Underwater Oil Droplet Research Articles

Preparation of SiO2/chitosan composite coating on stainless steel mesh for efficient and stable oil/water separation

In recent years, the application of underwater superoleophobic coatings in the field of oil/water separation has received increasing attention, and chitosan (CS) coatings have become a research hotspot due to their advantages of hydrophilicity, non-toxicity, biodegradability, and antibacterial properties. In this work, a SiO2/CS underwater superoleophobic composite coating was prepared on 304 stainless steel mesh by a simple dip-coating method. The underwater oil droplet contact angle (OCA) of the SiO2/CS coating was 151.2°, and the oil droplet showed a stable underwater Cassie state on the coating. The stainless steel mesh with SiO2/CS coating could effectively separate different kinds of oil/water mixtures, and the separation efficiency for the kerosene/water mixture was 99.2 %. After 20 cycles of separation, the separation efficiency was more than 98 %, indicating that the SiO2/CS coating had excellent recycling performance. In addition, the prepared underwater superoleophobic coating exhibited excellent long-term stability, acid and alkali resistant stability and UV resistant stability. This SiO2/CS coating has promising practical application, which could expand the use of underwater superoleophobic coatings.

Preparation of carbon-based active nanofluids with temperature and salt resistance for enhanced oil recovery

The abundant hydrophilic surface groups endow carbon-based nanoparticles (CDs) with intrinsic great dispersibility, however, the strong hydrophilicity limits the oil–water interface activity, restraining the performance in enhanced oil recovery. Developing CDs with simultaneously ultra-small sizes, high temperature and salt tolerance, as well as superior interfacial activity is of great significance. In this work, CDs with the size of 3.3 nm and excellent temperature and salt resistance were prepared by hydrothermal reaction, using citric acid (CA) and ethylenediamine (EDA) as precursors. Carboxyl ether carboxylate-6Na (APEC-6Na) was selected as the compound agent to construct carbon-based active nanofluids (CANs) with excellent oil–water and oil-solid interfacial activities. CANs can reduce the oil–water interfacial tension to 8.9 × 10-4 mN m−1, withstand a temperature up to 90°C and a salinity up to 6.3 × 104 mg·L-1. Even at the highest tolerant salinity, the underwater oil contact angle of 155° can be achieved, demonstrating excellent wettability control capability, which could be attributed to the synergistic effect of the compound CANs system. The increase of the contact angle of underwater oil droplets facilitates the coalescence of oil film on the lipophilic surface to form oil droplets. The electrostatic repulsion generated by the adsorption of CDs and APEC-6Na at the oil–water interface and rock surface makes the coalesced oil droplets more easily detached from the rock surface, and the oil film reduction rate exceeds 97.7 % within 24 h. The excellent oil–water and oil-solid interface properties and the structural separation pressure endows CANs with remarkable performance in core displacement experiments. Compared with simulated formation water, CANs can improve the recovery rate by 26.3 %, exhibiting important application value for improving the recovery rate of ultra-low permeability reservoirs.

Self-driven oil/water separator with super-high separation rate

Oil/water separation is of great significance due to the severe threats of oily wastewater to the environment and human health. However, the existing methods still cannot spontaneously and continuously separate the oil/water mixture with high rates and low costs. Here, a novel oil/water separation method based on a self-driven oil jet phenomenon was proposed to resolve the aforementioned problem. We designed a self-driven oil/water separator (SOWS) composed of an oil container with some oil and a water container with some water. The oil container, whose whole bottom was covered with underwater superoleophilic pores, underwater superoleophilic ring region, and other underwater superoleophobic region, was fixed in the water container. The underwater oil droplet was easily captured by the superoleophilic region and spontaneously penetrated the pore, generating an oil jet. The resultant force from Laplace pressure and hydrostatic pressure drove the droplet to generate the jet and affected the droplet flow rate. Based on the self-driven oil jet and suitable optimization, the SOWS realized the spontaneous and continuous oil/water separation with the separation rate of 42929.4 L∙m−2∙h−1 which was much faster than the reported oil/water separation methods. Moreover, the SOWS could efficiently separate the oil/water mixture with different oil content and oil viscosity. This method shows an outstanding practical application prospect in the oily wastewater treatment.

