Meaningful descriptions of thixotropy and extensibility for yield-stress fluid drop impact on thin films

Yield-stress fluids are non-Newtonian materials pervasive in daily life as well as industrial applications, one of which is sprays and fire suppression. Using a yield-stress fluid as a fire-retardant additive puts out fires quicker, because a finite yield stress prevents run-off and improves coating retention, and aids in environmental conservation by minimizing water usage. It is thus of great interest to study the drop impact fluid mechanics and rheology of yield-stress fluids. The objective of this work is to study the flow behavior of non-Newtonian fluids, yield-stress fluids in particular, and the role of rheologically complex phenomena of thixotropy and extensibility, in drop impact. We report the first-ever experimental study of microstructure, thixotropic aging, and extensibility effects in viscoplastic drop impact, generating a large data set covering hundreds of different impact conditions. The work reveals fundamental insight into rarely studied short-timescale flow conditions with such rheological effects. We analyze the drop impact regimes of yield-stress fluids on coated substrates using high-speed imaging and dimensionless groups from fluid mechanics with appropriate modifications, reducing the multi-dimensional parameter spaces into a few tractable quantities that offer deeper physical insight. Throughout this dissertation, we use the idea that such higher-order rheological effects arising from complex material microstructures can be described using a small number of meaningful parameters, and the great challenge is to identify those parameters. The rheology of these complex fluids is studied using timescale distributions and constitutive equations that further simplify the complex rheological properties and flow conditions involved in drop impact tests. Consequently, each project in this dissertation requires shear and extensional rheology, namely, characterizing the thixotropy and viscoplasticity of colloidal and microgel suspensions in shear and extension, modeling of extensibility, and developing analysis methodologies for robust, quantitative descriptions of thixotropy and extensibility. The utilization of high-speed imaging to capture millisecond-scale phenomena occurring during the droplet impact event complements the rheometric discoveries. Coupled with rheometry, short-time video capture allows access to a myriad range of flow and rheological properties, which yield the simpler metrics for describing their behavior. A major drawback of this method of representing complex rheological phenomena by their reduced descriptions, also known as summary statistics, is that we lose information about the details of the exact nature of the thixotropic, drop impact, or extensibility behavior. Data with the same reduced description could be very different in the details. Nevertheless, such parameters are valuable since they can serve as excellent descriptors of more complex phenomena, readily usable in applications involving viscoplastic fluids. Many of the quantities derived here have been used within the dissertation itself, and have a broader scope in applications such as fire suppression, 3D printing, and spray coatings, among many others.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Sen, Samya

Contributors dc:contributor

• Ewoldt, Randy H
• Tawfick, Sameh
• Rogers, Simon A
• Feng, Jie

• yield-stress fluids
• viscoplasticity
• thixotropy
• extensibility
• droplet impact
• design

Statement dc:rights

• Copyright 2022 Samya Sen

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/116176

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F116176