Marangoni Convection Research Articles

Ground tests for a future space experiment: Evaporation rates through a centimetric square chimney by vapor interferometry and simulation

In this paper, results of preparatory tests for a space experiment “Marangoni in Films” on board International Space Station are reported. The setup is thermalized at a setpoint temperature so that (thermal) Marangoni convection is exclusively a result of evaporative cooling of the film, here composed of a Hydrofluoroether (HFE)-7100, evaporating into nitrogen at a given setpoint pressure. The present paper is entirely devoted to the evaporation rates, whereas the Marangoni convection itself will be analyzed in future studies. Apart from the temperature and nitrogen pressure, various setpoints for the initial relative saturation in HFE-7100 vapor are explored. A special design of the test cell with a square chimney surrounding a square opening of the liquid layer permits to largely stabilize the evaporation rates during a longest-lasting middle stage (beyond the initial transients and the final film dry-out). In addition, it facilitates the use of vapor (digital-holographic) interferometry for measuring the evaporation rates, the results of which are compared with independent measurements by means of the pressure evolution within our sealed cell of a fixed volume. An excellent agreement is found before the final dry-out stage for all setpoint parameter combinations tested. The study is accompanied by numerical finite-element simulations in an axisymmetrized geometry, neglecting a possible influence of the Marangoni convection and verifying certain premises used in interferometric post-processing. Even if the simulation results might somewhat over-/underpredict in each particular case, the overall dependence of the evaporation rate on the setpoint parameters is well reproduced and the role of the buoyancy convection is thereby explored.

Investigation of surfactant-laden bubble migration dynamics in self-rewetting fluids using lattice Boltzmann method

Self-rewetting fluids (SRFs), such as aqueous solutions of long-chain alcohols, show anomalous nonlinear (quadratic) variations of surface tension with temperature involving a positive gradient in certain ranges, leading to different thermocapillary convection compared to normal fluids (NFs). They have recently been used for enhancing thermal transport, especially in microfluidics and microgravity applications. Moreover, surface-active materials or surfactants can significantly alter interfacial dynamics by their adsorption on fluid interfaces. The coupled effects of temperature- and surfactant-induced Marangoni stresses, which arise due to surface tension gradients, on migration bubbles in SRFs remain unexplored. We use a robust lattice Boltzmann method based on central moments to simulate the two-fluid motions, capture interfaces, and compute the transport of energy and surfactant concentration fields, and systematically study the surfactant-laden bubble dynamics in SRFs. When compared to motion of bubbles in NFs, in which they continuously migrate without a stationary behavior, our results show that they exhibit dramatically different characteristics in SRFs in many different ways. Not only is the bubble motion directed toward the minimum temperature location in SRFs, but, more importantly, the bubble attains an equilibrium position. In the absence of surfactants, such an equilibrium position arises at the minimum reference temperature occurring at the center of the domain. The addition of surfactants moves the equilibrium location further upstream, which is controlled by the magnitude of the Gibbs elasticity parameter that determines the magnitude of the surface tension variation with surfactant concentration. The parabolic dependence of surface tension in SRF is parameterized by a quadratic sensitivity coefficient, which modulates this behavior. The lower this quantity, the greater is the role of surfactants modifying the equilibrium position of the bubble in SRF. Furthermore, the streamwise gradient in the surfactant concentration field influences the transient characteristics in approaching the terminal state of the bubble. These findings provide new means to potentially manipulate the bubble dynamics, and especially to tune its equilibrium states, in microchannels and other applications by exploiting the interplay between surfactants and SRFs.

Pagoda solar evaporators with an alternating hot-cold pattern for rapid, scaling-free desalination of hypersaline brines

Solar-driven evaporation holds significant potential for desalinating hypersaline brines, which pose challenges for traditional reverse osmosis. However, achieving both rapid evaporation and scaling prevention remains a key obstacle. In this study, we introduce a pagoda-like solar evaporator designed to optimize heat and mass transfer during the evaporation process of hypersaline brines. The interconnected multi-layer structure creates an alternating hot–cold pattern that integrates solar and environmental evaporation, enhancing the overall evaporation rate. This pattern also induces thermal Marangoni convection within the evaporator, mitigating local ion accumulation and providing durable scaling resistance. The pagoda evaporator sustains an evaporation rate of approximately 1.70 kg·m−2·h−1 over 8 h with a 20 wt% NaCl solution and produces 7.78 L·m−2·day−1 of fresh water without surface scaling in outdoor experiments. This innovative and facile design marks a significant advancement in the sustainable and efficient desalination of highly saline brines.