An Experimental Study on the Influence of the Fractal Characteristics of X80 Steel Surface Morphology on Water Ring Stability

The surface morphology of X80 steel with hydrophilic underwater oleophobic characteristic is described greater comprehensively and quantitatively in this work by combining fractal dimension and multifractal. X80 steel with hydrophilic underwater oleophobic surface characteristics was constructed using a chemical etching method. Then, with the aid of three wettability parameters—contact angle, rolling angle, and adhesion work—this study investigated the relationship between the surface fractal dimension of X80 and the stability of the water ring in the core annular flow. The results showed that: (1) the fractal dimension of X80 steel specimens increased first and then decreased with the increase of reaction time. Besides that, the value of it was greater than 2, indicating that the surface had obvious fractal characteristics. The spectral difference, Δf(α), and the spectral width, Δα, supplemented the description of the X80 steel surface morphology, which was consistent with the scanning electron microscope results. (2) When the maximum fractal dimension was 2.0808, the minimum contact angle of distilled water on its surface was 50.2°, and the maximum contact angle of underwater oil droplets was 166.4°. The larger the fractal dimension of X80 steel with hydrophilic underwater oleophobic properties, the more hydrophilic and underwater oleophobic it is. This illustrated that there was a strong binding force between the water and the X80 steel pipe wall, and hence the quality and efficiency of the core annular flow was improved, which was more conducive to the promotion of this technology in the field of heavy oil transportation.

Modeling of self-driven directional movement of underwater oil droplets on bio-inspired nano-coated 3D-printed conical models

The contamination of water with oil from industrial processes has become a worldwide problem that requires the development of efficient, reliable and economical separation methods. Inspired by droplet movements on spider silk, we present in this contribution a model of the separation of oil and water that is based on the effect that liquid droplets move on cone-shaped structures, driven by capillary forces. We extend existing models to barrel-shaped oil droplets that move, completely surrounded by water, along suitably shaped filaments and derive the forces acting on the droplets from first principles of physics. By bringing together the results of the theoretical model and the corresponding experiments, the underlying mechanism of the directional movement of droplets in another liquid medium is thoroughly investigated and modeled. The resulting model predicts the dependence of the droplet motion on time and on “system defining” parameters, such as droplet size, viscosity, and dynamic contact angles formed between droplet and filament surface. We expect this work to pave the road for the design and preparation of bionic structures in the field of droplet transport phenomena, which is of great interest in various applications.

Interlaced wetting surfaces with switchable wettability for manipulating underwater oil droplets

Biomimetic surfaces that can manipulate droplets have attracted intense attention because of their application prospects in the fields of drug detection, cell sorting, bio-sensor, etc. Although many promising advances have been made, it is still a challenge to achieve lossless, no cross-contamination and on-demand controllable manipulation of underwater oil droplets without external energy input. Herein, SHI & SLIP interlaced wetting smart surfaces were fabricated by a simple strategy for manipulating underwater oil droplets. Depending on the switchable wettability and reasonable alternating wetting, the lubricant-infused area and superhydrophilic area can be on-demand controlled on the same surface. Numerical calculation and theoretical analysis reveal that the energy barrier between different interfaces is an important factor to realize underwater oil droplet manipulation. By the rational design of the interlaced wetting pattern, the capture, lossless transfer, fusion and directional transportation of underwater oil droplets were realized. Besides, the as-prepared surface has cross-contamination resistance, stable wettability and rewritability, which expands the application field and improves the potential value. We expect this work to open new avenues for flexible and controllable manipulation of underwater oil droplets and promote the development of smart microfluidic platform. It is also expected to be used in unpowered delivery systems, micro-reactions and energy transfer.

Simulation investigation of the spontaneous motion behaviors of underwater oil droplets on a conical surface.

