Engineering magnetorheological fluids with an external field is motivated by building colloidal structures with an enhanced packing and periodicity. Superparamagnetic colloidal particles are assembled in toggled magnetic fields by switching on and off the field repeatedly. At sufficiently high field strengths, the suspension begins to form a microstructure, and its fluidic behavior experiences a solid to gel-like state transition. The suspension is then further condensed into droplet-like domains by a competition between the time-averaged bulk and surface energies. In this study, the effect of the duty cycle of the toggled field is investigated where four general structures are observed after enough assembly time is given: fluid, columnar, percolated, and ellipsoidal-shaped structure. Two distinct kinetic pathways are observed. The microstructure forms the ellipsoidal state by diffusion-driven particle aggregation in an early stage and the growth rate increases at a breakthrough time. By contrast, for columnar- and percolated-states, initially aggregated structures are arrested and no rate transition occurs. Interestingly, under a small range of assembly conditions (1.5-3.5 Hz, 0.2 duty cycle ratio), a new class of wavy-shaped microstructures is formed which exhibits the rotational emergent dynamics with continuous coalescence and breakup processes. These dynamics are analyzed by toggling external field that causes an asymmetric combination of ballistic and diffusive motion.

Effect of duty cycle on self-assembling superparamagnetic colloids in the toggled-field

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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