Study on heat transfer and flow of molten pool during the deposition process of laser wire additive manufacturing

In the laser wire additive manufacturing process, the molten pool acts as the critical link between the wire and the deposited part. The heat transfer and flow behavior within the molten pool predominantly determine the quality of the final deposition. Under laser power ranging from 2400 to 3000 W, traverse speeds between 0.01 and 0.04 m/s, and wire feeding speeds from 0.03 to 0.08 m/s, three distinct flow states—single-swirl, double-swirl, and no-swirl—were observed with increasing heat input. Under the optimum process parameters, the molten pool with stable temperature distribution and orderly flow was obtained. In multilayer deposition, the implementation of a laser decay strategy mitigates steep temperature gradients, diminishes the Marangoni effect within the molten pool, and effectively reduces both heat accumulation and lateral flow. Consequently, the flow mode transitions from no-swirl to swirl, and the maximum flow velocity decreases by 40%.

Effect of the self-rewetting fluids on heat transfer performance of gravity heat pipes with sintered porous wick

The distribution of the working fluid in a heat pipe directly affects the phase change heat transfer process. This paper explores how self-rewetting fluids (SRWF) and porous wicks affect the distribution of working fluids and heat transfer efficiency in gravity heat pipe (GHP). We sintered a porous wick onto the evaporator section and tested two working fluids: water and SRWF (3 % butanol aqueous solution). The filling ratio of the working fluid φ was varied from 11.5 % to 46 %. We found that at large filling ratios, porous wick surface will immerse in liquid, and at small filling ratios the heating surface will lack of liquid. Consequently, the GHP operates at a lower temperature at a filling ratio of 23 % compared to 11.5 % and 46 %. The type of working fluid significantly impacts the operating temperature and temperature uniformity. The Marangoni effect of the SRWF enhances the rewetting of the fluid in the porous wick, but this effect is only observed after the temperature reaches the inflection point (where the SRWF exhibits minimum surface tension). Before this point, the GHP operating temperature with SRWF is relatively higher than with water. Correspondingly, the temperature uniformity on the evaporator section is better when using SRWF. At φ = 23 %, Q = 500 W the temperature uniformity improved by 66 %. At Q = 660 W the evaporator heat transfer coefficient increased from 3991.6 W/m2K to 6130.5 W/m2K which is a 53.6 % improvement over water. We used an infrared camera to visualize the rewetting phenomenon of SRWF in the heated porous wick, confirming that SRWF can rewet the dry-out area, thus validating our hypothesis.

Study on Enhancing the Quantitative Performance of LIBS for Cu Element in Liquids Based on the Chitosan-Parafilm Enrichment Method

Abstract To enhance the accuracy of laser-induced breakdown spectroscopy (LIBS) in detecting heavy metal elements in solutions, a Chitosan (CS)-Parafilm (PM) enrichment method is proposed. This method involves drying chitosan-heavy metal complexes on a Parafilm substrate. During the drying of droplets, electrostatic attraction and chelation by CS effectively concentrate analytes. Additionally, the hydrophobic effect of the PM substrate induces Marangoni flow, which drags analytes from the bottom edge to the central top of the droplet surface, effectively suppressing the coffee ring effect (CRE).LIBS spectra were collected in a uniform array on the surface of sediments, and spectral stability was assessed through cumulative excitation spectra measurements. The results showed that the spectral stability of emission lines Cu I 324.754 nm and Cu I 327.396 nm decreased to 3.85% and 3.78%, respectively, indicating that the CS-PM enrichment method allows for uniform deposition of analytes within samples, effectively enhancing detection repeatability.Quantitative analysis of Cu elements using the CS-PM enrichment method was conducted using PSO-SVM, PSO-BPNN, and RF algorithms. The RF algorithm demonstrated the best predictive performance with Rp² of 0.979, RMSEP of 4.60 mg/L, MAE of 3.19 mg/L, and RPD of 6.75. These results indicate that CS-PM effectively improves spectral stability and quantitative analysis accuracy, providing a technical reference for enhancing the stability and quantitative performance of element analysis in liquids using LIBS.

