Paper Number
IN37
Session
Flow Induced Instabilities and Non-Newtonian Fluids
Title
Thixotropic yield-stress fluid drop splash: Testing a dimensionless group across different microstructure classes
Presentation Date and Time
October 23, 2019 (Wednesday) 2:45
Track / Room
Track 4 / Room 305B
Authors
- Sen, Samya (University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering)
- Morales, Anthony G. (University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering)
- Ewoldt, Randy H. (University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering)
Author and Affiliation Lines
Samya Sen, Anthony G. Morales, and Randy H. Ewoldt
Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801
Speaker / Presenter
Sen, Samya
Text of Abstract
We use high-speed imaging to study drop impact and splash behavior of thixotropic yield-stress fluids. Aqueous suspensions of Laponite and Carbopol are both systematically studied, allowing tests of different microstructures against a proposed microstructure-agnostic dimensionless group to identify regime boundaries of stick-splash behavior. The effect of composition, drop diameter, velocity, and coating thickness are studied systematically and specific regimes of impact behavior are identified. The proposed dimensionless group, a ratio of droplet inertia and dissipative flow stresses during the splash event, successfully separates the different impact regime behavior of all fluids studied. Remarkably, we observe a constant critical value of this group for the onset of splashing for all impact conditions and all fluids tested, independent of chemistry or microstructure.
Drop impacts with thixotropically aged fluids give the same outcomes except that 'aged' properties are required for the dimensionless group. However, characterizing flow properties of aged samples is challenging since any deformation inherently rejuvenates and breaks down the microstructure and the rheological properties change, now resembling those of a fresh or ‘unaged’ sample. Steady-state flow properties are therefore undefined for aged samples, and we propose alternative rheological techniques for characterizing flow properties of aged samples, further using these in dimensionless groups that rationalize the stick-or-splash regime maps.
The results here provide a dramatically simplified understanding of the fluid dynamics of droplet impact of these rheologically complex fluids, with diverse applications including spray coating, painting, and fire suppression.