Paper Number
SC14 

Session
Suspensions and Colloids

Title
Rayleigh-Plateau instability of magnetorheological suspensions in toggled fields

Presentation Date and Time
October 6, 2014 (Monday) 5:15

Track / Room
Track 1 / Commonwealth A

Authors (Click on name to view author profile)

1. Bauer, Jonathan L. (University of Delaware, Department of Chemical and Biomolecular Engineering)
2. Swan, James W. (Massachusetts Institute of Technology, Department of Chemical Engineering)
3. Furst, Eric M. (University of Delaware, Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Jonathan L. Bauer¹, James W. Swan², and Eric M. Furst^1
1Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716; ²Department of Chemical Engineering, Massachusetts Institute of Technology, Boston, MA 02139

Speaker / Presenter 
Bauer, Jonathan L.

Text of Abstract

In a direct magnetic field, paramagnetic colloids form system-spanning, kinetically arrested networks. It is possible to phase separate and condense the suspension by toggling the external field [1]. In its structural evolution, the suspension undergoes a Rayleigh-Plateau instability for a range of field strengths and toggle frequencies. The particles initially chain together to form a percolated network that coarsens diffusively. As time progresses, the surface of the columns in the network become unstable. When the amplitude of the waves reaches a critical value the columns pinch off and condense into ellipsoidal structures.

First, we study the effects of toggle frequency and field strength on the breakup dynamics. The frequency controls the fluid-like nature of the breakup. By measuring the wavelength of the instability, we use Tomotika’s analysis of viscous thread breakup [2] to infer the apparent viscosity contrast of the colloidal columns. Second, we characterize the thinning of the strands as a function of time [3-4]. These point out contrasting dynamics of smooth phase separations at low frequencies versus rupturing mechanisms at high frequencies.

Second, we demonstrate that the field strength controls the surface energetics of the columns and thus the timing of the instability. At a given frequency, there is a critical field strength at which breakup occurs. Increasing the field strength causes the instability to proceed more quickly. The data scales onto a master curve that predicts the time necessary for breakup to occur as a function of field strength relative to the critical field strength. These results can be used to estimate the self-assembly kinetics of polarizable colloidal suspensions in a toggled field.

1. J.W. Swan et al., Soft Matter, 10, 1102-1109, 2014.
2. S. Tomotika, Proc. Royal Soc. London A, 150, 332-337, 1935.
3. G.H. McKinley and A. Tripathi, J. Rheol., 44, 653-670, 2000.
4. K. Niedzwiedz et al. Rheol. Acta, 49, 1103-1116, 2010.