Droplet Control in Aqueous and Hydrocarbon Fluids: Long, End-Associative Polymers Dictate Fluid Behavior Under Elongational Flows

Citation

Learsch, Robert Whitson (2023) Droplet Control in Aqueous and Hydrocarbon Fluids: Long, End-Associative Polymers Dictate Fluid Behavior Under Elongational Flows. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zkpd-bq33. https://resolver.caltech.edu/CaltechTHESIS:05102023-041835603

Abstract

Modifying elongational flows seen in sprayed mists, turbulent flows, and droplet spreading and retraction following impact, is of interest in diverse industries, including agriculture and aviation. Long flexible polymers (with fully extended lengths 1 to 10 µm) modify the elongational flow behavior of a fluid to which they are added. At low concentrations (1 to 10% of their overlap concentrations), their effect is mild under shear flow (shear viscosity increases < 50%), but dramatic under elongational flows (extensional viscosity increases ≥ 300).

These long polymers are not widely used in practice because they degrade under strong flows, such as passing through pumps and filters, that typically precede spray. Pairwise end-associative polymers can overcome this limitation. Pulling apart non-covalent associative bonds under such strong flow conditions relieves the tension along the polymer backbone. The pairwise end-associative polymers that are effective in mist control and drag reduction are individually short enough to avoid chain scission in flows that would break long covalent polymers, yet long enough that 6 to 8 associative polymers connected end-to-end create supermolecules that are as effective as their long covalent counterparts.

This thesis systematically compares the effect of long covalent and long end-associative polymers on the fluid’s extensional flow properties and the polymers' performance in controlling droplet impact and spray breakup. To measure the elongational flow properties, I implemented and enhanced the Dripping onto Substrate Extensional Rheometry (DoSER) technique (Chapter 2) and applied it to long covalent polymers (Chapter 3) and to end-to-end associative polymers (Chapter 4). Preparing solutions in which the polymers negligibly affect the interfacial tension (< 10%) allows us to explore the relationship between extensional flow properties and droplet impact (Chapter 5) and spray (Chapter 6).

By combining the quantitative measurements of extensional viscosity and extensional relaxation time with the corresponding behavior in impact and spray, I correlate the structure of polymers to the solution behavior in droplet rebound and spray breakup. This work has the potential to reduce pesticide contamination of soil, water, and air from agricultural sprays and fire hazard associated with hydrocarbon lubricants.

Thesis Files