Biomimetics Inspired Surfaces for Superhydrophobicity, Self-cleaning, Low Adhesion, and Drag Reduction

The liquid phase response of quartz crystal microbalances (QCMs) with a thin coating (∼9 µm) of epoxy resin with and without a carbon nanoparticles top layer is reported. The nanoparticles convert the epoxy surface to a superhydrophobic one with a high static contact angle (∼151°–155°) and low contact angle hysteresis (∼1°–3.7°) where droplets of water are in the suspended Cassie–Baxter state. The frequency decrease of the fully immersed QCM with the superhydrophobic surface is less than with only epoxy layer, thus indicating a decoupling of the QCM response. A wettability transition to a liquid penetrating into the surface roughness state (for droplets a high contact angle hysteresis Wenzel state) was triggered using a molarity-of-ethanol droplet test (MED) and electrowetting; the MED approach caused some surface damage. The electrowetting-induced transition caused a frequency decrease of 739 Hz at a critical voltage of ∼100 V compared to the QCM in air. This critical voltage correlates to a contact angle decrease of 26° and a high contact angle hysteresis state in droplet experiments. These experiments provide a proof-of-concept that QCMs can be used to sense wetting state transitions and not only mass attachments or changes in viscosity–density products of liquids.

In this work, we have studied the wettability of zinc oxide (ZnO) nanorods grown on fluorine-doped tin oxide (FTO) by highlighting the effect of polar and non-polar ZnO facets on contact angle (CA) results. The variation in the wettability behaviors of the synthesized surfaces is mainly related to physical and chemical surface texturing which influenced the liquid drop penetration. Indeed, three main penetration states can be deduced: total, partial, and null-penetration. Where, low CA (100.9°) with high contact angle hysteresis (CAH) (13°) is observed for total penetration of the liquid drop. While, high CA (139.6°) with low CAH (7°) is observed for null-penetration. Moreover, we have found that the chemical texturing of ZnO, especially the hydrophilicity of ZnO tips, responsible for liquid drop sticking, prevents the liquid slipping over the surface. In order to promote the liquid rolling on the ZnO surface, we reported the physical modification of the ZnO structures. Therefore, the rolling of the liquid drop on the inclined surface of ZnO is achieved by using a new structure based on double scale roughness. This surface exhibits superhydrophobic behavior with a CA of 153° and CAH of 3°.

We have taken advantage of the native surface roughness and the iron content of AISI 316 stainless steel to directly grow multi-walled carbon nanotube (MWCNT) random networks by chemical vapor deposition (CVD) at low-temperature () without the addition of any external catalysts or time-consuming pre-treatments. In this way, super-hydrophobic MWCNT films on stainless steel sheets were obtained, exhibiting high contact angle values () and high adhesion force (high contact angle hysteresis). Furthermore, the investigation of MWCNT films with scanning electron microscopy (SEM) reveals a two-fold hierarchical morphology of the MWCNT random networks made of hydrophilic carbonaceous nanostructures on the tip of hydrophobic MWCNTs. Owing to the Salvinia effect, the hydrophobic and hydrophilic composite surface of the MWCNT films supplies a stationary super-hydrophobic coating for conductive stainless steel. This biomimetical inspired surface not only may prevent corrosion and fouling, but also could provide low friction and drag reduction.

We present an efficient and simple approach for preparing superhydrophobic-superoleophilic polystyrene (PS) fibers via electrospinning. Bead-on-string fibers from a 5% PS solution and micro-sized fibers from a 20% PS solution were combined to achieve a surface having very high contact angle (about 160°) and low contact angle hysteresis. The presence of bead-on-string fibers increases the superhydrophobicity of the sorbent. The micro-sized PS fibers improve the mechanical properties of the electrospun mat through their elastic and flexible behavior. An evaluation of wettability at a low temperature (-10 oC) showed a delayed freezing time for water droplets on the superhydrophobic surface. Water droplets on a polished aluminum surface froze more quickly (about 6 seconds) than droplets on the fabricated superhydrophobic surface (about 500 seconds). Finally, the oil adsorption capacity of the developed superhydrophobic PS fibers, which have a porous surface structure, showed values of 69.1, 69.3 and 61.2 g/g for canola oil, olive oil and motor oil, respectively.

