Viscosity-dependent Janus particle chain dynamics.

Iron oxide (Fe3O4) Janus particles assemble into staggered chains parallel to the field lines in an ac electric field. Subsequent application of an external magnetic field leads to contraction of the staggered chains into double chains. The relation between the viscosity of the surrounding solution and the contraction rate of the iron oxide Janus particle chains is studied. Further, the influence of particle size and chain length (i.e., number of particles in chain) on the contraction rate is investigated. The base material for the Janus structure is silica (SiO2) with particle sizes of 1, 2, and 4 μm, and the cap material is Fe3O4. Addition of increasing amounts of glycerol to the aqueous system reveals that the contraction dynamics strongly correlate with the viscosity of the solution. The average chain contraction rate for each particle size can be fitted in the low viscosity range from 1 to 30 mPa·s with a power function of the form A/μ(0.9) - B/μ, in which the coefficients A and B are particle size, electric field, and magnetic-field-dependent constants. Using this function, the viscosity of an unknown solution can be determined, thereby pointing to the potential application of these Janus particle chain assemblies as in situ microviscometers.