Liquid–Liquid Interface Motion of a Capsule Motor Powered by the Interlayer Marangoni Effect

On-the-Fly Formation of Polymer Film at Water Surface

The self-propulsion of bodies floating in water is of great interest for developing new robotic and intelligent systems at different scales, and whenever possible, Marangoni propulsion is an attractive candidate for the locomotion of untethered micro-robots. Significant cases have been shown using liquid and solid surfactants that allow an effective propulsion for bodies floating on water to be achieved. Here, we show for the first time a strategy for activating a twofold functionality where the self-propulsion of a floating body is combined with the formation of a polymer thin film at the water surface. In fact, we demonstrate that by using polymer droplets with an appropriate concentration of solvent and delivering such drops at specific locations onto freely floating objects, it is possible to form “on-the-fly” thin polymer films at the free water surface. By exploiting self-propulsion, a polymer thin film can be formed that could cover quite extensive areas with different shapes depending on the motion of the floating object. This intriguing twice-functionality activated though a single phenomenon, i.e., film formation and related locomotion, could be used in perspective to perform complex operations at water surfaces, such as dynamic liquid packaging, cleaning, and moving away floating particles, monolayer films, or macro-sized objects, as discussed in the text.

Multimotion of Marangoni Propulsion Ships Controlled by Two-Wavelength Near-Infrared Light

Multimode motion of Marangoni propulsion ships on a water surface as per a near-infrared, two-wavelength selective response is achieved for the first time. The ships are rhombus- or propeller-shaped polyacrylamide or siloxane resin-based gels in which Nd₂O₃ and Yb₂O₃ nanoparticles are incorporated separately for photothermal conversion at 808 and 980 nm, respectively. The rhombus geometry is for straight locomotion, and the propeller geometry is for rotation. On/off remote control of the forward and backward locomotion of a rhombus-shaped ship and of the clockwise and counterclockwise rotations of a propeller-shaped ship via irradiation with 808 or 980 nm near-infrared light is demonstrated. The nanoparticles are incorporated into the desired locations of the gels, enabling selective local heating of the gels without focusing the light. The temperature gradient of the ships by local heating, based on a photothermal conversion, generates a Marangoni propulsion force to move the ship in the desired direction.

Micro/nano-machines for spilled-oil cleanup and recovery: A review

High-efficiency, safe and economically viable nano-engineered platforms for oil spill cleanup and recovery are of great importance. This review takes account of the concept of nanomotors and micromotors and their most advancements in use for oil spill treatment. The fundamental facets of artificial micro- and nano-machines/nanobots/nanomotors (MNMs) are first documented, followed by the most recent influencing developments in chemical engineering approaches toward their specific utilizations. The surface chemistry of these MNMs, their behaviors in different water matrices and their roles in the removal of oil are examined, revealing great rooms for improvement. The strategies for surface and structural modification of these tiny machines toward enhancing their reactivity in the removal of oil and coupled tasking are discussed in details, highlighting the significance of fit-for-duty design and tailored fabrication. The engineering limitations and practical implementation barriers of this emerging technology and how it can be overcome are also considered. Finally, some engineering boundaries and perspectives of this fast-evolving field are proposed at the end.

Binding of Lignin Nanoparticles at Oil-Water Interfaces: An Ecofriendly Alternative to Oil Spill Recovery

Synthetic amphiphiles used for managing large-scale oil spills have a toxic impact on the environment and marine life. Developing new oil spill recovery technologies is critical to minimize the environmental and ecological impact of such disasters. Here, we show that a mixture of lignin nanoparticles and 1-pentanol forms a biocompatible alternative to nondegradable, synthetic amphiphiles used for oil spill recovery. The pentanol in the mixture generates initial Marangoni flow and confines the spilled oil into a thick slick on the surface of water. While the alcohol solubilizes, lignin nanoparticles irreversibly adsorb onto the oil-water interface. We find that the lignin nanoparticle adsorption to the oil-water interface is governed by a combination of electrostatic, van der Waals, and hydrophobic interactions between the particles and the interface. These interactions, combined with interparticle electrostatic repulsion between nanoparticles adsorbed at the oil-water interface, drive the formation of a submonolayer. The submonolayer transforms into a film of jammed nanoparticles due to compressive stress acting on the interface upon the solubilization of pentanol. This interfacial layer of lignin nanoparticles restricts oil from respreading and locks the oil in its confined state. The herded state of the oil with the interfacial layer of nanoparticles facilitates safe removal of the spilled oil using mechanical methods. The study presents a new principle of using a mixture of heavy alcohol and biocompatible nanoparticles for oil herding applications, thus providing an ecofriendly alternative to oil spill recovery.

Metal-Organic Framework "Swimmers" with Energy-Efficient Autonomous Motility

Placed at a water/air interface, particles of porphyrin-based MOFs (metal-organic frameworks) cut from large-area films display efficient, multiple-use autonomous motility powered by release of solvents incorporated in the MOF matrix and directionality dictated by their shapes. The particles can be refueled multiple times and can achieve speeds of ca. 200 mm·s^-1 with high kinetic energy per unit of chemical "fuel" expended (>50 μJ·g^-1). Efficiency of motion depends on the nature of the fuel used as well as the microstructure and surface wettability of the MOF surface. When multiple movers are present at the interface, they organize into "open" structures that exhibit collective, time-periodic motions.

