Development and application of microsieves

Microsieves are a fairly recent (1994) innovation in membrane-filtration technology, especially at the start of the Ph.D. work (1996) that led to this thesis. Further development and application of such a recent innovation demand a high degree of pioneering. The field of subjects and possibilities was vast and largely unexplored. As a result the contents of this thesis diverge strongly and cover various areas of science, such as (micro)mechanics, hydrodynamics, optics, chemistry and biology. A key issue in microsieve technology is the formation of pores in the membrane. Up to now these pores were made using conventional photolithographic methods, which are limited by diffraction of the used UV-light. In this thesis it is shown that the resolution limits can be decreased with an alternative exposure method called laser-interference lithography. Using this method pores with a 65 nm diameter were obtained, which is more than an order of magnitude smaller than was at that moment obtained with contact-mask lithography. The method may be considered low-cost and is applicable for large surfaces. A critical step in microsieve manufacturing is the release of the perforated membrane from the silicon support. Hydrogen gas that is created during KOH etching of the silicon builds up a pressure that might damage the membrane. Especially for submicron-perforated membranes rupture is likely to occur. Two different approaches were investigated to avoid the pressure problem. The first approach is based on plasma etching instead of KOH etching. Since no liquids are involved, the gaseous reaction products do not have to exceed a bubblepoint pressure to escape through the pores. Using an SF6/O2 plasma and cryogenic substrate cooling, submicron perforated membranes were succesfully released. A second approach is the formation of gas-escape channels to the back side of the wafer. This was achieved by using wafers with a <110> orientation, which allows for the possibility to etch channels with vertical sidewalls. With this second method also membranes with submicron pores were successfully released. The advantage over plasma etching is the possibility to process large batches. Furthermore, thanks to the vertical sidewalls thick (and thus strong) wafers can be used, while small (and thus strong) membrane fields are obtained. Perforations in the sieves are usually circularly shaped and placed in a square array under an angle of 45° with the edges of the membrane fields. Other shapes or distributions might be stronger, but the correlation between these variables and membrane strength was not known. This correlation was investigated and it was found that membranes with square arrays of pores placed under an angle of 90° with the edges are stronger, because they are less flexible and will therefore show a decreased bending stress on the edges. A resulting rule of thumb for the design of perforation patterns is that the bars in-between the perforations should be as stiff as possible and hence be placed perpendicular to the longest membrane edges. For membranes with slit-shaped perforations it was found that a 4-5 fold decrease in flow resistance can be obtained in comparison with circular perforations, while the membranes are of comparable strength. Up to now microsieves are fabricated using high-quality ceramic materials and technology, making them especially appropriate for applications where durable filters are required. For applications where disposable filters are preferred, microsieves made with lowcost materials (like polymers) and easy fabrication methods would be highly desirable. We investigated two methods that may be used to fabricate polymeric microsieves. The first method is based on photolithographical techniques applied to a photosensitive polyimide, whereas the second method is based on an imprinting process. It was demonstrated that both methods are suitable for the fabrication of microsieves. In potention the imprint method is applicable for large-scale production of polymer microsieves with submicron pore size at low costs. The flow resistance of microsieves is up to several orders of magnitude smaller than that of other microfiltration membranes. Obstruction of the pores by retained particles leads therefore more than for other membranesto a strong increase in flow resistance. In order to keep the membrane surface void of particles, a force-balance analysis was made for spherical particles obstructing circular pores. The model was verified with yeast cells and polystyrene spheres. It was found that for the conditions decribed in this thesis it gives a fairly accurate description of the crossflow required to keep the pores free. Furthermore, a rule of thumb was obtained for the design of crossflow modules for cake-layer free filtration. There are numerous application areas where microsieves may become (or are already) successful. Examples are blood-cell separation, clarification of beverages, particle-analysis systems and support structures for gas separation. In this work we chose to investigate the clarification of lager beer. Insufficient fluxes, poor permeate quality and severe membrane fouling have up to now prevented large-scale replacement of kieselguhr filtration with membrane filtration. Earlier experiments have shown that microsieves may help to overcome these problems. In this work the experiments were continued in a more fundamental way. A crossflow-microfiltration rig was built in order to study fouling of microsieves through in-line microscope observations. The fouling process appeared to start with the formation of loosely attached flocks, gradually followed by pore clogging underneath these flocks. Most of the flocks could be removed during filtration by a strong temporary increase of the crossflow. Using this in-line cleaning method, filtration times of over 10 hours were achieved with average fluxes of more than two orders of magnitude higher than is commonly obtained with membrane filtration. Permeate turbidities were comparable to those obtained with kieselguhr filtration. The results are very promising for replacement of kieselguhr with microsieves in the near future.