We propose a research field "mechanics of liquid crystals," in which liquid crystals are studied from a mechanical viewpoint. The unsteady behaviors of a liquid crystal between parallel plates under an electric field are investigated. The imposition of the electric field on the liquid crystal induces flows, whose profile and magnitude strongly depend on the twist angle of the director at the plates. A visualization experiment confirms the generation of flows. The mechanism of such generation can be explained by considering that the rotation of molecules generated by the imposition of the electric field induces a local velocity gradient. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2840673͔Since the discovery of the applicability of liquid crystals to display devices in the 1960s, many studies on liquid crystals have been carried out both fundamentally and practically. As a result, liquid crystal displays have established a strong position as a substitute for cathode ray tube displays. The manufacturing of such displays has grown into a large industry with a global scale and further development is expected. However, the technical development has reached a plateau except for a few trivial improvements.A research field, such as "mechanics of liquid crystals," should exist and various studies concerning liquid crystals should be performed from a mechanical viewpoint, because in mechanical engineering, solid mechanics and fluid mechanics are established in the fields of solid, and gas and liquid, respectively. However, there is no such research field, while there are some studies where a liquid crystal is regarded as a fluid. [1][2][3][4] Figure 1 shows the conceptual applications of liquid crystals; that is, we input some factors, such as electromagnetic field, flow, heat, and light, into liquid crystals and output different factors by utilizing the easy change in orientation state, which is the most representative feature of liquid crystals. An example of such applications is a liquid crystal display in which an electric field is input and a change in transmitted light is output. We can regard a liquid crystal as a medium of converting energy by combining input and output factors appropriately. The applications of liquid crystals are assumed to increase by changing the combination of input and output factors or by proposing other factors.Although it is well known that the imposition of a velocity gradient makes liquid crystalline molecules rotate and reorient to a different direction, its reverse phenomenon, such that the rotation of the molecules ͑by imposing, for example, an electric field͒ induces a velocity gradient, is not clearly understood. Since this phenomenon corresponds to the energy conversion from electrical to kinetic energies, to utilize such induced flow leads to development of microactuators. Some studies 5,6 were made for the generation of a flow, which was called "back flow;" however, in these studies, the mechanical applications of the back flow were not considered, although its effect ...
