John Frostad

Emulsions occur in various industries such as food, pharmaceutical, agricultural, and oil and gas. Regardless of the application, controlling emulsion stability is crucial for increasing customer satisfaction, reducing manufacturing costs, and preserving the environment. Previously, it has been shown that emulsion droplets coalesce under shear, which finally results in emulsion phase separation. However, the underlying mechanism that causes droplet coalescence under shear has not been explained thoroughly. In this thesis, we first employed a Cantilevered Capillary Force Apparatus (CCFA) to capture a pair of droplets and collide them in two configurations: head-on and shearing. The aim is to find an emulsion composition that enables us to trigger coalescence under the shearing droplet collision scenario. We used surfactants and a range of particles, spherical and Janus silica, plate-like kaolin, and rod-shaped cellulose nanocrystals (CNCs), to stabilize droplets against coalescence. We showed that for all surfactant types, silica, and kaolin, shear-triggered coalescence is not possible. However, particle rolling on the interface results in a slightly higher coalescence probability under shear for droplets stabilized by Janus silica particles. A significant increase in the coalescence probability (more than two times higher) under shear occurs for droplets stabilized by CNC particles, which is due to particle reorientation under shearing flow. Without any salt in the CNC suspension, the repulsive force between individual CNC particles prevents them from forming a tight network at the interface, resulting in an unstable emulsion. With 10 mM salt in the suspension, the CNC network at the interface is strong enough to prevent coalescence in the head-on collision, but under shear, particles are oriented in the flow direction, leaving some fresh interface for coalescence. Finally, we showed that the increase in the coalescence probability causes a faster phase separation in model emulsions. When CNCs with 10 mM NaCl were used to stabilize emulsions, full phase separation occurred within two days when the emulsion bottle is placed on a roller. However, bottles on the shelf underwent no phase separation even after 4 months.

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Starch is a widely used ingredient in food products, acting as a thickening, clouding, and gelling agent. Starch gelatinization is an important process that can influence the texture of food products, therefore, it has been studied extensively by many researchers. A Particle Cohort Study (ParCS) apparatus was used to observe the gelatinization process of individual starch granules from four types of legume starch: yellow pea, red bean, chickpea, and green lentil. This new method allows us to capture and understand the variability between individual granules during the swelling that occurs due to gelatinization. The size as a function of time was measured for a large number of individual granules in order to quantify the intra-sample variability for each type of starch, as this information is not available from the standard techniques for characterizing gelatinization: starch pasting and differential scanning calorimetry (DSC).The swelling of individual starch granules under non-isothermal conditions was recorded and subjected to image analysis for quantifying their sizes. For each type of legume starch, around 180 granules were collected for data analysis. The cumulative size distribution measurements using image analysis were similar to that obtained from the laser diffraction method, except for red bean starch, which showed a smaller size by image analysis. We demonstrate that an empirical model, the Gompertz function, is highly effective at describing the size vs. time data.Using the Gompertz function, the data from image analysis are fitted to obtain and extract two new parameters related to gelatinization that we define for the first time in this manuscript: granule-swelling temperature and granule-swelling time scale. The accuracy of these new parameters is demonstrated by comparison with standard techniques. After proposing an alternative method for interpreting starch pasting data we show a very good correlation between all three techniques. The results indicate that these legume starches have a remarkably low variability in gelatinization properties. This new method of characterization is expected to enable optimization of starch gelatinization properties during large-scale processing of food products.

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In pesticide and fertilizer applications, large spray droplet sizes (300-500 um)are commonly used in the field for reduced spray drift. However, retention of spraydroplets after they reach the target surface can be limited by droplets splashing,rebounding, or rolling off of the surface due to their high impacting velocity andinertial energy. While polymer additives were proposed to dissipate energy duringthe droplet impact process, whether they can enhance the retention efficiency of cropsprays in practical field conditions is still not clear. This research work focuses on enhancing the retention efficiency with polymer additives,which is carried out in four major steps: The first step is to screen polymerssuitable for spray applications. The second step is to develop an approach that providesdetailed physical insight in a setup that is representative of real spray conditionsto quantify retention efficiency. The third step determines the effect of extensionalrheological properties on retention efficiency at various spray conditions. The workcarried out in the first, second, and third steps forms a comprehensive study onthe relationship between extensional rheological properties of polymer solution andretention efficiency. Then the fourth step focuses on exploring the extensional rheologicalproperties of selected polymer additives at different solvent conditions and inagrochemical solutions.The results demonstrate that increasing the extensional relaxation time of thespray solution can increase the retention efficiency by up to 20% and in some casesachieve a total efficiency greater than 95%. It’s also suggested for a particular polymer,surface, and droplet size, the extensional relaxation time alone could be sufficientto predict retention efficiency. The results also relate the extensional relaxation timeto important influencing parameters including pH, ionic strength, type of ions, etc.

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Stem cell-derived 3D tissues such as spheroids are excellent models for investigating mechanisms of tissue formation and responses to physiological and mechanical cues. Neuronal spheroids, also known as neurospheres, have attracted particular interest. A lot is now known about the differentiation and maturation of neurospheres, as well as their responses to biochemical cues. However, understanding about their mechanical properties pales in comparison, which is all the more galling in light of newfound insights about how mechanical stimuli trigger the onset of neurodegenerative conditions. In the current study, we have taken formative steps to fill this knowledge gap. We generated neurospheres from murine neural stem cells, treated with hydrogen peroxide to simulate oxidative stress, and subjected them to compressive forces. We observed that neurospheres exhibit viscoelastic behaviour at low strains and plastic deformation at larger strains. We also evaluated the suitability of the Tatara model for characterizing the mechanical properties of neurospheres. There was an observable dependence of the mechanical properties of the neurospheres on the their sizes, with smaller samples being stiffer. When comparing neurospheres treated with a mild peroxide treatment to untreated samples, no significant differences in the mechanical properties were detected Our study is the first to investigate the mechanical properties of living neurospheres under uni-axial compression. However, the results demonstrate the need for further method development in order to account for biological variability and sample heterogeneity.

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Historically, foam studies have focused on foamability, foam stability, and foam inhibition as opposed to foam density. Hence, the impact of various factors on foam density is still not well understood. Previous experimental work shows that concentrated surfactant solutions with similar surface tensions produce foams with different liquid fractions. These results contradict the common assumption held that these foams would have near-identical liquid fractions. Therefore, this study probes for correlations between equilibrium and dynamic surfactant adsorption parameters at air-liquid interfaces for aqueous foams made from solutions well above the critical micelle concentration. A protocol was developed for measuring foam density using a cylindrical foaming apparatus with porous filter plate for gas sparging. The foams were created from aqueous solutions of small-molecule surfactants known to have negligible surface shear viscosity. Equilibrium parameters of the maximum surface concentration, equilibrium adsorption/desorption rate constant, and effectiveness (critical micelle concentration) showed weak evidence of a correlation as quantified by linear and ranked correlation coefficients. The surfactant efficiency (concentration needed to reduce the surface tension 20 mN/m) showed some evidence of linear correlation between the ranking of the variables. In contrast, the time required to reduce the surface tension 35% and 50% of the way from the pure water value to the equilibrium value (t_35 and t_50) showed strong evidence of correlation of increasing liquid fraction with faster surfactant adsorption times. The results of this study highlight the influence of dynamics even for highly concentrated surfactant solutions.

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