The pressure stability of microfluidic glass chips was tested experimentally, with a special focus on the inserts for glued capillary connections.Destructive high-pressure experiments with demineralized water conducted at room temperature showed a difference in mean fracture pressure between the two tested glass types BF33 and D263T, with values of 192 ± 25 and 159 ± 25 bar, respectively.For BF33, hydrofluoric acid (HF) etching of the powder blasted (abrasive jet machined) chip insert increased the mean fracture pressure with 43 ± 9 bar, whilst for D263T a decrease of -22 ± 8 bar resulted.Contrary to the expected surface smoothening of the HF treatment, a rougher surface was obtained, particularly for the case of D263T, which is thought to be due to the opening of median (radial) cracks caused by the powder impact during the blasting process.The roughness obscures the effect of the tapering of the insert, preventing that factor from having a statistically significant effect on the mean fracture pressure.Nevertheless, a decrease in the mean fracture pressure and a decrease in the variance of the mean fracture pressure was observed when a taper is introduced, whereas the fracture location tends to move away from the insert-microchannel intersection towards the glue meniscus.A practical solution for cases where a high-pressure stability is required is found in applying a metal clamp around the capillary insert section.This significantly increased the fracture pressure of the chip insert section with 50 ± 21 bar, by preventing bond release.
This paper reports the modeling of the stationary flow-structure interaction of different types of micro check valves. Mathematical indirect, domain coupled and decoupled, finite element models as well as analytical approximations are made. The obtained simulation results are validated by performed measurements on fabricated micro valves. Simple analytical formulas are derived. These can be used for micro fluid handing component design purposes as well as for input for numerical lumped element system simulation programs.
Two types of microfluidic systems, a porous hollow fiber and a thin supported membrane with an array of micromachined holes, are investigated for concentrating mass-limited analyte samples. Water evaporation is driven by the partial pressure difference across the hydrophobic membrane, induced by dry sweeping gas on the permeate side. An analytical model permitting clarification of the contribution of design and process parameters on acquisition of concentrated solution and prediction of achievable concentration factors is presented. Concentrating an exemplary solution utilizing the two systems has been studied at different experimental conditions to validate the model. The results show that the hollow fiber gives controllable concentration factors of more than 10. For the micromachined membrane concentrator concentration factors of 6-8 were achieved, at much lower flow rates than predicted by the model. Because of the asymptotic dependence of concentration factor on flow rate, accurate control of the liquid feed is extremely critical in the flow rate range where high concentration factors are obtained, and the smallest variations in liquid flow rate may easily lead to supersaturation and deposition of solutes in the pores. This changes membrane porosity in an unpredictable way and limits the maximum attainable concentration factor.
In this paper, new possibilities are shown to fabricate micro structures with use of a simple anisotropic wet chemical etching method. The method is very accurate, cheap and puts low demands on cleanroom facilities. Due to the possibility to obtain structures like valves which have their entrance and exit channels in one wafer in-line, the method might be very valuable for integration of different fluid handling components such as valves, channels and pumps. Several structures are fabricated and a summation of other possible designs is given. These show that exploitation of the well-known anisotropic etching methods in alkaline solutions can result in more sophisticated structures than used up to now.
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