The Requirements for the Degree of Master of Science in Mechanical Engineering

This thesis presents a study of the in-situ precipitation of polyelectrolytes during droplet impacts and its applications in enhancing the retention efficiency of sprays. Large amounts of agricultural sprays are wasted worldwide, due to the poor retention on hydrophobic plants. As the harmful effects of pesticides in particular are more and more pointed out, there is an increasing pressure to reduce their use and make their spraying more efficient. Current solutions, mainly based on surfactants, all have limitations. Here, we present a novel idea based on the modification of the surface of the plant. By precipitating opposite polyelectrolytes, in-situ, we create sparse pinning sites that pin the contact lines of the impacting droplets from the spray and prevent them from bouncing off. We first study the behavior of the impact of two droplets containing oppositely charged polyelectrolytes on a hydrophobic surface. We then study the precipitation process of two polyelectrolytes and develop a model that predicts the outcome of a double drop impact. Finally, we show the macroscopic applications of this study, by using simultaneous spraying. Simultaneously spraying dilute opposite polyelectrolytes on a superhydrophobic surface leads to a large increase in the liquid retention and the coverage of the surface. The behavior has ben shown to hold for different polyelectrolytes and surfaces, making this method suitable for a range of applications. Thesis Supervisor: Kripa K. Varanasi Title: Associate Professor of Mechanical Engineering