Variation of Surface Tension of Liquid Metal with Fluorides in Tungsten Inert Gas Welding

Real-time measuring and obtaining surface tension of liquid metal during the arc welding process is critical for studying the behavior of the weld pool, such as Marangoni flow and flow velocity. The dynamic variation of surface tension of weld pool during tungsten inert gas welding process with and without fluoride flux was measured by weld pool oscillation sensing system, and the effect of fluoride flux on surface tension and pool behavior was analyzed. The results show that the surface tension gradient of liquid metal can convert from a negative to a positive value in a critical arc time. The flow velocity of liquid metal and critical arc time significantly increased and decreased, respectively, with fluoride flux. The variation of surface tension, flow velocity, and critical arc time with fluoride flux was induced by arc temperature increasing.

Transferable Percolated Particle Monolayers Derived from Marangoni Flows for Thermally Conductive Dielectric Coatings.

Thermal management is becoming one of the most significant design and size limitations for high power density electronics, including motherboards, power converters, and phased array antennas for 5G communications. There are few options for conducting heat away with dielectric materials that avoid shortening or distorting the performance of these electronics. Certain highly thermally conductive 2D and 3D materials, including hexagonal boron nitride and diamond, offer ideal material properties to address these issues but are extremely challenging to process. This work studies highly oriented single-particle thick films of hexagonal boron nitride, manufactured through a modified Langmuir-Blodgett process and densified further using the Marangoni effect to attain remarkable thermal conductivity enhancement with minimal coating thickness. High loadings of hexagonal boron nitride (∼60 vol %) in dense, castable films are also produced to compare thermal spreading ability in a comparatively simpler but less-coordinated percolated system to the highly percolated particle monolayers. These procedures were applied to glass fiber reinforced polymers used in aerospace and radome applications as well as to a single-board computer to demonstrate enhanced thermal management.

Graphene Microflower by Photothermal Marangoni-Induced Fluid Instability for Omnidirectional Broadband Photothermal Conversion.

2-D carbon-based materials are well-known for their broadband absorption properties for efficient solar energy conversion. However, their high reflectivity poses a challenge for achieving efficient omnidirectional light absorption. Inspired by the multilevel structures of the flower, a Graphene Microflower (GM) material with gradient refractive index surface was fabricated on polymer substrates using the UV-intense laser-induced phase explosion technique under the synergistic design of the photothermal Marangoni effect and the fluid instability principle. The refractive index gradient reduces light reflection and absorbs at least 96% of light at incident angles of 0-60° across the entire solar wavelength range (200-2500 nm). Over 90% absorption even at 75° angle of incidence. The light absorption is enhanced by the multiple interferometric phase cancelation and localized surface plasmon resonance, resulting in a steady-state temperature 60 °C higher than ambient conditions under one solar irradiation. The max rate of temperature rise can reach up to 62 °C s-1. The device is then integrated at the hot end of the temperature difference generator at high altitude to ensure continuous and efficient power generation, producing a steady-state power of 196 mW.

On the self-similarity of unbounded viscous Marangoni flows

The Marangoni flow induced by an insoluble surfactant on a fluid–fluid interface is a fundamental problem investigated extensively due to its implications in colloid science, biology, the environment and industrial applications. Here, we study the limit of a deep liquid subphase with negligible inertia (low Reynolds number, $Re\ll {1}$ ), where the two-dimensional problem has been shown to be described by the complex Burgers equation. We analyse the problem through a self-similar formulation, providing further insights into its structure and revealing its universal features. Six different similarity solutions are found. One of the solutions includes surfactant diffusion, whereas the other five, which are identified through a phase-plane formalism, hold only in the limit of negligible diffusion (high surface Péclet number $Pe_s\gg {1}$ ). Surfactant ‘pulses’, with a locally higher concentration that spreads outward, lead to two similarity solutions of the first kind with a similarity exponent $\beta =1/2$ . On the other hand, distributions that are locally depleted and flow inwards lead to similarity of the second kind, with two different exponents that we obtain exactly using stability arguments. We distinguish between ‘dimple’ solutions, where the surfactant has a quadratic minimum and $\beta =2$ , from ‘hole’ solutions, where the concentration profile is flatter than quadratic and $\beta =3/2$ . Each of these two cases exhibits two similarity solutions, one valid prior to a critical time $t_*$ when the derivative of the concentration is singular, and another one valid after $t_*$ . We obtain all six solutions in closed form, and discuss predictions that can be extracted from these results.

