Multi-Scale Structure and Directional Hydrophobicity of Titanium Alloy Surface Using Electrical Discharge

Titanium alloys with special macro-micro composite structures of directional hydrophobicity are difficult to prepare due to poor thermal conductivity and good corrosion resistance, inhibiting the wide engineering applications for aerospace, marine engineering, and biomedicine. To prepare macro-micro composite structures on the surface of titanium alloys and achieve directional hydrophobicity, the sub-millimeter structures with an edge width of 150 μm, a groove width of 250 μm, and a depth of 250 μm were fabricated on the titanium alloy by wire electrical discharge machining (WEDM) technology, and high voltage-induced weak electric arc machining (HV-μEAM) was used to fabricate micro-scale feature size micro-structures on the processed macro-structure edges. The influence of process parameters on the morphology of microstructures was studied experimentally. The smooth surface of the titanium alloy is isotropically hydrophilic, and its contact angle is 68°. After processing the macrostructure on the titanium alloy surface, it shows directional hydrophobicity after being modified by low surface energy materials. The macro-micro composite structure formed by HV-μEAM realizes a directional hydrophobic surface with contact angles (CA) of 140° (parallel direction) and 130° (perpendicular direction), respectively. This surface has been modified with low surface energy to achieve contact angles of 154° and 143°. The results of the abrasion resistance test show that under the load of 100 g, it retains directional hydrophobicity at a friction distance of 700 mm with 600# sandpaper. The existence of the sub-millimeter macrostructure is the reason for the directionality of surface hydrophobicity. The microstructure can realize the transformation of the titanium alloy surface from hydrophilic to hydrophobic. Under the combined effects of the macro and micro composite structure, the surface of the titanium alloy shows obvious directional hydrophobicity.

Temporally Arrested Breath Figure

Since its original conception as a tool for manufacturing porous materials, the breath figure method (BF) and its variations have been frequently used for the fabrication of numerous micro- and nanopatterned functional surfaces. In classical BF, reliable design of the final pattern has been hindered by the dual role of solvent evaporation to initiate/control the dropwise condensation and induce polymerization, alongside the complex effects of local humidity and temperature influence. Herein, we provide a deterministic method for reliable control of BF pore diameters over a wide range of length scales and environmental conditions. To this end, we employ an adapted methodology that decouples cooling from polymerization by using a combination of initiative cooling and quasi-instantaneous UV curing to deliberately arrest the desired BF patterns in time. Through in situ real-time optical microscopy analysis of the condensation kinetics, we demonstrate that an analytically predictable self-similar regime is the predominant arrangement from early to late times O(10-100 s), when high-density condensation nucleation is initially achieved on the polymer films. In this regime, the temporal growth of condensation droplets follows a unified power law of D ∝ t. Identification and quantitative characterization of the scale-invariant self-similar BF regime allow fabrication of programmed pore size, ranging from hundreds of nanometers to tens of micrometers, at high surface coverage of around 40%. Finally, we show that temporal arresting of BF patterns can be further extended for selective surface patterning and/or pore size modulation by spatially masking the UV curing illumination source. Our findings bridge the gap between fundamental knowledge of dropwise condensation and applied breath figure patterning techniques, thus enabling mechanistic design and fabrication of porous materials and interfaces.

How Droplets Move on Surfaces with Directional Chemical Heterogeneities

The nature of adhesion of droplets to surfaces is a long pending scientific question. With the evolution of complex surfaces, quantification and prediction of these adhesion forces become intricate. Nevertheless, understanding these forces is highly relevant for explaining liquid transport in nature and establishing design guidelines for manmade interfaces. Here, it is shown that adhesion of droplets is highly sensitive to the direction of chemical heterogeneities, both in the static and dynamic regimes. This dependency is quantified by bending beam and droplet roll-off experiments. The shape of the fluid contact line on the microscale elucidates the origin of the direction-dependent adhesion. Namely, the droplet receding part pins to a higher number of patches when moving toward to the apex in comparison to the opposite direction. These findings improve the understanding of droplet adhesion to surfaces with chemical heterogeneities and directional transport phenomena.

Multi-Scale Simulation of Injection Molding Process with Micro-Features Replication: Relevance of Rheological Behaviour and Crystallization

The possibility of tailoring key surface properties through the injection molding process makes it intriguing from the perspective of sustainability enhancement. The surface properties depend on the replication accuracy of micro and nanostructures on moldings; such an accuracy is enhanced with cavity temperature. The simulation of the injection molding process is very challenging in the presence of micro and nanostructures on the cavity surface; this does not allow for the neglect of phenomena generally considered not to influence the overall process. In this paper, a multiscale approach was proposed: in the first step, the simulation of the overall process was conducted without considering the presence of the microstructure; in the second step the outputs of the first step were used as an input to simulate the replication of the microfeature. To this purpose, a lubrication approximation was adopted, and the contribution of the trapped air, which slows down the polymer advancement, was accounted for. A modification of the viscosity equation was also proposed to describe the rheological behavior of isotactic polypropylene at very low temperatures. Concerning the microcavity filling simulation, the modification of the viscosity description at low temperatures consistently describes the process, in terms of polymer solidification. Concerning the replication accuracy, it increases with the cavity surface temperature, consistently with the experimental observations.

