Alignment of Complex Fluids under Confinement and Flow

Abstract

We report on a new application of the X-ray Surface Forces Apparatus [1][2][3][4] (XSFA) to the study of the alignment of complex fluid thin films under confinement and for flow using synchrotron X-ray scattering. The smectic A liquid crystal BCB (4-cyano-4'-octylbiphenyl) and the hexagonal phases of a zwitterionic polyisoprene melt and of a lyotropic liquid crystal composed of sodium dodecyl sulfate (SDS), pentanol and water were investigated in the regime where the distances between the confining surfaces range from o.4 to tens of microns. In this paper, we demonstrate that this new technique allows preparation of highly-oriented complex fluid samples suitable for high resolution X-ray diffraction and scattering studies. In addition, we have directly measured forces across thin films of BCB using the conventional Surface Forces Apparatus (SFA). The force measurements complement well the direct X-ray imaging and provide indirect information on the sample alignment in films a few tens of lngstr6ms thick. 1.

An X-ray scattering study of flow-aligned samples of a lyotropic liquid-crystalline hexagonal phase

“…Some hexagonal mesophases spontaneously grow into single domains or may be oriented by a magnetic field [6] but these are rather exceptional cases. Sophisticated techniques such as "strand pulling" [7], the "X-ray Surface Force Apparatus" [8] or shear cells [9] have already been successfully applied to produce aligned samples but simpler methods are clearly needed in order to easily invesa e-mail: imperor@lps.u-psud.fr b Laboratoire associé au CNRS. tigate a larger number of hexagonal mesophases.…”

The x-ray surface forces apparatus for simultaneous x-ray diffraction and direct normal and lateral force measurements

“…4 In the initial experiments, the XSFA was successfully used to study effects of shear and confinement in liquid crystalline films. [5][6][7] These experiments demonstrated the unique advantage of combining the SFA methodology with in situ x-ray small angle scat-tering, which provides direct information on molecular arrangements in the confined sample. However, the first generation XSFA did not permit any direct normal or lateral force measurements to be made simultaneously with XRD experiments.…”

Direct Observation of Shear-Induced Orientational Phase Coexistence in a Lyotropic System Using a Modified X-Ray Surface Forces Apparatus

“…The first generation of the x-ray surface forces apparatus (XSFA), adapted directly from the SFA-III surface forces apparatus (SFA) [5], allowed for the first time x-ray structural measurements to be carried out on confined complex fluid samples using a synchrotron source [6]. In the initial experiments, the XSFA was successfully used to study effects of shear and confinement in liquid crystalline films [7][8][9]. These experiments demonstrated the unique advantage of combining the SFA methodology with in situ x-ray small-angle scattering, which provide direct information on molecular arrangements in the confined sample.…”

“…A side port (not shown) was used to inject the sample in between two molecularly smooth mica surfaces. The back of the mica sheets was coated with a semitransparent silver layer, attached to ultrathin quartz sheet supports and mounted onto curved aluminum disks fitted with 500 mm pinholes [7]. For measuring the sample thickness T , the fringes of equal chromatic order obtained were analyzed in a portable grating spectrometer and recorded using a low-light silicon intensified tube camera.…”

“…The XSFA-II was used for studying a four-component lyotropic model lubricant system composed of sodium dodecyl sulfate [SDS (surfactant)], pentanol (cosurfactant), dodecane (oil), and water. This system has a well-characterized, rich phase diagram, which allows one to access a variety of molecular structures by simply changing the composition [1,7]. The data shown below were obtained from a lamellar (L a ) phase sample which was composed of 28.18% SDS, 20.91% petanol, 7.19% dodecane, and 43.72% water (in percent by weight).…”