Crystal Patterning from Aqueous Solutions via Solutal Instabilities.

Fluid instabilities can be harnessed for facile self-assembly of patterned structures on the nano- and microscale. Evaporative self-assembly from drops is one simple technique that enables a range of patterning behaviors due to the multitude of fluid instabilities that arise due to the simultaneous existence of temperature and solutal gradients. However, the method suffers from limited controllability over patterns that can arise and their morphology. Here, we demonstrate that a range of distinct crystalline patterns including hexagonal arrays, branches, and sawtooth structures emerge from evaporation of water drops containing calcium sulfate on hydrophilic and superhydrophilic substrates. Different pattern regimes emerge as a function of contact line dynamics and evaporation rates, which dictate which fluid instabilities are most likely to emerge. The underlying physical mechanisms behind instability for controlled self-assembly involve Marangoni flows and forced wetting/dewetting. We also demonstrate that these patterns composed of water-soluble inorganic crystals can serve as sustainable and easily removable masks for applications in microscale fabrication.

Ultra‐Durable Solar‐Driven Seawater Electrolysis for Sustainable Hydrogen Production

AbstractIons in seawater hinder direct sewage electrolysis due to the extreme corrosion of Cl− to the anode and reaction of Mg2+ and Ca2+ on the cathode producing solid substances, which reduce the electrolytic efficiency. However, traditional desalination consuming fossil fuel with massive CO2 emissions threatens human survival. Therefore, zero‐carbon emission, ultra‐durable, large‐scale production of freshwater from seawater for water electrolysis is urgently needed. Herein, a multifunctional system for seawater is demonstrated electrolysis based on ultra‐durable solar desalination outdoors. The solar evaporators reach an evaporation flux of 1.88 kg m−2 h−1 with a photothermal conversion efficiency of solar energy as high as 91.3% with excellent ultra‐durable salt resistance even for saturated saltwater due to the Marangoni effects. Moreover, the condensation of pure water from solar desalination based on the evaporation system reaches 0.54 L m−2 h−1 outdoors, which is suitable for a 20 cm × 20 cm engineered electrode equipped with a Janus membrane powered by a solar panel to produce H2 outdoors. The ultrafast unidirectional transport of H2 bubbles enabled by Janus membranes can greatly improve the H2 production efficiency at a rate approaching 85 mL h−1 for continuous 24 h outdoors.

Microvascular Engineering for the Development of a Nonembedded Liver Sinusoid with a Lumen: When Endothelial Cells Do Not Lose Their Edge.

Microvascular engineering seeks to exploit known cell-cell and cell-matrix interactions in the context of vasculogenesis to restore homeostasis or disease development of reliable capillary models in vitro. However, current systems generally focus on recapitulating microvessels embedded in thick gels of extracellular matrix, overlooking the significance of discontinuous capillaries, which play a vital role in tissue-blood exchanges particularly in organs like the liver. In this work, we introduce a novel method to stimulate the spontaneous organization of endothelial cells into nonembedded microvessels. By creating an anisotropic micropattern at the edge of a development-like matrix dome using Marangoni flow, we achieved a long, nonrandom orientation of endothelial cells, laying a premise for stable lumenized microvessels. Our findings revealed a distinctive morphogenetic process leading to mature lumenized capillaries, demonstrated with both murine and human immortalized liver sinusoidal endothelial cell lines (LSECs). The progression of cell migration, proliferation, and polarization was clearly guided by the pattern, initiating the formation of a multicellular cord that caused a deformation spanning extensive regions and generated a wave-like folding of the gel, hinged at a laminin-depleted zone, enveloping the cord with gel proteins. This event marked the onset of lumenogenesis, regulated by the gradual apico-basal polarization of the wrapped cells, leading to the maturation of vessel tight junctions, matrix remodeling, and ultimately the formation of a lumen─recapitulating the development of vessels in vivo. Furthermore, we demonstrate that the process strongly relies on the initial gel edge topography, while the geometry of the vessels can be tuned from a curved to a straight structure. We believe that our facile engineering method, guiding an autonomous self-organization of vessels without the need for supporting cells or complex prefabricated scaffolds, holds promise for future integration into microphysiological systems featuring discontinuous, fenestrated capillaries.

