Microvalves are useful components for several microfluidic applications in which they control the fluid flow in a microfluidic system. Most microvalves to date are made by using silicon micro-machining, which is a complex manufacturing process, or soft lithography using Polydimethylsiloxane (PDMS) which has a high gas permeability. Next to that most microvalves in literature have small actuation forces resulting in small pressure ranges and large leakages at low pressure levels. In this paper, a normally open microvalve which is fabricated by only using 3D printing techniques with a bio-compatible resin is presented, making it more easy and accessible to manufacture. The novelty is the integrated micro-channels, membrane and microfluidic connections in a single 3D printed piece. The utilized actuator is a commercially available piezo stack, which has a displacement of 34 µm at 150V. Due to its large actuation force the microvalve can modulate fluids from -600 to 600 mbar, with measured flow-rate levels between 0-90 µl/min and projected flow-rate levels between 0-410 µl/min. In fully closed state the leakage-rate of the microvalve is 1.67 µl/min at 600 mbar, with a static power consumption of 442.5 mW. Subsequently it is shown that after using higher clamping torques (> 0.7 Nm) the microvalve can operate leakage free up to 1.5 bar. Additionally a 3-to-1 fluid selector is designed using three microvalves, which can be integrated into a portable microfluidic platform for Organ-on-Chip (OoC) applications.

The aim of this research was to evaluate the feasibility of direct hollow fiber nanofiltration membranes for drinking water purposes. The experiments were performed on the dNF40 pilot provided by NXF. The fouling potential and the ion retention of the dNF40 pilot were determined by continuous filtration experiments using raw IJssellake water under different operational conditions. The performances (ion retention and fouling potential) of the dNF40 on raw IJssellake water were compared with the dNF40 performances on pre-treated water from Waterwinstation Prinses Juliana (WPJ) (previously done at PWNT). The WPJ pre-treated water has undergone extensive pre-treatment consisting of drum screens, flocculation, sedimentation, rapid sand filtration (RSF) and granular activated carbon (GAC). The OMP retention of the dNF40 pilot was determined by full recirculation experiments using WPJ pre-treated water under two different operational conditions with elevated concentrations 'spiked solution'.
Limited to no fouling impact was observed on the membrane performance when feeding the pilot with raw IJssellake water. The membrane performance parameters (mass transfer coefficient (MTC), trans membrane pressure (TMP) and normalized pressure drop (NPD)) were stable over time. In addition, limited to no fouling impact was observed on the membrane when feeding the pilot with WPJ pre-treated water. However, membrane performance (i.e. MTC) was better for raw IJssellake water (1 year old membrane) compared to WPJ pre-treated water (virgin membrane). This implies that the active outer layer of the membrane has undergone a change in properties leading to these higher MTC values.
An increase in recovery, flux and crossflow velocity resulted in a decrease in ion retention. However, a decrease in ion retention with elevated crossflow velocity is unusual. Higher crossflow velocities should actually lead to an increase in ion retention due to reduced ion build-up next to the membrane surface (i.e. lower concentration polarization effect). However, the lower ion retention can be attributed to the elevated MTC during experiments. The removal of natural organic matter (NOM) was consistently above 90% and was not influenced by a change in operational condition. Ion retention was higher for WPJ pre-treated water (virgin membrane) compared to raw IJssellake water (1 year old membrane). This can be attributed to the increase in MTC of 1.5 LMH/bar in raw IJssellake water (compared to WPJ pre-treated water) potentially caused by a change in the properties of the active outer layer.
For determining the OMP retention of the dNF40 membrane, a spiked solution containing per- and polyfluoroalkyl substances (PFAS) and pharmaceutical compounds was analyzed. The PFAS compounds of the spiked solution were retained very well (above 80%). As expected, the retention increased with increasing MW. The adsorption percentage of PFAS was between 40%-90%. The pharmaceutical retention was around 30%, although all pharmaceuticals analyzed had a MW below the MWCO of the membrane.
A 5-stage full-scale dNF40 plant was designed based on a permeate flow of 15 M m3/year, a total hardness concentration in the permeate stream below 1.4 mmol/L and a recovery percentage of 85%. Based on the 5-stage full-scale dNF40 plant an economical analysis was performed and compared to the full-scale UF-RO in Heemskerk. The total cost (OPEX and CAPEX) were cheapest for the full-scale dNF40 plant fed with raw IJssellake water (12 ct/m3), followed by the dNF40 plant fed with WPJ pre-treated water (32 ct/m3) and most expensive for the UF-RO plant (35 ct/m3). The major factors in the OPEX was the membrane replacement cost for the dNF40 plant and the energy and chemical cost for the UF-RO plant.

The coalescence of two droplets suspended in a viscous fluid is the focus of this study. When two droplets that are suspended in a shearing flow come in contact with each other, it is currently uncertain if the contact would result in coalescence. To be able to make such predictions, it is necessary to study the effects of the system parameters, such as the angle of collision between the droplets and the properties of the flow and the two fluids, on the process of coalescence. This necessitates the need for a robust experimental setup in which such studies can be performed.

The characteristic of a systematic study is repeatability and control over experimental conditions. In coalescence studies, the impact angle between the droplets is a parameter that has proved difficult to control. In this study, a microfluidic device is developed that uses the concept of surface energy wells to achieve repeatability in the impact angles of droplet collisions. Using the device, droplet coalescence experiments were performed. An interesting observation was made from the experiments which guided the further course of this research work. It was seen that the droplets did not coalesce upon approach, but coalescence was driven by the separation of the droplets. In the literature, this phenomenon is referred to as 'separation-driven coalescence'. Furthermore, it was suspected that an experimental condition could be defined based on a non-dimensional parameter, namely, the Capillary number Ca, such that for Ca > Ca_cr, separation of droplets ceases to trigger coalescence.

The effect of the system parameters, namely the impact angle, θ_i, and the viscosity ratio, λ, over the Ca_cr was investigated. In this study, the presence of a Ca_cr for separation-driven coalescence is confirmed, both experimentally and through a scaling argument. The results of the study indicate that the thickness of the film on the onset of separation influences the Ca_cr. However, a dependency between Ca_cr and λ was not found experimentally. Large experimental uncertainties prevent any further conclusions to be made regarding the Ca_cr.

In this thesis, a framework is developed for the investigation of the Ca_cr for separation-driven coalescence. With more experimental data, a deeper understanding of separation-driven coalescence can be obtained.