Nano-long chains were grafted over the replicated micro-grooves of shark skin in a novel attempt to replicate bio-synthetic drag reduction structure with high precision through synthetic bio-replication. Pre-treated shark skin was used as casting template to prepare a flexible female die of silicone rubber by soft die formation. A waterborne epoxy resin was then used to graft long-chains of drag reduction agent and prepare a synthetic drag reduction shark skin with nano-long chain drag reduction interface and lifelike micro-grooves. Replication precision analysis shows that this technology could replicate the complicated three-dimensional morphology of a biological drag reduction surface with high precision. Drag reduction experiments show that the material had an excellent synthetic drag reduction effect, with a maximal drag reduction rate of up to 24.6% over the velocities tested.

Mechanical models of drillstrings with drag reduction oscillators and optimal design methods of vibration parameters in horizontal drilling

The organic vapor sensitivity of a quartz crystal microbalance (QCM) with an epoxy resin–carbon soot coating designed to be tolerant to humidity is reported. This is achieved using nonengulfed, but attached, hydrophobic carbon soot nanoparticles, coated in an irregular surface topography composed of islands and cavities. The root mean square roughness ( $R_{\mathrm {{rms}}}$ ) of 130 nm, together with the hydrophobic soot, convert the epoxy surface to a superhydrophobic (SH) one with high static contact angle ( $\sim 151^{\circ }$ ) and low contact angle hysteresis ( $\sim 1.1^{\circ }$ ). The frequency shift of the SH QCM at 100% relative humidity is $\sim 7$ times lower compared with an uncoated device, and thus indicating low water vapor adsorption due to superhydrophobicity. In addition, the SH QCM shows between three and six times higher gas sensitivity and lower detection limit compared with the conventional polymer coated QCMs. These results correlate well with the $R_{\mathrm {{rms}}}$ and surface topography of the coating, which ensure enhanced sensing area. Furthermore, the sensor demonstrates reproducibility, reversibility, and fast response-recovery time ( $\sim 10$ s) to ethanol, methanol, and isopropanol vapor. These experiments reveal that superhydrophobicity increases the organic vapor sorption at the expense of water vapor sorption, and thus allowing operation of the QCM gas sensors in an uncontrolled humidity environment.

Polymers with high molecular weights can significantly reduce the frictional drag in turbulent flow. However, under the strong hydrodynamic force of turbulent flow, chains of the polymer's macromolecules may undergo significant mechanical degradation, leading to a loss of drag reduction effectiveness. This paper proposes using oil-soluble surfactants for turbulent drag reduction in oil transport. By disk rheometer experiments and molecular dynamics simulations, four oil-soluble surfactants-heptadecenyl hydroxyethyl imidazoline (HEHI), sorbitan monolaurate (Span20), didecyl phosphate (AP10), and ditridecyl hydrogen phosphate (DHP) were selected to elucidate the coupling mechanism between the dynamic micellar network structures and rheological properties. Our study establishes quantitative links between surfactant microstructure and macroscopic drag reduction. Molecular dynamics simulations show that the optimal mean square displacement for HEHI surfactant is 83 Å. Exceeding the critical micelle concentration of 250–500 ppm for HEHI negatively impacts its drag reduction efficiency. Disk rheometer experiments measured a maximum drag reduction (DR) of 17.8% at 250 ppm for HEHI, while DHP achieved 10.9% DR at 500 ppm. HEHI and Span20 buffer shear stress via hydrophobic chains. DHP and AP10 use charge repulsion to limit aggregation and sustain stable performance under high shear. All four surfactants exhibit strong shear resistance. This work demonstrates the critical role of surfactant type and concentration in oil drag reduction, revealing excellent shear resistance to guide industrial transport optimization.

Self-cleaning mechanisms according to the wettability of the surface and deposition material

Investigation on drag characteristics of heavy oil flowing through horizontal pipe under the action of aqueous foam

The ability to create superoleophobic surfaces repellent toward low-surface-tension liquids is important for various applications, and has been recently demonstrated using re-entrant or doubly re-entrant microtopography. Liquid droplets on such surfaces feature composite liquid-solid-air interfaces, whereas composite liquid-lubricant-air interfaces would have potential for additional repellency. Here, the development of a novel slippery superoleophobic surface with low adhesion is demonstrated via combining doubly re-entrant microtopography with slippery lubricant-infused porous surfaces. This is realized by using 3D direct laser writing to fabricate doubly re-entrant micropillars with dedicated nanostructures on top of each pillar. The top nanostructures stabilize the impregnated slippery lubricant, while the re-entrant geometry of the micropillars prevents lubricant from spreading. The slippery layer reduces the adhesion of liquid to the pillars, as proved using scanning droplet adhesion microscopy (SDAM), while the doubly re-entrant micropillars make the surface superoleophobic. This novel interface combining two extremes, superoleophobicity and slippery lubricant-infused surface, is of importance for designing superoleophobic and superhydrophobic surfaces with advanced liquid repellent, anti-icing, or anti-fouling properties.