Stearic Acid as Polymerization Medium, Dopant and Hydrophobizer: Chemical Oxidative Polymerization of Pyrrole.

In recent years, fatty acids have garnered significant attention as a natural phase-change material and a hydrophobizer due to their biocompatibility and biodegradability. In this study, the utilization of fatty acid is proposed as a polymerization medium for the first time. As a specific reaction, chemical oxidative polymerizations of pyrrole is conducted using ferric chloride as an oxidant in a stearic acid medium. The polymerizations resulted in the production of micrometer-sized polypyrrole (PPy) grains, which are aggregates of atypical primary particles with submicrometer size. The PPy grains are doped with stearic acid, suggesting that the stearic acid functioned as a dopant and a hydrophobizing agent as well as a polymerization medium. The dried PPy grains can adsorb at the air-water interface and function as a liquid marble stabilizer with light-to-heat photothermal properties. The liquid marble can move on a planar air-water interface by Marangoni flow induced by NIR laser light irradiation.

Stability analysis of Rayleigh–Bénard–Marangoni convection in fluids with cross-zero expansion coefficient

Cross-zero expansion coefficient Rayleigh–Bénard–Marangoni (CRBM) convection refers to the convective phenomenon where thermal convection with stratified positive and negative expansion coefficients in a liquid layer is coupled with the Marangoni convection. In the Bénard convection, fluids with a cross-zero expansion coefficient contain a neutral expansion layer where the expansion coefficient (α) is zero, and the local buoyancy-driven convection is coupled with the Marangoni convection, leading to unique flow instability phenomena. This paper uses linear stability theory to analyze the CRBM convection in a horizontal liquid layer under a vertical temperature gradient and performs numerical calculations for fluids under different Bond numbers (Bd) in both bottom-heated and bottom-cooled models, obtaining the critical destabilization conditions and modes. In the bottom-heated model, different combinations of buoyancy instability mechanism (BIM), tension instability mechanism, and coupled instability mechanism (CIM) appear depending on the dimensionless temperature for the neutral expansion layer (Tα0) and the Bd. In the bottom-cooled model, two mechanisms occur according to the variation of Tα0: BIM and CIM.

Critical radius deviated from Leidenfrost state of droplets on liquid layer

The levitation of Leidenfrost droplets on liquid pool is fascinating, but its final stage is lack of understanding. Here, we found that a droplet levitated on liquid layer eventually deviated from Leidenfrost state once its radius was lower than a critical radius due to evaporation. The critical radius of ethanol droplet deviated from Leidenfrost state on silicone oils with a thickness ranging from 2.0 to 15.0 mm was determined by experiment. The influences of the initial radius of droplet, viscosity, and thickness of liquid layer on critical radius were analyzed. In addition, the critical radius decreases with increase in superheat for ΔT lower than 25.0 °C, whereas it does not significantly vary after ΔT exceeding 25.0 °C. The bottom temperature Tb of droplet does not approach to saturation temperature even under a high superheat. The experiment found that Marangoni convection existed inside droplet. Based on a theoretical model considering Marangoni convection, the reason for droplet deviated from Leidenfrost state was explained. These findings are helpful for understanding the final state of Leidenfrost droplet on liquid layer and would provide a potential practical application such as extinction of oil pool fires with liquids.

Step flow mechanism in dissolutive wetting Cu/Ni systems

The Cu-Ni system is a typical dissolutive system due to its mutual dissolution across a wide range of temperatures and compositions. We characterized the effects of Ni dissolution on the wetting behavior of liquid Cu by combining high-temperature wetting experiments, in-situ observation of spreading and solidification, microstructure analysis of the quenched droplets, and computational fluid dynamic (CFD) simulations. In the very early moment, at 1100 °C, when the Cu droplet is brought in contact with the Ni substrate, it oscillates due to capillarity and is dampened by inertial effects, while the significant Ni dissolution at 1150 °C largely reduced the initial oscillations. Later, a peculiar spreading behavior is observed and we propose to describe it through a 4-step mechanism: pinning of the contact line by a newly formed solid solution layer at the interface acting as a physical barrier, driving of liquid towards the solidified edge due to a Ni-concentration induced Marangoni flow, forming of a precursor film ahead of the solidified edge caused by the strong Cu-Ni interactions and Marangoni flow, and finally depinning due to overflow as a result of liquid accumulation at the solidified edge. The formation of a solid solution layer is confirmed by in-situ observation and quenching. The Ni-concentration induced Marangoni flow is characterized experimentally and further investigated by CFD simulations. The proposed step flow mechanism can be potentially relevant to other dissolutive wetting systems (e.g. Bi/Sn, Ag/Cu and Cu/Fe systems), which are crucial for high-temperature processing techniques.

