Magnetic Janus Particle Aggregation Rate at Long Times

Magnetic Janus particles are spherical colloids coated asymmetrically on a single hemisphere with a thin iron oxide cap (cap volume approximately 1% of particle volume). The iron oxide gives the particles a magnetic dipole moment shifted away from the center of the particle. When placed in a uniform magnetic field, these particles aggregate into aligned chains, macroscopically creating a magnetorheological fluid. The rate of this aggregation is an essential component in understanding the fluid’s properties.

At long times, the system switches from long chains mainly aggregating with small chains to long chains aggregating with each other and small chains at equal probabilities. The average chain length at these long times follows a power law relationship with time:

< l > ~ t ^ z

Here, we quantify when the system reaches these “long times” by drawing analogies with polymer chain growth statistics. “long time” data from experiments and simulations are used to find the z-coefficient. Contrary to previous results, the coefficient is found not to be limited to the range 0.6<z<0.7, but instead ranges from 0.4<z<0.8 and is found to depend on concentration and dipole strength.