A conical surface can realize the spontaneous transportation of micro-sized oil droplets in an aqueous environment without energy input, exhibiting great potential for applications in microfluidics, chemical micro-reactors, water remediation, etc. However, the precise manipulation of an oil droplet on a cone is still very challenging because the dynamic behavior of a droplet on a cone is not fully understood. Herein, the dynamic behavior of oil droplets on a cone is quantitively studied via numerical simulations, and the effects of wettability, apex angle, and droplet size on the droplet's dynamic behavior are systematically analyzed. The results show that the moving velocity and transport distance of the droplet on the cone are highly related to the droplet shape on the cone. It was found that a clamshell-shaped droplet moves faster than a barrel-shaped droplet. Besides, the clamshell-shaped droplet with a larger size, on the cone with a smaller apex angle and smaller contact angle tends to obtain a faster moving speed and a longer transportation distance. The droplet shape adopted on the cone was determined by the cone wettability and the size of the droplet relative to the local curvature of the cone. It was found that the oil droplet tends to form a barrel shape on the cone with a highly oleophilic and small apex angle, and tends to form a clamshell shape on cones with a highly oleophobic and large apex angle. In addition, the droplet might transit from a barrel shape to a clamshell shape when it moves from the cone tip to the cone base, and the trigger time of the transit is negatively correlated with the contact angle and apex angle of the cone. This work provides a microscale understanding of the dynamic behavior of an underwater oil droplet on a cone, and also offers theoretical guidance for manipulating the behavior of a droplet on a cone and for the rational design of cone surfaces for spontaneous droplet transport and droplet collection.

Modeling of Self-Driven Directional Movement of Underwater Oil Droplets on Bio-Inspired Nano-Coated 3d-Printed Conical Models

Modeling of Self-Driven Directional Movement of Underwater Oil Droplets on Bio-Inspired Nano-Coated 3d-Printed Conical Models

Biomimetic Janus photonic soft actuator with structural color self-reporting.

Soft actuators with variable signal/color play an important role in the fields of targeted locomotion, artificial phototropism, drug screening, cargo transportation, and interactive sensing. The ability to achieve rapid response, large curvature, wide bending angle, and full-color display continues to be an unresolved challenge for artificial actuating materials. Inspired by the angle-dependent structural color of broad-tailed hummingbird and the Janus wettability of the lotus leaf, a Janus photonic soft actuator (JPSA) was fabricated by integrating an underwater super-oleophilic copper micro-nano array and oil-phobic inverse opal through a Laplace channel. The JPSA exhibits unidirectional permeability to underwater oil droplets. Attractively, with the combination of a swellable super-oleophilic surface and photonic crystals, JPSAs were endowed with oil-controlled reversible bending behavior with self-reporting angle-dependent color indication. We described for the first time the directional actuating mechanism induced by underwater oil unidirectional penetration and revealed the corresponding actuating kinetics and the inner-stress distribution/transfer by using structural color. As an extension of such theory, a rapid responsive JPSA with a wide bending angle and full-color self-reporting is further fabricated. This work provides an efficient strategy for oil directional transportation and separation in aqueous media and inspires the fabrication of a soft actuator/sensor with structural color self-reporting.

Formulating Multiphase Medium Anti-wetting States in an Air-Water-Oil System: Engineering Defects for Interface Chemical Evolutions.

