Probing the Directional Properties of Liquid Crystals Via Actuating Assemblies of Microcubes

Microbots, which we define as field-powered actuators consisting of self-assembled and magnetically responsive microcubes, show big promise for probing the viscoelastic properties of fluids and microscopic materials such as cells and vesicles. One system of particular interest is complex fluids such as liquid crystals, which have different viscoelastic properties depending on the direction of the applied force. We have prepared a class of 10 μm cubes that possess a ferromagnetic coating along one face that, when exposed to a magnetic field, assemble into chains that can reversibly reconfigure when the applied field is removed. Based on the sequence of cubes, these “microbots” display a wide range of actuation patterns. These chains are typically comprised of different sequences from two fundamental types of base segments: the flexible “AA” segment and the rigid “AB” segment. In the AA segment, the magnetic faces are aligned end-to-end and the cubes are on an adjacent side, creating a hinge that allows the cubes to fold when the field is removed. In the AB segment, the magnetic faces are also aligned end-to-end, but the cubes are on opposite sides of the magnetic film, thus retaining their configuration when the field is removed. We focus our study on the so-called “ABBA” sequence, consisting of two rigid AB configurations joined by a flexible AA joint allowing for reversible actuation, as the two “AB” ends sterically prevent irreversible folding of the “AA” hinge. In this work, we show that the speed of folding of these actuations is dependent on the viscosity of the fluid medium and the strength of the magnetizing field. We investigate the angular velocity of ABBA microbots in isotropic solutions of polyvinyl alcohol, which have known viscosities, and we infuse a fixed amount of energy into the microbots such that they close with equivalent force upon each actuation. We use these findings to assess the apparent viscosity of 5CB liquid crystals along three axial directions of the liquid crystal. Looking forward, we believe these microbots could be used as a tool to probe the properties of complex fluids, extending beyond liquid crystals, for which the viscosity changes depending on the direction or speed of the applied force.