Interactions between the liquid crystal molecules and solid surfaces can be manipulated to control the molecular alignment, for example to orient all the liquid crystal molecules in a container in a particular direction. Conversely, if a small particle is suspended in a liquid crystal, these liquid-solid interactions can be exploited to manipulate the particle. In recent work, we have shown that it is possible to employ these effects to orient micrometer sized, needle-shaped metal particles in a liquid crystal and even to levitate them against the force of gravity.

Levitation of a nickel wire within a twisted nematic liquid crystal (LC). A: The LC orientation (green), twists uniformly as a function of height to match the directions of planar anchoring at the upper and lower surfaces. When a magnetic field aligns a nanowire parallel to LC at some height in the cell (e.g., at the center of the cell in the figure), the elastic energy becomes a function of the wire's height, leading to a levitating force on a wire resting on the bottom substrate. B: Measurements demonstrate the height of a wire initially resting at the bottom rises as a function of time in response to this force.

These effects arise because in our system the liquid crystal molecules prefer to lie at the particle surface parallel to the particles' long axis. Thus, the minimum energy configuration for these elongated particles occurs when the particles point parallel to the liquid crystal alignment direction. Rotating a particle away from this direction costs elastic energy as it produces a distortion in the orientation of the liquid crystal near the particle. As we have shown, using a magnetic nanowire forced by strong magnetic fields to point in various directions in a liquid crystal, the size of this energy cost can be understood through an elegant analogy with the basic laws of electrostatics. Further, when the field reorienting the particle is removed, a torque on the particle from the liquid crystal swings the particle back into alignment.