Studies which regulate macroscopic wetting states on determined surfaces in multiphase media are of far-reaching significance but are still in the preliminary stage. Herein, inspired by the wettability subassembly of fish scales, Namib desert beetle shell, and lotus leaf upper side, interfaces in the air-water-oil system are programmed by defect engineering to tailor the anti-wetting evolution from double to triple liquid repellency states. By controlling the visible light irradiation and plasma treatment, surface oxygen vacancies on CuxO@TiO2 nanowires (NWs) can be healed or reconstructed. The original membrane or the membrane after plasma treatment possesses abundant surface oxygen vacancies, and the homogeneous hydrophilic membrane shows only double anti-wetting states in the water-oil system. By the unsaturated visible light irradiation time, the surface oxygen vacancy partially healed, the heterogeneous hydrophilic-hydrophobic components occupied the membrane surface, and the anti-wetting state finally changed from double to triple in the air-water-oil system. After the illumination time reaches saturation, it promotes the healing of all surface oxygen vacancies, and the membrane surface only contains uniform hydrophobic components and only maintains double anti-wetting state in the air-oil system. The mechanism of the triple anti-wetting state on a heterogeneous surface is expounded by establishing a wetting model. The wetting state and the adhesion state of the CuxO@TiO2 NW membrane show regional specificity by controlling the illumination time and region. The underwater oil droplets exhibit the "non-adhesive" and "adhesive" state in a region with unsaturated irradiation time or in an unirradiated region, respectively. Underwater oil droplet manipulation can be accomplished easily based on switchable wettability and adhesion. Current studies reveal that defect engineering can be extended to anti-wetting evolution in the air-water-oil system. Constructing an anti-wetting interface by heterogeneous components provides reference for designing the novel anti-wetting interface.

Femtosecond laser-induced shape memory polymer micropillar with tunable wettability and reversible adhesion for underwater oil droplet lossless transfer

Shape memory polymers (SMPs) with tunable wettability and reversible adhesion through adjusting the surface morphology have great prospects in functional devices. However, current functional devices based on SMPs are mainly concentrated on the manipulation of diverse liquid droplets in the air; it is still a challenge to achieve the reversible tuning of underwater oil wettability and adhesion on the microstructured SMP surface. Herein, we fabricate a kind of SMP micropillar array through one-step femtosecond laser irradiation, which could achieve the reversible tuning of underwater oil wettability and adhesion between the tilted and upright states. It is worth noting that the deformed micropillar arrays show underwater oleophobicity and have a larger adhesion force to the oil droplet than that of upright micropillar arrays, which is mainly related to the variation of the contact area between the oil droplet and the micropillar array. Finally, we demonstrate the lossless oil droplet transfer underwater by using the micropillar array surface with different states. We believe the present work will bring some insights for researchers in the field of dynamically responsive tunable underwater oil droplet manipulation.

Development of durable superhydrophobic and UV protective cotton fabric via TiO2/trimethoxy(octadecyl)silane nanocomposite coating

This work was focused on the development of superhydrophobic and UV protective cotton fabric by simple approach. Cotton fabric was coated with TiO2 particles by one step hydrothermal reaction. TiO2 coated fabric was subsequent treated with Trimethoxy(octadecyl)silane (OTMS). The effect of hydrothermal reaction time on synthesis of nanoparticles was studied. The results showed TiO2 particles were uniformly distributed on the fiber surface with a high coating density. The scanning electron microscopy (SEM), EDS analysis and Raman spectroscopy techniques were used to investigate the surface structure of coated fabrics. The analysis of superhydrophobicity, UV protection and oil/water separation was performed. The water contact angle was increased to 146°, 153° and 159° for deposition duration of 4, 8 and 12 h, respectively. The robust superhydrophobic fabric exhibited good stability against physical and chemical insults including mechanical abrasion test, washing durability, and exposures to extreme aqueous conditions (e.g. acidic and alkaline). Further, the superhydrophobic cotton fabrics with special wettability showed great potentials for oil/water separation under various conditions (e.g. floating oil layer or underwater oil droplet or even oil/water mixtures). In addition, this facile approach can be widely used to construct multifunctional textiles with excellent self-cleaning, UV protection, and oil/water separation properties.

The highly efficient collection of underwater oil droplets on an anisotropic porous cone surface via an electric field

The highly efficient collection of underwater oil droplets on an anisotropic porous cone surface via an electric field was demonstrated.

