Smart materials for subsea buoyancy control
Buoyancy control is needed in small autonomous underwater devices to enable greater flexibility in measurements in the ocean. This project has examined a number of ways in which buoyancy changes might be achieved. Firstly, an extensive review of the mechanisms by which various marine organisms control their buoyancy was undertaken. There is a tremendous diversity of natural buoyancy control mechanisms, but most of these mechanisms produce only slow (and small) changes in buoyancy. Studies were carried out on the behaviour of polymer gel systems that exhibit large volume changes under the influence of solvent composition and/or temperature. The effects of salinity were investigated, from 5 parts per thousand (ppt) to 35ppt, on hydrolysed polyacrylamide gels, over the temperature range of 5°C to 40°C. It was found that the gels decreased in volume in the solutions, this effect being most pronounced in the 35ppt solution. As temperature increased, the volume changes were observed to decrease. The cyclical volumetric strain behaviour of the polyacrylamide gels, by alternate exposure to saline solutions and distilled water, resulted in significant (~200%) volume changes induced over periods of 2 days. In a second study, the density change associated with the volumetric strain of polymeric materials was investigated in poly(N-isopropylacrylamide), NIPA, gels. The temperature-sensitive NIPA gels, immersed in distilled water or seawater solutions at temperatures ranging from 5°C to 50°C, exhibited volume changes of over 800%, and density changes of 30-40%. NIPA gels exhibit a faster response time than polyacrylamide gels, and their density and volume changes have potential application in buoyancy change. Experiments were also performed on NiTi shape memory alloys (SMA), which change in length and mechanical properties with temperature. A controllable parallel-plate device was constructed, linked by four helical SMA springs, which exerted significant axial forces with the application of temperature. The device is capable of producing substantial volume changes if contained in a suitable enclosure. It is currently on loan to the Science Museum, London, as part of a new exhibition of the Wellcome Wing.