Marangoni-driven pattern formation in an absorbing binary mixture

We investigate the evolution of convective instability in a LiBr–water binary mixture, driven by thermal and solutal Marangoni stresses, through numerical simulations. A small perturbation in absorption at the surface disturbs the equilibrium, generating surface tension gradients that drive Marangoni flows. To isolate and better understand the interplay between the Marangoni effect and absorption, we extended the previous study by investigating the LiBr–water binary system in the absence of gravity. For the first time, we observed the formation of a stable rim in an absorbing binary mixture, which underwent a slight contraction followed by rapid Marangoni spreading. This behavior shows similarities with the flow patterns seen in the “coffee-ring” and Marangoni spreading phenomena in evaporating binary mixtures. On the subsurface, convective motion breaks into several vortices, accompanied by the formation of plumes with reduced mass fraction. As the system evolves, the symmetry of the flow pattern around the cell center breaks down. The absence of buoyancy-driven forces eliminates a key counterforce to Marangoni flows, transforming the previously ordered patterns into non-periodic oscillations, followed by the development of non-stationary but regular patterns. These results complement our earlier findings [P. F. Arroiabe et al., Phys. Fluids 36, 022119 (2024)] where gravitational forces obscured these phenomena.

Morphological evolution mechanism of microstructures involved in shaping micro-pillar arrays into microlens arrays on fused silica surfaces by CO2 laser polishing

The periodic micro-pillars on the hard-brittle fused silica surface can be shaped into the smooth and defect-free microlens arrays (MLA) by CO2 laser polishing, which can be applied in imaging and beam-shaping. However, the morphology evolution mechanism of the micro-pillars is not clear during polishing, which seriously affects the rapid preparation of high-quality MLAs. Herein, a multi-physics coupling model is established to investigate the interaction between CO2 lasers and micro-pillars on the fused silica surface, and is verified from the perspectives of temporal and spatial temperature. The steady surface temperature evolution law with laser power and spot radius is explored, based on which, the parameter ‘Window’ for polishing is determined to make the material melt and flow without ablation removal. The morphology evolution of micro-pillars is investigated as well as laser-material interaction. It is found that under laser irradiation, the micro-pillar height first increases and then decreases, and its top curvature radius increases. The surface tension makes the dominant contributions to the melting and flow of the micro-pillar materials, while the Marangoni effect and the gravity have little effect. Furthermore, to fabricate the target MLAs with the specific dimensions, the influence of micro-pillar aspect ratio (φ) on microlens surface morphology is investigated. It is found that there are two critical φ for the target MLAs. For the required MLA with the target period of 200 μm and the target height of 40 μm, when the φ is larger than 5.1, the micro-pillar array fails to be shaped into the target MLA by CO2 laser polishing. As the φ decreases, the junction of micro-pillar and the substrate sags downwards significantly, and the deviation between the shaped- and the ideal spherical contours increases. To successfully prepared the target MLA and guarantee the geometry accuracy, the minimum φ is selected to be 0.71. This work has important theoretical significance for obtaining the smooth and defect-free target fused silica MLAs with the specific dimensions by CO2 laser non-evaporative polishing.

Molecular modelling of active oil droplet propulsion: Insights from dissipative particle dynamics simulation

This study employed dissipative particle dynamics (DPD) simulations to investigate the self-propelled motion of oil droplets in water–oil–surfactant systems. It is the first attempt to replicate self-propulsion models of oil droplets at the molecular level, contrasting previous simulations focused on Brownian motion and hydrodynamic behavior of colloidal particles. The DPD model reproduced droplet propulsion and visualized internal Marangoni flow, showing that larger droplet radii and greater interfacial tension differences increase propulsion speeds. Additionally, surfactants with stronger oil–oil repulsion enhanced propulsion speed, suggesting that surfactant-induced local structures are crucial for the self-propulsion mechanism.