Nanotextured Surfaces with Underwater Anisotropic Sliding Resistance for Oil Transfer and Coalescence

Underwater oil droplet manipulations have attracted considerable attentions due to the increasing need of miniature organic reactions. In this work, nanotextured surfaces (underwater superoleophobic) patterned with smooth regions (underwater oleophilic) was prepared via simple through-mask alkalinity oxidation. Relationship between underwater sliding resistance of oil droplets and the pattern shape/size was investigated. Results indicate that the modified Furmidge equation with correction coefficient can be well applied to predict the underwater sliding resistance; underwater anisotropic sliding, which means sliding resistance varies with sliding directions, is readily achievable by design of Y-shaped and straight line pattern. With the help of gravity, the anisotropic sliding resistance can guide oil droplets to move exactly along the line pattern; and by linearly moving surfaces with line patterns, underwater oil droplet transfer and coalescence were realized. This strategy of underwater oil droplet manipulations may have promising applications in the development of new miniature platform for organic reactions.

Understanding the coalescence and non-coalescence of underwater oil droplets

Oil collection requires effective methods to accelerate coalescence of oil in water. The coalescence behaviors of n-hexane, n-heptane, n-octane, dichloromethane and carbon tetrachloride droplets on oleophobic surfaces in water are investigated by experiments and simulations. Experimental results show that the n-hexane, n-heptane and n-octane droplets coalesce in water while the dichloromethane and carbon tetrachloride droplets do not coalesce. Theoretical calculations indicate that the critical thickness of water film between oil droplets to rupture in experiment is several nanometers, which is exactly in the size scale of molecular dynamics simulation. To investigate the mechanisms underlying the coalescence/non-coalescence, molecular dynamics simulations on the oil/water/oil interface are carried out. The simulation results indicate that the orientated oil molecules and increased number of hydrogen bonds in water are main reasons why oil/water/oil interface is stable. To undermine the stability, oleophilic surface in water is constructed, resulting in the coalescence of carbon tetrachloride droplets.

Unidirectional liquid transportation and selective permeation for oil/water separation on a gradient nanowire structured surface

Liquid transport and permeation on solid surfaces with gradient wettability had attracted tremendous research attention to solve some important problems of interface science and technology. On account of the existing problem of film pore blocking during the oily water separation process, however, it still remains a challenge to achieve rapid, continuous, high flux liquid mixture separation, e.g., oil/water separation, on a micro-structured surface. Here we demonstrate a strategy to achieve unidirectional motion and selective permeation of underwater oil droplet and water on the Cu(OH)2 nanowires structured copper foil/mesh surface with a density and length gradient. The gradient Cu(OH)2 nanowires fabricated by anodic oxidation method with anodization time gradient, shows wettability gradient from hydrophilic/oleophobic to superhydrophilic/superoleophobic and adhesion force gradient to underwater oil from high to low. As a result, the underwater oil droplet can only move unidirectionally from the superhydrophilic/superoleophobic scope to the hydrophilic/oleophobic area on the nanowires structured copper foil/mesh surface, meanwhile, water can only permeate on the superhydrophilic area, but not permeate on the hydrophobic area of the nanowires structured copper mesh. Therefore, a new way was presented to provide more contact area of oil/water mixture, owing to the unidirectional motion of underwater oil droplets and selective permeation of water, for rapid, continuous, high flux oil/water mixture separation with different interfacial tension in engineering field, which would also be promising to develop smart interface materials for microfluidic devices.

Underwater Anisotropic 3D Superoleophobic Tracks Applied for the Directional Movement of Oil Droplets and the Microdroplets Reaction

AbstractIn this paper, a simple way to fabricate underwater anisotropic superoleophobic tracks is reported for manipulating underwater oil droplets by the femtosecond laser etching and the oxygen plasma treatment. Laser ablation is able to generate micro/nanoscale hierarchical structures on the poly(dimethylsiloxane) (PDMS) surface. The textured PDMS surface is further turned from superhydrophobicity to hydrophilicity and underwater superoleophobicity by subsequent oxygen plasma irradiation. Underwater anisotropic 3D superoleophobic tracks can also be fabricated on the PDMS surface by the laser etching method. The width of the tracks depends on the laser‐treated area, while the depth of the tracks increases with increasing the laser power and the scanning number and with decreasing the laser scanning speed/space. The underwater superoleophobic 3D tracks show ultralow adhesion and anisotropic sliding property to oil droplets, thereby allow the underwater oil droplets to move just along the track. A microdroplets reaction is proposed based on the underwater superoleophobic tracks. There is no loss of the reactants during the whole reaction process. The anisotropic sliding property of underwater organic droplets will potentially have enormous applications in droplet manipulation, microfluidics system, surface lab‐chip devices, and chemical engineering.

