(316h) Fluid-like Instabilities of Magnetorheological Suspensions in Toggled Fields

In a steady magnetic field, paramagnetic colloids form system-spanning, kinetically arrested networks. From this state, it is possible to phase separate and condense the suspension by toggling the external field [1]. In its evolution towards the equilibrium state, 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.  With time, the surface of the columns in the network become unstable. The amplitude of the waves eventually reaches a critical value and the columns pinch off and condense into ellipsoidal structures.

The toggle frequency controls the fluid-like nature of the breakup. By measuring the wavelength of the instability, Tomotika’s analysis of viscous thread breakup [2] is used to infer the apparent viscosity contrast of the coarsening colloidal columns. The thinning of the strands as a function of time also points to contrasting dynamics of smooth phase separations at low frequencies versus rupturing mechanisms at high frequencies. Second, 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 kinetics of self-assembly from 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.