Geothermal energy plays a role in advancing clean energy solutions for a sustainable future, which is why we investigate the sustainability of geothermal systems in an aquifer. The geothermal system examined in this research operates using two wells, positioned 1 km apart. One well pumps cold water into the porous medium and the second well pumps warm water out of the aquifer. As warm water is extracted, the temperature of the aquifer decreases. This cooling disturbs the chemical balance of the minerals in the water, which can lead to precipitation reactions. This precipitation reduces the porosity and consequently the permeability of the aquifer. The temperature distribution is modeled using the pressure and velocity field. This showed that the aquifer had cooled to 40˝C in 6 years. According to this result, the use of a geothermal aquifer is not sustainable. The cooling within 6 years is not accurate, because the boundary conditions that were selected were not fully chosen to reflect the complexities of the system and we didn’t properly consider the heat coming from Earth’s core. Precipitation reactions lead to an increase or decrease in concentration of various minerals in the water. These reactions cause deposits of solid minerals on the grains or porous rock, causing the aquifer to clog, allowing less and less water to flow through. In this model, the amount of precipitation was very small and therefore had little effect on the water flow and the sustainability of the aquifer.

The salinity profile in a coastal sea-river system is the result of balancing of salt transport due to mixing of the water by tides and the freshwater discharge from the river. When for example during times of drought freshwater discharge suddenly declines, the salinity in the river may increase significantly and threaten drinking water supplies and agriculture. The time it takes for the salinity to reach the new equilibrium may be considerable. Thorough understanding of the adaptation process is essential for taking appropriate prevention measures. However, much about this adjustment process is still unknown, in particular regarding the interaction between the river and the adjacent sea. In this study, a simplified linear model is developed to describe a well-mixed river and the adjacent sea where the water flows radially away from the mouth of the river. The unique assumptions, most notably a time-independent intrusion length, allow for a unique analytical approach using the method of eigenfunction expansion. The smallest eigenvalues are found to define a time scale of the adjustment process. The eigenfunctions provide insight about how the salinity adjustment time varies within the system. The adjustment time does not only depend on the parameters that describe the new equilibrium, but also on the initial salinity. The eigenvalue time scales corresponding to the sea and to the river can then be used to determine the time scale of the coupled system. It is found that when the sea adjusts much faster than the river, the time scale of the river is leading the adjustment. When the eigenvalue time scale of the sea is similar to or larger than the eigenvalue time scale of the river, the adjustment time is significantly increased by the sea. When salinity is increasing, the sea restricts the inflow of salt in the river, while for decreasing salinity, the sea keeps transporting salt to the river by mixing which delays the adjustment. The model is applied to analyse a sudden decrease in freshwater discharge in the Rotterdam Waterway, which is part of the Dutch Rhine-Meuse delta. Results show that the salinity response is delayed compared to freshwater discharge, indicating that the adjustment time of the river indeed plays an important role. Further research should consider a more general dispersion relation or an extra vertical dimension to better describe estuaries that are not well-mixed. Using numerical methods there are many other possibilities to extend the model. The effect of tides on the presence and dynamics of the fresh water bulge in the sea is of specific interest.

Oil-in-water emulsions are very common in industrial processes, and understanding the fac- tors governing emulsification and de-emulsification is crucial. A mechanism through which de-emulsification commonly takes place is coalescence, being the act of two dispersed-phase droplets coming together to create a single larger droplet. Due to density differences, it is common for dispersed phase droplets to form a creaming layer at the top of the emulsion, in- creasing the likelihood of coalescence and therefore de-emulsification. The goal of this thesis is to use the lattice Boltzmann method to simulate the rising of a dispersed phase oil droplet towards a creaming layer due to buoyancy effects and to quantify the effect of the viscosities of both phases on the velocity with which aforementioned droplet rises. This is done using the Shan-Chen psuedopotential method. After introducing gravity into the simulation the the droplet is allowed to reach a terminal velocity vt. This is done for independently varying viscosities for both the dispersed phase and continuous phase (νd and νc respectively). A weakly inverse relation was found between vt and νc. An estimation for the terminal velocity of a droplet rising due to creaming behavior is found by cancelling drag force found from Stokes’ law against the buoyancy force. Found results were not in agreement with said estimation. Different explanations for this discrepancy are varying droplet diameters, varying droplet densities, droplet deformation and high velocity fluctuations. The terminal velocities were also compared to the ratio between the viscosity of both phases, but no correlation was found.

