Unravelling colloid filter cake motions in membrane cleaning procedures.

The filtration performance of soft colloid suspensions suffers from the agglomeration of the colloids on the membrane surface as filter cakes. Backflushing of fluid through the membrane and cross-flow flushing across the membrane are widely used methods to temporally remove the filter cake and restore the flux through the membrane. However, the phenomena occurring during the recovery of the filtration performance are not yet fully described. In this study, we filtrate poly(N-isopropylacrylamide) microgels and analyze the filter cake in terms of its composition and its dynamic mobility during removal using on-line laser scanning confocal microscopy. First, we observe uniform cake build-up that displays highly ordered and amorphous regions in the cake layer. Second, backflushing removes the cake in coherent pieces and their sizes depend on the previous cake build-up. And third, cross-flow flushing along the cake induces a pattern of longitudinal ridges on the cake surface, which depends on the cross-flow velocity and accelerates cake removal. These observations give insight into soft colloid filter cake arrangement and reveal the cake's unique behaviour exposed to shear-stress.

A new bioinspired method for pressure and flow sensing based on the underwater air-retaining surface of the backswimmer Notonecta.

In technical systems, static pressure and pressure changes are usually measured with piezoelectric materials or solid membranes. In this paper, we suggest a new biomimetic principle based on thin air layers that can be used to measure underwater pressure changes. Submerged backswimmers (Notonecta sp.) are well known for their ability to retain air layers on the surface of their forewings (hemelytra). While analyzing the hemelytra of Notonecta, we found that the air layer on the hemelytra, in combination with various types of mechanosensitive hairs (clubs and pins), most likely serve a sensory function. We suggest that this predatory aquatic insect can detect pressure changes and water movements by sensing volume changes of the air layer under water. In the present study, we used a variety of microscopy techniques to investigate the fine structure of the hemelytra. Furthermore, we provide a biomimetic proof of principle to validate our hypothesis. The suggested sensory principle has never been documented before and is not only of interest for sensory biologists but can also be used for the development of highly sensitive underwater acoustic or seismographic sensory systems.