Paper Number
AD9 

Session
Rheology of Active Matter and Directed Systems

Title
A numerical study on the magnetization of dilute magnetic emulsions under small amplitude oscillatory shear

Presentation Date and Time
October 12, 2022 (Wednesday) 4:45

Track / Room
Track 4 / Michigan AB

Authors (Click on name to view author profile)

  1. Abdo, Rodrigo F. (University of Brasília, Laboratory of Energy and Environment)
  2. Abicalil, Victor G. (University of Brasília, Laboratory of Energy and Environment)
  3. Cunha, Lucas Hildebrand (Rice University, Chemical and Biomolecular Engineering)
  4. Oliveira, Taygoara F. (University of Brasília, Laboratory of Energy and Environment)

Author and Affiliation Lines (in printed abstract book)
Rodrigo F. Abdo1, Victor G. Abicalil2, Lucas Hildebrand Cunha3 and Taygoara F. Oliveira4
1Laboratory of Energy and Environment, University of Brasília, Brasília - DF, Brazil; 2Laboratory of Energy and Environment, University of Brasília, Brasília - DF, Brazil; 3Chemical and Biomolecular Engineering, Rice University, HOUSTON, TX 77005; 4Laboratory of Energy and Environment, University of Brasília, Brasília - DF, Brazil

Speaker / Presenter 
Cunha, Lucas Hildebrand

Keywords
computational methods; active matter; emulsions

Text of Abstract
The magnetization properties of a dilute magnetic emulsion under the combined action of a uniform external magnetic field and a small amplitude oscillatory shear are studied using numerical simulations. We consider a three-dimensional domain with a single ferrofluid droplet suspended in a Newtonian non-magnetizable fluid. The magnetic field is calculated considering Maxwell's equations in the magnetostatic limit, and the level set method tracks the droplet interface in the flow. The magnetic field in the main velocity direction confines the droplet to a region of lower effective shear. Consequently, although the amplitude of the magnetic torque increases as the magnetic field became stronger, the amplitude of the magnetization is reduced. Conversely, when the magnetic field is in the main velocity gradient direction, the droplet is stretched in a higher effective shear region, causing higher amplitudes in the periodic response of the bulk magnetization as the magnetic field increases, leading to even higher magnetic torques. We separate the in-phase and the out-of-phase (in relation to the shear) components of the periodic response of the bulk magnetization and the magnetic torque. We found that for both magnetic field orientations, the in-phase component is greater than the out-of-phase at lower frequencies, and the opposite occurs at higher frequencies. However, for all studied cases, the magnetic field oriented in the velocity gradient direction produces higher in-phase and out-of-phase components than the magnetic field in the main velocity direction. Additionally, the existence of an out-of-phase component for the periodic response of the magnetization indicates a finite relaxation time of the emulsion’s bulk magnetization, related to the fluid mechanical response to the periodic shear. This fact leads to the conclusion that the mechanism of magnetization relaxation in dilute magnetic emulsions possibly originates from the periodic variations of the droplet shape and orientation in the micro-scale.