Ellipsometric profiles of nematic microdroplets

Thickness profiles of microdroplets of the very common 5CB liquid crystal on bare, oxidized silicon wafers appear on Fig. 5.6. The nematic-isotropic (Nl) transition temperature Tni is close to 35°C. The scenario is complex, both at the molecular and at mesoscopic scale. 

A remarkable feature is the vertical part on the thickness profiles, whose upper boundary (UB) and lower boundary (LB) are fixed at a given temperature, whatever the drop volume and time. On a given profile, the part above the UB is smooth. Steps are visible below the LB in a narrow range of temperature. This corresponds to a complex shape of the disjoining pressure a Maxwell construction leads to a horizontal part between the LB and the UB, defining a forbidden range of thickness. Below the LB, steps, if any, reveal surface-induced layering and therefore a smectic-like structure. 

Far below Tni (Fig. 5.6a), ellipsometric measurements agree with X-ray reflectivity to show that the LB is a trilayer of molecules. The trilayer is built as the superposition of a monolayer with ferroelectric order (the molecules are parallel to each other with the same orientation) and a bilayer of molecules with head-to-foot orientation. 

Closer to the transition, the number of bilayers on top of the monolayer increases (Fig. 5.6b,c), and then the profile becomes 

The height of the vertical part in the profile increases when the Nl transition is approached, like in a wetting transition, which is actually the case the isotropic phase above Tni does not wet the wafers. What is specific here is that both LB and UB increase close to Tni (Fig. 5.7). 

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