In many analytical chemical applications, it is desired to detect analytes in water at very low concentrations. A good example is water quality control, where certain undesired pollutants are present in micromolar concentrations, or even less. To detect such low concentrations, very sensitive instruments are needed. A way to increase the limit of detection of an instrument is by concentrating the target molecule by removing the solvent. Prrevious research in the MCS group has focused on two types of concentrators , a hollow porous fiber, and a microfluidic membrane with defined, 200 nm wide pores, both based on the principle of evaporation of solvent through the membrane. The figure below shows the microfluidic device (left: membrane with pores, right: one pore). The hollow fiber works reasonably well, with a concentration factor of ca. 16, the performance of the microfluidic membrane is far below expectations, with a concentration factor of ca. 6 to 8. The probable reason for this problem is that dissolved substances do not have enough time to diffuse away from the pore openings in the membrane, where the solvent is evaporating quickly, and the solution becomes supersaturated. As a result of this, crystals will form in the opening, which block the pores. This changes membrane porosity in an unpredictable way and limits the maximum attainable concentration factor.
Previously an analytical model was developed for the microfluidic preconcentrator , which predicts the concentration factor for the hollow fiber membrane quite well. However, for the microfluidic membrane the model and the experimental results are very different, most likely because the evaporation is much faster than the model predict. The main goal of this assignment is to come to an optimized microfluidic concentrator, by modeling flow, diffusion and evaporation, and designing and testing new designs. This assignment consists of the following tasks:
- Develop a (numerical?) model which describes the evaporation of solvent at a circular pore in combination with the diffusion of the dissolved molecules away from the pore.
- Include the flow of the liquid that is present on one side of the membrane, as well as the flow of sweeping gas on the other side of the membrane, into the model.
- Formulate design rules for an improved membrane-based preconcentrator.
- Make a new design and test it on a model substance (e.g. a coloured liquid)
The assignment can be extended with experimental (microscopic or microfluidic) work on the evaporation through (single or multiple) pores.
- H. Zhang et al., In-line sample concentration by evaporation through porous hollow fibers and micromachined membranes embedded in microfluidic devices, Electrophoresis 37 (2016) 463-471; DOI: 10.1002/elps.201500285
Han Gardeniers; Email: email@example.com