In the human body calcium is the most abundant mineral and is involved in a lot of vital processes. Calcium deficiency occurs when there is not enough calcium present in the bloodstream. In the Netherlands one in three women will face this problem in their lifetime. A major factor contributing to this problem is that the intake of calcium on average is below the recommended amount which is around 950 mg/day for adults in the Netherlands. Calcium deficiency when left unattended can lead to osteoporosis. Therefore it is important to consume enough calcium rich products or take calcium supplements. Milk is a good source of calcium and is considered a staple food. This makes bovine milk a good candidate for calcium fortification. Milk can be fortified by using calcium salts to increase the bioavailability the calcium present. In this thesis in vitro methods were used to study the effects of calcium salt fortification on the phase distribution of calcium in bovine skimmed milk under different pH conditions and by using the radioactive isotope 45Ca to track the exchange between the phase casein micelles, serum proteins and the soluble phase when fortifying the milk. For unfortified milk it was found that 36.0±0.9% was present in the soluble phase, 3.7±2.9% was present in the serum proteins phase and 60.3±3.0% in the casein micelles. Acidifying the milk to simulate the gastric environment resulted in an increase of the soluble phase from 55.1±2.7% to 97.0±3.3% which corresponds to a decrease in pH from pH 6 to pH 2. For the fortification of the milk, five calcium salts were selected for synthesis. The salts were calcium carbonate, tricalcium di-citrate, calcium gluconate, calcium lactate and tri-calcium phosphate. The fortification was performed with all of the synthesized salts except calcium lactate and tri-calcium phosphate. The starting material for the syntheses, calcium chloride, was also used for fortification. It was found that for the fortification of CaCO3 under gastric conditions a bioavailability of 100% could be achieved. However the acidic environment also resulted in coagulation of the milk and therefore the bioavailability could not be accurately assessed. Calcium gluconate had the only increase in bioavailability under normal conditions. In conclusion a better understanding of fortification with calcium salts was achieved and the influence of pH on the bioavailability of calcium in milk and the effects on the salt fortification.

The study of adhesion mechanisms in animals such as the gecko has helped mankind with the understanding and development of novel adhesives. The adhesion mechanism of the tree frog is not fully understood, it is a wet adhesion mechanism which works differently than the dry adhesion mechanisms of animals, such as the gecko, studied previously. The wet adhesion mechanism of the tree frog may be based on the capillary force present in liquids. Researching the adhesion mechanisms of the tree frog could help develop new capillary based
adhesives. Although there are several capillary measurement methods and devices used to study the interactions between small volumes of liquids and surfaces, they would require the frog to be immobilized for accurate measurements, which constitutes as harm. This thesis begins the design and development of a system to measure the capillary force created by tree
frogs without harming them. The selected concept functions with the use of a pressure sensor connected to the capillary pressure source with capillary channels. This concept allows for a precision of 12Pa determined by the pressure sensor and a responsiveness of 30 ms determined by fluidic circuit while allowing the frog to roam free around the system input. The components were tested to verify their functionality. A controlled pressure source is used to verify the calibration of the sensor, the time constant and hydraulic capacitance; which are
the parameters that determine the accuracy and responsiveness of the system. The sensor was successfully calibrated to measure pressure linearly up to 10 kPa but the time constant and hydraulic capacitance were experimentally measured to be three orders of magnitude larger than the estimated parameters of the components. In order to solve the disparity between the experimental and theoretical values, different tubing and connections were suggested to reduce the hydraulic capacitance and a method verifying the presence of air bubbles was advised. A Sesille drop experiment was performed on the pressure sensor in order to support the principle of measurement, the results suggests the principle of measurement is valid but the issues surrounding the fluidic circuit needs to be resolved in order to complete the system.