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Droplet-Droplet, Droplet-Wall and Microorganism-Wall Interactions

Krishnan, K Ghokulla Haran
Thesis/Dissertation; Online
Krishnan, K Ghokulla Haran
Loth, Eric
The first study investigates the outcomes in droplet-droplet collision. Droplet-droplet collisions in a gas occur when droplets move towards each other in flight and come into direct contact. The “outcome” is defined herein to be the state of the liquid matter after the collision process is completed and generally can be classified as an agglomeration (two drops become one drop), reflection (two drops stays as two drops), or a break-up (more than two drops result from the interaction). The outcome can be important for many energy systems, particularly fuel sprays in the near injector region, where there can be many collisions between droplets. The outcomes for these interactions are critical to the overall size distributions, which in turn can affect overall combustion performance. Models for predicting the outcome are typically based on inviscid stability criterion and are validated using experimental data of water droplets colliding in atmospheric pressure conditions. However, combustion sprays generally include hydrocarbon droplets (with lower surface tension and higher viscosity) impacting in high pressures gasses and often with elevated temperatures (where gas densities and viscosities are also elevated). As such, a robust set of models to predicting the collision outcome under conditions of variable gas pressure and viscosity as well as variable viscosity and surface tension of the liquid is proposed in the study. The second study investigates the anti-wetting models, namely Wenzel and Cassie-Baxter models. The usage and limitations of the models are compared with experimental data through comprehensive literature analysis. Both models predict the contact angle limits accurately. However, the transition from Wenzel to Cassie-Baxter is not accurately predicted by the models and it is found that the receding contact angle had the least dispersion. The third study investigates the all possible outcomes of droplet-wall collision. Some new outcomes were observed during the experiments. Besides that, the contact angle effect and Oh effect (viscosity) are coupled. The receding contact angle seems to be controlling parameter if the viscosity is fixed. This study proposes a qualitative model to capture both of the effects to predict the critical number of deposition (when deposition will no longer happen). The fourth study investigates the anti-wetting effects on biofouling inception. The focus of the study is to prevent the inception of biofouling as opposed to a biofouling film. Various surfaces were exposed to Rivanna lake water for 45 minute periods and the silicon dioxide based superhydrophobic surface that was developed in-house, SH-3 had the least fouling. The amount of fouling both in terms of number of fouling elements and the height of fouling elements correlated to the contact angle of the surface. This study shows that biofouling inception could be prevented by use of superhydrophobic surfaces.
University of Virginia, Department of Mechanical and Aerospace Engineering, MS, 2013
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