STUDY OF FLOW IN A SMECTIC LIQUID-CRYSTAL IN THE X-RAY SURFACE FORCESAPPARATUS

We describe the newly invented X-Ray Surface Forces Apparatus (X-SFA) which allows the simultaneous measurement of forces and collective str uctures of confined complex fluids under static and flow conditions. T he structure of the smectic liquid crystal 8CB (4-cyano-4'-octylbiphen yl) confined between two mica surfaces with separation ranging from 40 00 to 20,000 Angstrom was measured. At small gaps and no shear, the sm ectic layers take on distinct stable orientations, including the bulk forbidden ''b'' orientation, which persist under low shear (gamma less than or equal to 30 s(-1)). However, at higher shear rates (gamma = 3 60 s(-1)), the shear acts to dramatically order and align the smectic layers into a single ''a'' orientation.