(728e) External Field Directed Co-Assembly of Rigid and Ultra-Flexible Permanent Chains

Authors: 
Bharti, B., North Carolina State University
Fameau, A. L., National Institut of French Agricultural Research
Findenegg, G. H., Technical University Berlin
Velev, O. D., North Carolina State University



Assemblies of colloidal particles are key components in the development of microscopically and nanoscale patterned or ordered materials. The most popular and widely used technique for tailoring linear structures is the application of an external field (electric or magnetic) to the colloidal dispersion. Application of external AC-electric or magnetic field causes a rapid arrangement of colloidal particles into linear chains by dielectrophoresis (DEP) or magnetophoresis (MAP). DEP (or MAP) induced structures resulting from similarly charged microparticles are not permanent, because of their surface charge repulsion, and last only as long as the field is present. We present two novel approaches to use DEP and MAP induced chaining for the formation of permanently linked structures. For achieving DEP assembly of rigid and permanent chain structures of dielectric particles, we used a bi-particle system, i.e. a dispersion of two types of colloidal particles having opposite surface charges. By replacing similarly charged micro-particles by oppositely charged binary mixtures, we introduce a short-range strongly attractive potential. DEP is then used to drag the particles near the surface contact point, where the strong attractive electrostatic potential binds them together and the structures formed survive even when the electric field is switched off. The results of the field-driven process depend on the size ratio as well as the number ratio of the two interacting species. We derived assembly rules for the resulting structures and a statistical model that is in excellent agreement with the experimental results. The oppositely charged biparticles yield permanent but stiff chains. In order to fabricate chains with soft and flexible connectors we used molecularly self-assembling fatty acid as a new type of binding agent. We fabricated chains of FeO nanoparticles connected via soft linkers using an initial burst of magnetic field. The composite chains subjected to 1D low frequency alternating magnetic field (0.1 Hz) showed an instantaneous response by reorienting themselves in the field following the minimum energy constraints. In addition, we showed that in a modulated alternating field, chains can reversibly wind/unwind into rings, hooks and other type structures. To the best of our knowledge, this is the first study where ultra-flexible magnetic/soft matter responsive chains have been shown to form and respond in a robust and reversible manner. Such structures could possess unusual magnetic, electric and rheological properties, which can be used in new functional soft materials.