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Title: Flows and torques in Brownian ferrofluids subjected to rotating uniform magnetic fields in a cylindrical and annular geometry

Ferrofluid flow in cylindrical and annular geometries under the influence of a uniform rotating magnetic field was studied experimentally using aqueous ferrofluids consisting of low concentrations (<0.01 v/v) of cobalt ferrite nanoparticles with Brownian relaxation to test the ferrohydrodynamic equations, elucidate the existence of couple stresses, and determine the value of the spin viscosity in these fluids. An ultrasound technique was used to measure bulk velocity profiles in the spin-up (cylindrical) and annular geometries, varying the intensity and frequency of the rotating magnetic field generated by a two pole stator winding. Additionally, torque measurements in the cylindrical geometry were made. Results show rigid-body like velocity profiles in the bulk, and no dependence on the axial direction. Experimental velocity profiles were in quantitative agreement with the predictions of the spin diffusion theory, with a value of the spin viscosity of ∼10{sup −8} kg m/s, two orders of magnitude larger than the value estimated earlier for iron oxide based ferrofluids, and 12 orders of magnitude larger than estimated using dimensional arguments valid in the infinite dilution limit. These results provide further evidence of the existence of couple stresses in ferrofluids and their role in driving the spin-up flow phenomenon.
Authors:
; ;  [1] ;  [2] ;  [3]
  1. Department of Chemical Engineering, University of Puerto Rico, Mayagüez, Puerto Rico 00681-9000 (United States)
  2. Department of Chemistry, University of Puerto Rico, Mayagüez, Puerto Rico 00681-9019 (United States)
  3. Department of Engineering Science and Materials, University of Puerto Rico, Mayagüez, Puerto Rico 00681-9044 (United States)
Publication Date:
OSTI Identifier:
22257098
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Fluids (1994); Journal Volume: 26; Journal Issue: 1; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; COBALT; CYLINDRICAL CONFIGURATION; DATA; EQUATIONS; FERRITE; FERRITES; FORECASTING; GEOMETRY; IRON OXIDES; LIQUIDS; MAGNETIC FIELDS; NANOSTRUCTURES; RELAXATION; SPIN; TORQUE; VELOCITY; VISCOSITY