<p>Biofilm associated infections are the fourth leading cause of death worldwide, within the U.S. about 2 million annual cases lead to more than $5 billion USD in added medical costs per annum. Therefore, it is important to control biofilm growth and reduce the instances of infections.  Physical strategies, in particular the use of rationally designed surface topographies or surface energies, have present us with an interesting approach to prevent bacterial adherence and biofilm growth without the requirement for antimicrobials.</p> <p>A variety of natural surfaces exhibit antibacterial properties. Examples of such surfaces include rose petals with hierarchical structures and Nepenthes pitcher plants with slippery liquid-infused porous surfaces.  </p> <p>In this study, we fabricated different  biomimetic surfaces (rose-petal surfaces and slippery liquid-infused porous surfaces).   We have demonstrated that rose-petal surface can delay early stage P. aeruginosa and S. epidermidis biofilms formation (2 days) by about 70% and control  biofilm  formation according to surface structures.  The mechanisms of hierarchical structures  of rose-petal influence biofilm formation are two folds: 1) Papillae microstructure block  the bacterial clusters in between the valleys, limiting the potential for cell-cell communication via fibrous networks, thereby resulting in impaired biofilm growth. 2) The secondary structure (nano-folds) on microstructures can align bacterial cells within the constrained grooves, thereby delaying cell clusters formation during short term growth of biofilm.</p> <p>While, the slippery liquid-infused porous surface(s) can prevent over 90% P. aeruginosa and S. epidermidis biofilms formation for a duration of 6 days.  These are mainly attributed to their high contact angle and extreme low contact angle hysteresis.</p>

The effective reduction of seawater drag is pivotal in enhancing the speed and minimizing the energy consumption of submarines, which has significant implications in the fields of energy and defense. Surface bionics has emerged as one of the leading techniques for drag reduction. Current research primarily focuses on replicating the groove-like structures observed on shark skins and the flexible properties of dolphin skins. However, the application of cone microstructures on submarine surfaces remains relatively underexplored. In this study, a novel arrangement of bionic drag-reducing microstructures is employed to modify the turbulence structure surrounding the submarine by incorporating bionic cone microstructures at both the front and rear ends of the submarine. Numerical simulations were performed using the SST k-ω turbulence model to evaluate the impact of these frontal microstructures on drag reduction under varying Reynolds numbers, spacings, and positions, as well as the tail microstructures’ effect at different Reynolds numbers, heights, and circumferential separation angles. The findings reveal that positioning microstructures at the submarine’s head increases the drag reduction rate proportionally with the distance from the apex, displaying an inverse relationship between spacing and drag reduction rate. Conversely, an increase in cone separation angle at the tail leads to a decrease in the overall drag reduction rate. At the same time, an inverse proportionality is observed between cone height and drag reduction rate. This suggests that cone microstructures play a dual role: mitigating friction drag greatly and augmenting pressure drag, thereby achieving overall drag reduction. Moreover, these cone microstructures disrupt eddy currents within the boundary layer surrounding the submarine, restraining the propagation of turbulent momentum transfer in both the head and tail regions. This research not only pioneers a novel drag reduction strategy for underwater vehicles but also sparks new avenues for their optimized surface design.

Shark skin has functionalities such as self-cleaning and antifouling; it also exhibits excellent drag reduction owing to a hierarchical structure of microgrooves and nanometer-long chain mucus drag reduction interfaces around the shark body. In this study, the wettability of a shark skin surface and its replicas are evaluated. First, a shark skin template is taken from a real shark. Then, shark skin replicas are produced directly from a shark skin template, using a micromolding technique. The quantitative replication precision of the shark skin replicas is evaluated by comparing the geometry of the shark skin template to the replica using 2D surface profiles. Contact angles at the solid-air-water interfaces are evaluated for the shark skin template and its replicas under two conditions: with and without hydrophobic coating. The results show that the microriblets on shark skin improve the hydrophobic feature and play a critical role in self-cleaning.

We conduct experimental investigations of macroscopic capillary forces between two flat rigid substrates characterized by their advancing and receding contact angles with water. Our results exhibit excellent agreement with theoretical predictions obtained by the numerical solution of the capillary equation. On the basis of this comparison, we use the measurements of the capillary force to investigate the phenomenon of contact angle hysteresis. We present examples of force measurements for surfaces that display low, moderate, and high contact angle hysteresis and compare results for a larger variety of substrates. Finally, we show that for the case of water, the role of viscosity is insignificant within the range of force and velocity measured in the present work.