Surface adhesion measurements in aquatic biofilms using magnetic particle induction: MagPI

Natural sediment stability is a product of interacting physical and biological factors, and whereas stability can be measured, few techniques allow sensitive assessment of the sediment surface as conditions change. For example, stability gradually increases as a biofilm develops or as salinity rises, or it might be influenced by toxic compounds. This article introduces a new technique (magnetic particle induction: MagPI) based on the magnetic attraction of specially produced fluorescent ferrous particles. The test particles are added to a surface and subjected to an incrementally increasing magnetic field produced by permanent magnets or electromagnets. There is a strong correlation between magnetic flux density (mTesla) and distance from the surface (r(2) = 0.99) for permanent magnets and between magnetic flux density and the current supplied to an electromagnet (r(2) > 0.95). The magnetic force at which the particles are recaptured is determined as a measure of surface adhesion. MagPI therefore determines the stickiness of the surface, whether a biofilm, sediment, or other material. The average magnetic flux density required to remove test particles from diatom biofilms (15.5 mTesla) was significantly greater than from cyanobacterial biofilms (10 mTesla). Controls of fine glass beads showed little adhesion (2.2 mTesla). Surface adhesion is an important bed property reflecting the sediment system's potential to capture and retain new particles and accumulate material. MagPI offers a straightforward and economic way to determine the surface adhesion of a variety of surfaces rapidly and with precision. The technique may have applications in physical, environmental, and biomedical research.