S. Kuiper

Development and applications of very high flux microfiltration membranes

Inorganic microfiltration membranes with a pore size down to 0.1 ?m have been made using laser interference lithography and silicon micro machining technology. The membranes have an extremely small flow resistance due to a thickness smaller than the pore size, a high porosity and a very narrow pore size distribution. They are relatively insensible to fouling, because they have a smooth surface, short pore channels and because they can be operated in cross flow configuration at very low transmembrane pressures. Experiments with yeast cell filtration of beer show a minimal fouling tendency and a flux that is about 40 times higher than in conventional diatomaceous earth filtration. The uniform pore distribution makes the membranes suitable for many other applications like critical cell to cell separation, particle analysis systems, absolute filtrations and model experiments.

Nanosieves with microsystem technology for microfiltration applications

A nanosieve with a very uniform pore size of 260 nm and a pore-to-pore spacing of 510 nm has been fabricated using multiple exposure interference lithography and (silicon) micromachining technology. The nanosieve filter consists of a thick silicon nitride membrane perforated with submicron diameter pores and a macroperforated inorganic silicon support. The calculated clean water flux is at least one to two orders higher than that of conventional inorganic membranes.

Microsieves made with laser interference lithography for micro-filtration applications

A microsieve with a very uniform pore size of 260 nm and a pore to pore spacing of 510 nm has been fabricated using multiple exposure interference lithography and (silicon) micro-machining technology. The sieve consists of a 0.1 µm thick silicon nitride membrane perforated with sub-micron diameter pores and a macro perforated silicon support. The calculated clean water flux is at least one to two orders higher than that of conventional inorganic membranes.

Filtration of lager beer with microsieves: flux, permeate haze and in-line microscope observations

Membrane fouling during filtration of lager beer with microsieves was studied through in-line microscope observations. It was observed that the main fouling was caused by micrometre-sized particles, presumably aggregated proteins. These particles formed flocks covering parts of the membrane surface. Most of the flocks could be removed by a strong temporary increase in crossflow. Underneath the flocks a permanent fouling layer was formed inside the pores. This made frequent removal of the flocks crucial in delaying the process of permanent in-pore fouling. Besides the fouling process the influence of pore size on permeate flux and turbidity was investigated. Centrifuged beer appeared to give a significantly clearer permeate than rough beer. For centrifuged beer and a microsieve with a pore diameter of 0.55 μm a haze of 0.23 EBC was obtained during 10.5 h of filtration at an average flux of 2.21×103 l/m2 h. For a sieve with slit-shaped perforations of 0.70 μm×3.0 μm a haze of 0.46 EBC was obtained during 9 h of filtration at an average flux of 1.43×104 l/m2 h. This flux is more than two-orders of magnitude higher than is commonly obtained with membrane-filtration of lager beer. Concentration of the beer by a factor of 12 hardly influenced the magnitude of the flux.

Fabrication of microsieves with sub-micron pore size by laser interference lithography

Laser interference lithography is a low-cost method for the exposure of large surfaces with regular patterns. Using this method, microsieves with a pore size of 65 nm and a pitch of 200 nm have been fabricated. The pores are formed by inverting a square array of photoresist posts with a chromium lift-off process and by subsequent reactive-ion etching using the chromium as an etch mask. The method has wider process latitude than direct formation of holes in the resist layer and the chromium mask allows for etching of pores with vertical sidewalls.

Ceramic microsieves: influence of perforation shape and distribution on flow resistance and membrane strength

Ceramic microsieves with slit-shaped perforations were compared to sieves with circular-shaped perforations, regarding flow resistance and membrane strength. Destructive tests show that the highest strength is obtained if the perforations are placed in a non-alternating pattern. Especially for slits, alternating patterns should be avoided as they make the structure unnecessarily flexible. The highest stress occurs at the edges of the membrane where it is attached to the support. Flexible structures bend stronger and therefore cause a higher stress at the edge, resulting in an easier rupture of the membrane. Our results show that ceramic microsieves with slits show a four to five-fold decrease in flow resistance for comparable strength related to sieves with circular pores.

Wet and dry etching techniques for the release of sub-micrometre perforated membranes

For the production of microsieves we studied the release of perforated silicon nitride membranes from a silicon substrate. During the release by KOH etching the pressure build-up due to hydrogen gas formation can be quite large and cause rupture of the membrane. We explored the use of anisotropic etching with an SF6/O2 plasma to replace KOH etching. For sub-micrometre pores excellent results were obtained.

Determination of particle-release conditions in microfiltration: a simple single-particle model tested on a model membrane

A simple single-particle model was developed for cross-flow microfiltration with microsieves. The model describes the cross-flow conditions required to release a trapped spherical particle from a circular pore. All equations are derived in a fully analytical way without any fitting parameters. For experimental verification of the model ultra-thin microsieves of uniform pore size and distribution were used. The release of trapped particles (polystyrene spheres and yeast cells) was determined by flux measurements as well as by in-line observation through a microscope. The results show that the model gives a fairly good indication of what cross-flow should be applied to keep the pores free for the conditions specified in this paper. In addition it provides us with a simple rule of thumb for the design of cross-flow modules for microsieves. It describes which geometrical demands have to be met to enable filtration without pore blocking, again for the conditions specified in this paper.