces. The former uses energy input at the particle level to propel particle motions and direct dynamic assemblies. The latter relies on the spontaneous adsorption of particles at fluid interfaces to modify the interfacial energy, rheology, and permeability of biphasic materials. Here, we address two key questions that connect these otherwise distinct fields of study. How do liquid interfaces influence the dynamics of active or driven colloidal particles? How can particle activity influence the dynamics of liquid interfaces? These questions motivate the pursuit of active particle surfactants that move and organize at fluid interfaces to perform useful functions such as enhancing mass transport or modulating interfacial properties. Drawing examples from the literature, we discuss how fluid interfaces can provide a unique environment for the study of active colloids, how surface tension can be harnessed to propel particle motions, and how capillary interactions can be activated to achieve dynamically tunable emulsions and foams. We highlight opportunities for the future study and application of active particles at liquid interfaces. [ABSTRACT FROM AUTHOR]

Liquid marbles, that is, liquid drops coated by a hydrophobic powder, do not wet any solid or liquid substrate, making their transport and manipulation both highly desirable and challenging. Herein, we describe the light-driven transport of floating liquid marbles and emphasize a surprising motion behavior. Liquid marbles are deposited on a water solution containing photosensitive surfactants. Irradiation of the solution generates photoreversible Marangoni flows that transport the liquid marbles toward UV light and away from blue light when the thickness of the liquid substrate is large enough (Marangoni regime). Below a critical thickness, the liquid marbles move in the opposite direction to that of the surface flow at a speed increasing with decreasing liquid thickness (anti-Marangoni). We demonstrate that the anti-Marangoni motion is driven by the free surface deformation, which propels the non-wetting marble against the surface flow. We call this behavior 'slide effect'. [ABSTRACT FROM AUTHOR]

A model paint consisting of resin dissolved in butanol was sprayed onto horizontal glass substrates to form films varying from 100 μm to 450 μm in thickness. The substrates were photographed at 5 s intervals and image analysis software used to measure the number, diameter and velocity of air bubbles trapped in the paint layer. Painted substrates were weighed to determine the rate of solvent evaporation. Bubbles escaped from the paint in 100–900 s, the time increasing with paint thickness. The Sauter mean diameter of bubbles in films less than 300 μm in thickness decreased with time because larger bubbles escaped faster than small bubbles, while the mean diameter of bubbles in a 450 μm thick layer increased due to bubble coalescence. Bubble velocities due to movement of the liquid increased with paint thickness and reached 30 μm/s. Bubbles were observed to escape from both upward and downwards facing surfaces. Concentration gradients due to solvent evaporation in a paint film create surface tension variations that drive Marangoni flows, which bring bubbles to the paint surface. An analytical one-dimensional model of solvent diffusion was used to calculate solvent concentration variations in the paint film and the Marangoni number. [ABSTRACT FROM AUTHOR]

Giant electrohydrodynamic instability of a partially immersed liquid‐metal body is triggered by tiny copper particles, resulting in long‐term surface convection and large‐scale shape transformation of the liquid metal. In article number 1700939, Jianbo Tang, Jing Liu, and co‐workers describe how complex flow patterns of the particle‐driven surface convection are further captured by tracing the motions of the particles accompanying the flow. [ABSTRACT FROM AUTHOR]

The article presents the inside back cover of the journal "Angewandte Chemie International Edition" is presented.

Transition metal oxide nanoparticles capsuled in amorphous carbon nanotubes (ACNTs) are attractive anode materials of lithium-ion batteries (LIBs). Here, we first designed a fast and universal method with a hydromechanics conception which is called Marangoni flow to fabricate transition bimetal oxides (TBOs) in the ACNT composite with a better electrochemistry performance. Marangoni flows can produce a liquid column with several centimeters of height in a tube with one side immersed in the liquid. The key point to induce a Marangoni flow is to make a gradient of the surface tension between the surface and the inside of the solution. With our research, we control the gradient of the surface tension by controlling the viscosity of a solution. To show how our method could be generally used, we synthesize two anode materials such as (a) CoFe 2 O 4 @ACNTs, and (b) NiFe 2 O 4 @ACNTs. All of these have a similar morphology which is ∼20 μm length with a diameter of 80-100 nm for the ACNTs, and the particles (inside the ACNTs) are smaller than 5 nm. In particular, there are amorphous carbons between the nanoparticles. All of the composite materials showed an outstanding electrochemistry performance which includes a high capacity and cycling stability so that after 600 cycles the capacity changed by less than 3%.

We present an experimental study of the micro- and mesoscopic structure of thin films of medium length n-alkane molecules on the native oxide layer of a silicon surface, prepared by dip-coating in a n-C32H66/n-heptane solution. Electron micrographs reveal two distinct adsorption morphologies depending on the substrate withdrawal speed v. For small v, dragonfly-shaped molecular islands are observed. For a large v, stripes parallel to the withdrawal direction are observed. These have lengths of a few hundred micrometers and a few micrometer lateral separation. For a constant v, the stripes' quality and separation increase with the solution concentration. Grazing incidence X-ray diffraction and atomic force microscopy show that both patterns are 4.2 nm thick monolayers of fully extended, surface-normal-aligned alkane molecules. With increasing v, the surface coverage first decreases then increases for v > v(cr) ∼ 0.15 mm/s. The critical v(cr) marks a transition between the evaporation regime, where the solvent's meniscus remains at the bulk's surface, and the entrainment (Landau-Levich-Deryaguin) regime, where the solution is partially dragged by the substrate, covering the withdrawn substrate by a homogeneous film. The dragonflies are single crystals with habits determined by dendritic growth in prominent 2D crystalline directions of randomly seeded nuclei assumed to be quasi-hexagonal. The stripes' strong crystalline texture and the well-defined separation are due to an anisotropic 2D crystallization in narrow liquid fingers, which result from a Marangoni flow driven hydrodynamic instability in the evaporating dip-coated films, akin to the tears of wine phenomenology.

Hydrodynamic instabilities at the interface between two partially miscible liquids impact numerous applications, including CO2 sequestration in saline aquifers. We introduce here a new laboratory-scale model system on which buoyancy- and Marangoni-driven convective instabilities of such partially miscible two-layer systems can easily be studied. This system consists of the stratification of a pure alkyl formate on top of a denser aqueous solution in the gravitational field. A rich spectrum of convective dynamics is obtained upon partial dissolution of the ester into the water followed by its hydrolysis. The properties of the convective patterns are controlled by the miscibility of the ester in water, the feedback of the dissolved species on its own miscibility, as well as the reactivity of given chemicals in the aqueous solution with the solubilized ester.