Progress of marine biofouling and antifouling technologies

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.

Bionic Research on Fish Scales for Drag Reduction

To reduce friction drag with bionic method in a more feasible way, the surface microstructure of fish scales was analyzed attempting to reveal the biologic features responding to skin friction drag reduction. Then comparable bionic surface mimicking fish scales was fabricated through coating technology for drag reduction. The paint mixture was coated on a substrate through a self-developed spray-painting apparatus. The bionic surface with micron-scale caves formed spontaneously due to the interra- cial convection and deformation driven by interfacial tension gradient in the presence of solvent evaporation. Comparative experiments between bionic surface and smooth surface were performed in a water tunnel to evaluate the effect of bionic surface on drag reduction, and visible drag reduction efficiency was obtained. Numerical simulation results show that gas phase de- velops in solid-liquid interface of bionic surface with the effect of surface topography and partially replaces the solid-liquid shear force with gas-liquid shear force, hence reducing the skin friction drag effectively. Therefore, with remarkable drag re- duction performance and simple fabrication technology, the proposed drag reduction technique shows the promise for practical applications.

Underwater drag reduction by gas

Publications on underwater drag reduction by gas have been gathered in the present study.Experimental methods,results and conclusions from the publications have been discussed and analyzed.The stable existence of gas is a requirement for underwater drag reduction induced by slippage at the water-solid interface.A superhydrophobic surface can entrap gas in surface structures at the water-solid interface.However,many experimental results have exhibited that the entrapped gas can disappear,and the drag gradually increases until the loss of drag reduction with immersion time and underwater flow.Although some other surface structures were also experimented to hold the entrapped gas,from the analysis of thermodynamics and mechanics,it is difficult to prohibit the removal of entrapped gas in underwater surface structures.Therefore,it is essential to replenish a new gas supply for continued presence of gas at the interface for continued underwater drag reduction.Active gas supplement is an effective method for underwater drag reduction,however,that needs some specific equipment and additional energy to generate gas,which limits its practical application.Cavitation or supercavitation is a method for passive gas generation,but it is only adaptive to certain vehicles with high speed.Lately,even at low speed,the evaporation induced by liquid-gas-solid interface of a transverse microgrooved surface for continued gas supply has been discovered,which should be a promising method for practical application of underwater drag reduction by gas.

Friction characteristics of floppy disks

This note presents the principle and structure of a tribological measure for floppy disks. The precision of the force measuring system is 1 mN in loading and 3×10-6 N in friction. The resolution of the film thickness between head and floppy disk is 0.5 nm in the vertical and 1.5 nm in the horizontal direction. in order to investigate the tribological characteristics of floppy disks, six types of floppy disks have been tested and the floating properties of these disks are also studied with film measuring system. The experimental results of the surface morphology and friction coefficient of these floppy disks using the atomic force microscope/friction force mcroscope (AFM/FFM) are in accordance with the conclusion made by our own measuring system. The experimental results show that the air film thickness between head and disk is of the same order as the surface roughness of floppy disks.