How To Obtain Six Different Superwettabilities on a Same Microstructured Pattern: Relationship between Various Superwettabilities in Different Solid/Liquid/Gas Systems.

A range of different superwettabilities were obtained on femtosecond laser-structured Al surfaces. The formation mechanism of each superwetting state is discussed in this paper. It is revealed that the underwater oil droplet and bubble wettabilities of a solid surface have a close relationship with its water wettability. The laser-induced hierarchical microstructures showed superhydrophilicity in air but showed superoleophobicity/superaerophobicity after immersion in water. When such microstructures were further modified with a low-surface-energy monolayer, the wettability of the resultant surface would turn to superhydrophobicity with ultralow water adhesion in air and superoleophilicity/superaerophilicity in water. The understanding of the relationship among the above-mentioned six different superwettabilities is highly important in the design of various superwetting microstructures, transforming the structures from one superwetting state to another state and better using the artificial superwetting materials.

Superhydrophobicity, UV protection and oil/water separation properties of fly ash/Trimethoxy(octadecyl)silane coated cotton fabrics

The presented study proposed simple and low-cost approach for improvement in UV protection and superhydrophobic properties of cotton fabrics by coating of mechanically activated fly ash particles. The maximum UV blocking was observed for 3 wt% fly ash, where UV transmittance decreased from 14.19% of untreated fabric to 0.11% of coated fabric. After subsequent treatment of Trimethoxy(octadecyl)silane (OTMS) on fly ash coated fabrics, the water contact angle was increased to 143°, 147° and 153° for fly ash concentration of 1, 2 and 3 wt% respectively. From Cassie-Baxter theories, the unwetted fraction of air pockets were estimated to be 43%, 55% and 67% respectively for 1, 2 and 3 wt% of fly ash particles. Furthermore, the coated fabrics showed great potentials for separation of floating oil layer, underwater oil droplet or oil/water mixture. The separation efficiency of 98%, 96%, 97% and 95% was obtained for selected model oils toluene, n-hexane, chloroform and petro ether, respectively.

Computer simulations of underwater oil adhesion of self-assembled monolayers on Au (111)

ABSTRACTMolecular dynamics and steered molecular dynamics (SMD) simulations are used to investigate the underwater adhesive forces of 1, 2-dichloroethane (DCE) on ionic and non-ionic self-assembled monolayers (SAMs) grafted on Au (111). The non-ionic SAMs containing hydroxyl group include ethanolamine-terminated SAM (ETA-SAM), dopamine-terminated SAM (DOPA-SAM), hydroxyl-terminated SAM (OH-SAM) and oligo(ethylene glycol)-terminated SAM (OEG-SAM); the ionic SAMs contain mixed-charged zwitterionic SAM and zwitterionic phosphorylcholine-terminated SAM . Simulation results show that the order of underwater oleophobicity of SAMs is OEG-SAM < ETA-SAM < OH-SAM < DOPA-SAM < < , while the order of underwater DCE oil droplet adhesion forces on different SAMs is < < ETA-SAM < DOPA-SAM < OH-SAM < OEG-SAM. The ionic SAMs show stronger underwater oleophobicity and lower oil adhesion than non-ionic SAMs. Generally, a more underwater oleophobic surface is less adhesive. The underwater oil droplet desorption behaviours on ionic and non-ionic SAMs are also different. With the oil droplet being pulled away from ionic SAMs, the oil contact lines gradually shortened and disappeared finally; whereas the oil contact lines on non-ionic SAMs almost kept an initial state even the main part of oil droplet separated from the SAMs. This work systematically proved that a zwitterionic surface was less underwater oil adhesive than a non-ionic surface and provided a new insight for their oil desorption behaviours. The above results will be helpful for the design of underwater oleophobic surfaces with tuneable adhesion.