Electrovariable gold nanoparticle films at liquid–liquid interfaces: from redox electrocatalysis to Marangoni-shutters

Control over the physical properties of nanoparticle assemblies at a liquid–liquid interface is a key technological advancement to realize the dream of smart electrovariable nanosystems. Electrified interfaces, such as the interface between two immiscible electrolytes solutions (ITIES), are almost an ideal platform for realizing this dream. Here, we show that the Galvani potential difference across soft interfaces can be effectively used to manipulate: (i) the reactivity of gold nanoparticle assemblies through varying the Fermi level (both chemically and electrochemically); (ii) the location distribution of the nanoparticles at the liquid–liquid interface. In the first case, in addition to our previous studies on electron transfer reactions (ET) across the ITIES, we used intensity modulated photocurrent spectroscopy (IMPS) to study the kinetics of photo-induced electrochemical reactions at the ITIES. As expected, the direct adsorption of gold nanoparticles at the interface modifies the kinetics of the ET reaction (so-called, interfacial redox electrocatalysis), however it did not lead to an increased photocurrent by “plasmonic enhancement”. Rather, we found that the product separation depends on double layer effects while the product recombination is controlled by the Galvani potential difference between the two phases. In the second case, we demonstrated that polarizing the ITIES caused migration of gold nanoparticles from the middle region of the cell to its periphery. We called such systems “Marangoni-type shutters”. This type of electrovariable plasmonic system did not experience diffusion limitation in terms of the adsorption/desorption of nanoparticles and the entire movement of nanoparticle assemblies happened almost instantly (within a second). It opens a fresh view on electrovariable plasmonics and presents new opportunities to create smart nanosystems at the ITIES driven with an electric field.

Long time and distance self-propelling of a PVC sphere on a water surface with an embedded ZnO micro-/nano-structured hollow sphere

In this research, a zinc oxide micro-/nano-structured hollow sphere (MNHS) with a large specific surface area is applied as energy storage material to encapsulate poly(vinyl chloride) solution and control the fuel release.

Self-Propelled and Long-Time Transport Motion of PVC Particles on a Water Surface

Driven by the Marangoni effect, a poly(vinyl chloride) (PVC) particle runs in its orbit (a) with high velocity due to the release of surfactant and heat. The PVC particles are also able to efficiently drive an aluminum bulk and to induce spinning and quick runs on a water surface (b).

Marangoni effect in nonequilibrium multiphase system of material processing

In a nonequilibrium multiphase system, the Marangoni effect has important influence on interphase heat and mass transfer induced by interfacial tension gradient associated with either solutal or thermal gradients in numerous applications. Although the basic knowledge of the Marangoni effect has made great progress, its mechanism is not yet fully recognized. An in-depth understanding of the basic principles of heat and mass transport induced by the Marangoni effect is important for better operation of interphase transfer units. This report aims to provide a systematic study of the mechanisms of the Marangoni effect. Special foci are concentrated on the field of material processing, which encompasses the process wherein the raw materials are transformed into useful engineering products. A series of experimental, theoretical, or simulated results are extracted from the literature to elucidate the mechanism of the Marangoni effect and to rationalize the interphase transport phenomena. We examine the major physical explanations and attempt to distinguish the working principle behind the process. Perspectives are also proposed to trigger further innovative thinking about the topics. This work provides new insight into material processing by taking advantage of the Marangoni effect and has far-reaching implications in the design of future nonequilibrium multiphase system.

Self-Propelled Enzyme-Based Motors for Smart Mobile Electrochemical and Optical Biosensing

A millimeter-sized tubular motor for mobile biosensing of H2O2 in environmental and relevant clinical samples is reported. The concept relies on the self-propelled motion by the Marangoni effect, where the asymmetric release of SDS surfactant induces fluid convection and rapid dispersion of horseradish peroxidase (HRP) enzyme into the sample solution. This efficient movement together with the continuous release of fresh enzyme leads to greatly accelerated enzymatic reaction processes without the need of external stirring or chemical and physical attachment of the enzyme as in common classical biosensing approaches. In this strategy, the use of a single millimeter-sized tubular motor during 120 s allows the reliable and accurate quantification of hydrogen peroxide in a set of different matrices such as tap and mineral waters, urine, plasma, and tumor cell cultures treated with antineoplasic Cisplatin without any previous sample preparation. Furthermore, detection can be performed electrochemically, optically, and via visual detection, which makes this approach a clear candidate as a point-of-care analytical tool.

Synthesis of conducting asymmetric hydrogel particles showing autonomous motion

In the present work, we introduce a new approach for the synthesis of asymmetric particles made from electrically conducting polyaniline-alginate hydrogels by using bipolar electrochemistry. Such an intrinsic break of symmetry allows the soft beads to exhibit tunable motion at the air/water interface when loaded with ethanol due to controllable directed release of the solvent.

A floating self-propelling liquid marble containing aqueous ethanol solutions

We report the behaviour of a self-propelling liquid marble containing an aqueous ethanol solution.

Acetylene bubble-powered autonomous capsules: towards in situ fuel

A fuel-free autonomous self-propelled motor powered by acetylene bubbles is illustrated.

Self-Induced Surfactant Transport along Discontinuous Liquid-Liquid Interfaces

While the Marangoni-stress-driven spreading of surfactants along continuous fluid interfaces is a well-studied problem, we demonstrate experimentally that swift and efficient surfactant transport can also occur along discontinuous interfaces. We used chemical surface patterning to create arrays of discrete drops and liquid bridges immersed inside a second immiscible liquid. Surface-active compounds introduced at one end of the linear array are transported along the array via surfactant-induced interfacial convection at a rate by far exceeding diffusion. We believe this mechanism to be relevant to the application of surfactants in enhanced oil recovery, where oil-water interfaces are likely to be discontinuous. Marangoni flows can provide access to dead-end pores and low-permeability regions that are otherwise bypassed by conventional pressure-driven waterfloods.