(358f) A New Class of Colloidal Swimmers Based on Magnetically Actuated Assemblies of Metallo-Dielectric Microcubes

Authors: 
Velev, O. D., North Carolina State University
Bharti, B., North Carolina State University
Shields, C. W. IV, Duke University
Lopez, G. P., University of New Mexico
Arratia, P. E., University of Pennsylvania
We present results and analyze the origins of motility of a new class of externally powered active clusters made by magnetic assembly and actuation of metallo-dielectric microcubes. We have previously shown how metallo-dielectric Janus/patchy spheres and microcubes acquire complex polarization patterns in magnetic fields, leading to multidirectional interactions and assembly into microclusters and microchains. The field-induced polarization of the ferromagnetic facets leads to dipole-dipole and field-dipole interactions and reconfiguration of the neighboring microcubes, which is directed by their conformational restrictions. In effect, the external field energy enables programmed cluster dynamics. The reversible magnetically controlled folding and unfolding of these clusters can be used to create novel active structures. In this talk, we show how pre-assembled microcube clusters form the basis of a new class of microswimmers. These assembled micro-cube clusters are able to achieve net motion if (i) the stroke pattern is made non-symmetric in time (by adjusting magnetic field sequence) and (ii) the fluid medium (i.e., polymeric solutions) possesses non-Newtonian rheological properties such as shear-thinning viscosity. The magnetically powered assemblies can be moved dynamically back and forth by inverting the time-asymmetric stroke sequence (e.g., rapid opening and slow closing vs. slow opening and rapid closing) or made to stay in place by applying time-symmetric fields. Surprisingly, the direction of their motion is dependent on the cluster sequence and the length of the re-folding chain. We believe that this behavior is connected to polymeric fluidâ??s extensional viscosities, which is known to be dependent on strain. The results will be interpreted on the basis of a model of â??coupled scallopsâ? configuration projected upon the metallo-polymer clusters. We will discuss the potential of making more complex dynamic structures that could self-propel in various complex fluids such as liquid crystals.
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