Liquid-state motion sensing

This paper demonstrates a liquid droplet-based motion sensing system which has the advantages of simple fabrication, low power consumption and digital signal processing. The sensor consists of a dielectric substrate patterned with an array of microelectrodes, and a saline droplet as the proof mass. Once an external linear acceleration is applied, the inertial force moves the droplet on the micropatterned substrate. The acceleration is determined from the movement profile detected by the microelectrodes. In order to enhance the threshold and the sensitivity of motion sensing, two surface treatment approaches are utilized to create superhydrophobic surfaces. The result shows that the minimal sliding angle that can move a 20 μl droplet on the superhydrophobic surface is lower than 1°, corresponding to a threshold of lower than 0.017 g. A lumped-parameter model is developed to estimate the dynamic behavior of the proposed system. The result shows that the frequency response of the droplet-based sensor is more significant at low frequencies than at high frequencies, which is distinct from solid-state accelerometers. Measurement under a constant acceleration shows that the predicted value derived from the measurement has a good match with the actual applied acceleration, validating the proposed system as a viable alternative for motion sensing.     

Development of a superhydrophobic polyurethane-based coating from a two-step plasma-fluoroalkyl silane treatment

A method of achieving a superhydrophobic surface based upon a highly filled polyurethane (PU) paint coating has been demonstrated through the use of a combined oxygen/argon plasma pretreatment and a fluoroalkyl silane (FAS) final treatment.The combined plasma-FAS treated PU surface has been investigated and characterised using: field emission gun secondary electron microscope (FEG-SEM); X-ray photoelectron spectroscopy (XPS); energy-dispersive X-ray spectroscopy (EDX); water contact angle analysis (WCA); atomic force microscopy (AFM), and; Fourier transform infrared spectroscopy (FTIR).It was found that the oxygen/argon plasma treatment increased both the surface roughness (R_a) and surface free energy (SFE) of the PU paint coating from approximately 60–320 nm, and, from ~52 to ~80 mN/m respectively. It was also found that the plasma process created a multiscale roughened texture through the process of differential ablation between the PU polymer and the barium sulphate solid content, which is present in the paint as an extender, and other additives. In addition, the process also imparted favourable polar groups into the PU surface from the ionised and radical oxygen species in the plasma.When the FAS coating was subsequently applied to the PU without prior plasma treatment, there was a significant increases in water contact angles. This parameter increased from approximately 60° on untreated PU to around 130° with FAS applied. In this case, the SFE decreased to ~7.5 mN/m and showed 42.0 at% fluorine present as indicated by XPS.However, subsequently applying the FAS polymer after plasma pretreatment takes advantage of the known synergistic relationship that exists between surface roughness and low surface free energy coatings. The two processes combined to create superhydrophobicity with a surface that exhibited water contact angles up to 153.1°. With this optimised process, the apparent SFE was 0.84 mN/m with a more highly fluorinated surface present. In this case 47.2 at% surface fluorine was observed by XPS.In addition to changes in SFE, plasma treatment was also observed to alter levels of surface gloss and colour. After exposure to 600 s of plasma gloss levels are shown to reduce from values of from ~50 to ~21 (GU), with small but significant corresponding increases in the lightness and yellowness of the surface.     

Superhydrophobic concrete surfaces with integrated microtexture

Ultra-High Performance Concrete (UHPC) is characterized by a densely packed mix-design, which can offer attractive surface properties for architectural building facades. A technical challenge for aesthetic applications is the protection against fouling. This work demonstrates that water-repellent concrete can be obtained just after demoulding by replicating the features of micro-pillared moulds made of polydimethylsiloxane (PDMS). Moreover, the negative replica of the microtextured UHPC surface can be used as a master to template for other UHPC samples, constituting a cost-effective route to fabricate large-scale microtextured concrete pieces. The chemical functionalization of UHPC with a low surface energy material is obtained by transferring residues from the PDMS mould or by spraying siloxane-based compounds to form a homogenous surface film. The latter preparation method showed superhydrophobic properties with static contact angles reaching up to 164° and contact angle hysteresis reaching as low as 2.5°. This process enables the manufacture of water-repelling, self-cleaning concrete. Raindrops slide off the concrete surface, carrying debris away.     

Durable,self-cleaning and superhydrophobic bamboo timber surfaces based on TiO2 films combined with fluoroalkylsilane

Decorative materials, including bamboo timber, have been proposed to exploit their superhydrophobic and self-cleaning properties, but a comprehensive appraisal of their environmental adaptability is still deficient. In this paper, a robust and durable superhydrophobic surface was formed on bamboo timber substrate through a process combining chemical solution deposition and chemical modification. The superhydrophobic surface resulted from micro-nanoscale binary-structured TiO₂ films and the assembly of low-surface-energy fluorinated components, which exhibited a water contact angle of 163±1° and a sliding angle of 3±1°. The surface maintained superhydrophobicity after mechanical abrasion against 1500 mesh SiC sandpaper for 800 mm at the applied pressure of 1.2 kPa, indicating good mechanical stability. Moreover, the superhydrophobic surface exhibited good chemical stability against both acidic and basic aqueous solutions (e.g., simulated acid rain). After exposure to atmosphere for more than 180 days, the obtained surface still maintained a contact angle of 155±2° and a sliding angle of 6±2°, revealing good long-term stability. In addition, the as-prepared superhydrophobic surface exhibited almost complete wet self-cleaning of dirt particles with water droplets. It is believed that the method presented in this study can provide a straightforward and effective route to fabricate a large-area, mechanically robust, anticorrosive and self-cleaning superhydrophobic surface on woody materials for a great number of potential applications.     

Elaboration of Al2O3/PTFE icephobic coatings for protecting aluminum surfaces

In order to protect aluminum ground wires and phase conductors of overhead power lines against ice adhesion and excessive accretion, for ensuring safe and reliable power transmission during winter periods, a new coating with icephobic characteristics and satisfactory mechanical properties was developed. The method consisted in depositing an extremely adherent poly(tetrafluoroethylene) or PTFE coating on an Al₂O₃ underlayer produced by anodisation in either a phosphoric or an oxalic acid electrolyte. PTFE impregnation was carried out at low temperature (320 °C) and coating adhesion was assessed using tape and bend tests. These treatments resulted in highly hydrophobic surfaces with water contact angles lying between 130° and 140°. Ice shear stress was reduced by almost 2.5 times, and the PTFE coatings remained active after several ice shedding events. Morphologies and chemical compositions were studied using scanning electron microscopy, energy dispersive X-Ray analysis, as well as Fourier Transform Infra Red and X-Ray photoelectron spectroscopy.