Based on interfacial convection in the presence of solvent evaporation, a novel method for the fabrication of a micro-structured surface is proposed to facilitate drag reduction. A mixture was coated on a substrate through a specially developed spray-painting system. Micron scale pits formed spontaneously in the coated surface because of interfacial convection and deformation driven by the gradient of the interfacial tension. Experimental results indicated that particles in the mixture played a crucial role in pit for-mation, and with a suitable selection of particle size and dosage, the characteristic parameters of the pitting could be controlled. The drag reduction experiments were first performed in a water tunnel, and the results showed that the micro-structured surface had a remarkable drag reduction performance over a great range of flow speeds.
Material damage of silicon wafer with different roughness and wettability was investigated by using the self-made vibration cavitation apparatus in de-ionized water. Various roughness and wettability of sili- con wafer were achieved by changing their morphology and depositing Au, diamond-like carbon films (DLC films) on them. Surface morphology was observed with a scanning electron microscope (SEM) and a surface profilometer, and wettability was characterized by the contact angle measurement. The cavitation erosion results showed that many tiny pits and cracks appeared on the wafer surface as a result of brittle fractures; the number and size of the pits and cracks increased with experiment time, which made material flake away finally; cavitation occurred more easily on the silicon wafer surface with the augment of roughness or contact angle by changing surface morphology or depositing Au, DLC thin film on it, which consequently aggravated cavitation damage.
The electronic properties of hydrogen- and oxygen-terminated diamond surfaces exposed to the air are investigated by scanning probe microscopy(SPM).The results indicate that for the hydrogen-terminated diamond surface a shallow acceptor above the valence-band-maximum(VBM) appears in the band gap.However,the oxygen-terminated diamond film exhibits a high resistivity with a wide band gap.Based on the density-functional-theory,the densities of states, corresponding to molecular adsorbate in hydrogenated and oxygenated diamond(100) surfaces,are studied.The results show that the shallow acceptor in the band gap for the hydrogen-terminated diamond film can be attributed to the interaction between the surface C-H bonding orbitals and the adsorbate molecules,while for the oxygen-terminated diamond film,the interaction between the surface C-O bonding orbitals and the adsorbate molecules can induce occupied states in the valence-band.
Adhesion of marine fouling organisms on artificial surfaces such as ship hulls causes many problems, including extra energy consumption, high maintenance costs, and increased corrosion. Therefore, marine antifouling is an important issue. In this review, physical and biochemical developments in the field of marine biofouling, which involves biofilm formation and macro-organism settlement, are discussed. The major antifouling technologies based on traditional chemical methods, biological methods, and physical methods are presented. The chemical methods include self-polishing types such as tributyltin (TBT) self-polishing co- polymer coatings, which despite its good performance has been banned since 2008 because of its serious environmental impact. Therefore, other methods have been encouraged. These include coatings with copper compounds and biocide boosters to replace the TBT coatings. Biological extracts of secreted metabolites and enzymes are anticipated to act as antifoulants. Physical methods such as modification of surface topography, hydrophobic properties, and charge potential have also been considered to prevent biofouling. In this review, most of the current antifouling technologies are discussed. It is proposed that the physical antifouling technologies will be the ultimate antifouling solution, because of their broad-spectrum effectiveness and zero toxicity.
It has been believed for about one hundred years that the cavitation directly induces the cavitation erosion. It is proposed in this research that cavitation is only the necessary condition but not the sufficient condition of the cavitation erosion. The experiment performed on the rotary disk cavitation system shows that the micro-particles in the fluid play indispensable roles in the cavitation erosion process, and the generation of the erosion pits on the steel surface is also affected by the particles' size. These cracks and deformations on the sample surface indicate that the erosion is the result of the mechanical behavior. Numerical calculations are also provided to support this mechanism.
WANG JiaDao CHEN HaoSheng QIN Li LI Yongdian CHEN DaRong
