The formation of Langmuir-Blodgett films

The technology of the formation of LB films has much in common with the trough technology discussed in Chapter 3. In addition to the equipment described there, one needs a mechanical device to raise and lower the substrate through the air/water interface at a predetermined rate. Various devices have been employed but it is usual to provide the vertical movement by driving a large micrometer screw by an electric motor via a reduction gear train. Practical velocities are such that they are usually measured in millimetres per minute. It is essential to be able to vary the dipping rate as there is an upper effective rate which can be employed for any particular material. This rate is determined by the speed at which water drains from the film as it is withdrawn from the subphase and by the viscosity of the film and hence the rate at which material can approach the substrate. For a material of high viscosity this procedure is difficult to carry out properly and a substantial difference in pressure may occur between the pressure sensor and the region immediately in contact with the substrate. In fully automated troughs the substrate is withdrawn from the subphase and maintained in this position while the film at the air/water interface is respread and compressed to a predetermined dipping pressure. In such systems it is necessary to carry through a cyclic compression and expansion process several times to arrive at a good approximation to an equilibrium situation before the dipping process is reactivated.