Flow-orientation effect in batch-produced Langmuir-Blodgett films: Observation of the unsteady flow around the stagnation point

The in-plane anisotropy of the Langmuir-Blodgett films of a merocyanine-fatty acid mixed system has been further studied for two different cases of batch production with the substrates aligned face to face and side by side, respectively, to clarify the limitations of the flow-orientation model based on the ideal-fluid approximation. For the side-by-side case, satisfactory coincidence is found between the model and experiment. The actual dichroic behavior deviates from that predicted for the face-to-face case with smaller intersubstrate distances. It was speculated that the stagnation point is associated with anomaly in velocity which is responsible for this discrepancy. In order to examine this, the flow of the monolayer on the water surface during the deposition has been observed using sulphur powder as marker in addition to the optical measurements. The actual flow deviates from that expected from the model and is associated with unsteady motion around the stagnation point as previously speculated. It is indicated that the ideal-fluid approximation is inappropriate for the face-to-face case with smaller distances, leading to the prediction inconsistent with the actual dichroic behavior.