Reconfigurable responsive structures assembled from magnetic Janus particles†‡

Responsive materials are used in many diverse applications including actuators, sensors, tunable viscosity liquids, and displays. In addition to materials with controlled properties, a number of ‘‘smart’’ building blocks are promising as components of responsive assemblies. Electroand magnetorheological fluids rely on such assemblies for their dynamic change in viscosity. The design of building blocks with novel and directional interactions has resulted in nanoparticle superlattice crystals based on light-induced dipole-switching, and colloid assemblies by AC electric field-induced polarization. The response of a material or structure is usually coupled to a change in the environment (temperature, pH, applied field) and when such a change occurs naturally, responsive structures can be used to detect it. In many cases, however, an environmental change is maintained solely to keep a structure/material in a given state, which wastes energy or requires good insulation from the surroundings. Some conditions required to sustain a certain structural response (e.g., electric fields of 1–2 kV/mm for electrorheological fluids) are also impractical to maintain for long periods of time. A solution to this problem can be found in bistable (or multistable) systems where the states of interest can occupy local potential energy minima, with high activation barriers between them prohibiting interconversion. Such macroscopic switches and latches are explored in energy-efficient robotics designs. Microscopically, such designs are found in MEMS actuators and optical switches. Miniaturization also requires one to take advantage of forces that dominate on the micron scale and below (magnetic, electrostatic, van der Waals), resulting in different device designs and opportunities. When bistability is engineered at the level of the building blocks, the whole structure formed by the blocks can assemble and disassemble in a bistable fashion.