Elastocapillary effect in self-repair of proboscises of butterflies and moths

HYPOTHESIS

Self-repair in living organisms, without tissue regeneration or regrowth, is rare. Recent discovery that butterflies can self-repair the proboscis after the two halves (galeae) have been separated raised a question about the physical mechanism allowing them to reunite the parts. We discovered that butterflies pump saliva during repair of their proboscises. We then hypothesized that saliva spreading along the food canal of the proboscis would create capillary forces capable of bringing the galeae together.

EXPERIMENT

To test the hypothesis, we distinguished capillary forces from muscular action of the galeae by sedating butterflies and video tracking retraction of the saliva menisci during galeal separation. To theoretically show capillary adhesion, the elastic moduli of the galeae were measured, and the galeal profiles were extracted from videos as a function of time. The values were then fitted with a mathematical model based on an augmented Euler-Bernoulli beam theory whereby each galea was treated as a beam bent by capillary forces due to saliva. We also evaluated friction forces that prevented disjoining of the galea at the tip of their separation.

FINDINGS

The results showed that butterflies use saliva to repair their proboscises via capillary adhesion, and theoretically supported the role of saliva in providing the necessary capillary forces to bring the galeae together. Tangential shear forces acting parallel to the galea at the tip of their separation are caused primarily by friction between the cuticular linking structures.

Photo-switchable smart superhydrophobic surface with controllable superwettability

An azobenzene-based smart superhydrophobic surface undergoes reversible transformations among multiple bioinspired superwetting states through photo-manipulation, demonstrating promising potential on a rewritable platform for droplet transportation.

High-Throughput Multiobjective Optimization of Patterned Multifunctional Surfaces

The realization and optimization of multifunctional materials is difficult, especially when the functionalities are directly incompatible. For example, it is challenging to make surfaces both enzymatically active and water repellent, as these two properties are directly competitive because of the hydrophilic nature of the enzyme-laden surfaces. Patterning discrete domains of distinct functionalities can represent a path to multifunctionality, but the innumerable possible domain permutations present a major barrier to optimizing performance. Here, we develop a high-throughput approach for exploring patterned multifunctional surfaces that is inspired by the microtiter plate architecture. As a model system, patterned surfaces are realized with horseradish peroxidase-decorated domains amidst a background of hydrophobic fluorinated self-assembled monolayers. In experiments exploring effects of pattern geometry, the measured enzyme activity is dependent only on the surface coverage. In contrast, roll-off behavior strongly depends on the parameters of the pattern geometry. Importantly, this finding enables the precise tailoring of distinct wetting behavior of the surfaces in a manner that is independent of their enzymatic activity. The high-throughput nature of the platform facilitates multiobjective optimization of surface functionalities in a general and flexible manner.

Guided droplet transport on synthetic slippery surfaces inspired by a pitcher plant

We show how anisotropic, grooved features facilitate the trapping and directed transport of droplets on lubricated, liquid-shedding surfaces. Capillary action pins droplets to topographic surface features, enabling transport along the feature while inhibiting motion across (or detachment from) the feature. We demonstrate the robustness of this capillary-based mechanism for directed droplet transport on slippery surfaces by combining experiments on synthetic, lubricant-infused surfaces with observations on the natural trapping surface of a carnivorous pitcher plant. Controlling liquid navigation on synthetic surfaces promises to unlock significant potential in droplet-based technologies. Our observations also offer novel insight into the evolution of the Nepenthes pitcher plant, indicating that the 'pitfall' trapping mechanism is enhanced by the lubricant-infused, macroscopic grooves on the slippery peristome surface, which guide prey into the trap in a way that is more tightly controlled than previously considered.

Fabrication of pixelated liquid crystal nanostructures employing the contact line instabilities of droplets

A liquid crystal (LC) droplet resting on a poly-dimethylsiloxane substrate could rapidly spread upon solvent vapour annealing to form a non-uniform film. While the solvophobic surfaces restricted the spreading of the droplet to form a thicker film upon solvent annealing, the solvophilic substrates allowed the formation of a thinner film under similar conditions. Withdrawal of the solvent exposure caused rapid evaporation of the solvent molecules from the film, especially near the retracting contact-line to form microscale LC-droplets, which shrunk into nanoscopic ones after evaporation of the excess solvent. The thinner films on solvophilic surfaces allowed the formation of droplets with smaller size and periodicity as small as ∼100 nm and ∼200 nm, respectively. Furthermore, the use of a patterned substrate could impose a large-area ordering on the nanodroplets. A theoretical model for an evaporating film of LC-solution revealed that the spacing of nanodroplets could be decided by the interplay of stabilizing and destabilizing components of capillary force while van der Waals interaction played a supportive role when the film was ultrathin near the contact line. The micro/nanodroplets thus formed showed an anomalous oscillatory rotational motion originating from the difference in the Laplace pressure near contact lines under the influence of an external electric field. The application of the Lorenz force to these droplets showed translation and rotational motions followed by ejection of satellite droplets.