(427a) Iron Oxide Janus and Patchy Particles

Iron oxide occurs in many oxidation states. Each iron oxide carries distinct magnetic and electric properties thereby creating a rich set of materials that can be explored in the field directed assembly of colloids. Over the past few years, we have studied the preparation, properties, and assembly of iron oxide capped Janus particles. Varying the deposition rate of iron in the presence of an oxygen:argon background mixture leads to the formation of three distinct iron oxides; Fe_1-xO, Fe₃O₄, and a-Fe₂O₃. Janus particles with Fe_1-xO and Fe₃O₄ caps respond to magnetic and electric fields by assembling into well-defined staggered and double chain structures, which has led to their application in an in situ micro-viscometer. Careful study of the chain assembly revealed that chain formation occurs via the formation of Janus particle doublets, which have been shown to show pre-alignment depending on whether they will form staggered or double chains. In addition, we have shown that the analysis of the doublet formation process gives access to the magnetic strength of the cap material. In order to explain the distinct assembly behavior of Fe_1-xO and Fe₃O₄, we have developed a simple dipole-dipole model which will be introduced in this talk. Using the model, we predict conditions at which a Janus particle chain should open and close reversibly and subsequently present Janus particles that show reversible chain expansion and contraction upon assembly with an external magnetic field and actuation with an electric field.