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Assembly behavior of Janus particle in a magnetic field

Magnetic particles have potential applications in position sensing, ultrahigh density magnetic sstorage media, clinical diagnosis and treatment, biological labeling, and separation applications. Janus particles exhibit an asymmetric magnetization making them suitable building blocks for directed three-dimensional assembly in external magnetic fields.

Figure 1.

Iron oxide capped Janus particles are prepared via physical vapor deposition of nanometer thin iron films onto a polystyrene or silica particle monolayer in an oxygen/argon atmosphere. Subsequently, the aqueous suspension of Janus particles is exposed to a magnetic field and monitored for 30 min. Three assembly behaviors are observed: (i) staggered chains, (ii) double chains, or (iii) no assembly. Parameter that may affect the assembly behavior such as deposition rate, iron oxide cap thickness, particle volume fraction, and assembly time are studied. A deposition rate diagram has been developed relating deposition rate and assembly behavior. The cap materials are determined by the physical properties of the iron oxide thin films as follows: Fe1-xO leads to staggered chains, Fe3O4 leads to double chains and α-Fe2O3 shows no assembly. More recently, we have focused on determining the mechanism of Janus particle chain formation and contraction in parallel electric and magnetic fields.

Figure 2.

In our experiments, the AC electric field is introduced first to assist formation of staggered Janus particle chains. Subsequent application of a strong external magnetic field contracts the staggered chains into a double chain structure. We have studied the relation between the viscosity of the surrounding solution and the iron oxide Janus particle chain dynamics as a function of particle size and chain length. The solution viscosity is adjusted by addition of specific amounts of glycerol to our aqueous system. We observes that chain dynamics are strongly correlated with the viscosity of the solution thereby pointing to the potential application of these Janus particle chains as in situ micro-viscometers.

Figure 3.

Current Graduate Students: Bin Ren


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• This work was supported in part by