Alignment of Hexatic Langmuir Monolayers under Shear

Abstract

We have studied the structural changes that fatty acid monolayers in the Ov phase undergo when a simple shear flow is imposed. A strong coupling is revealed by the changes in domain structure that are observable using Brewster angle microscopy, suggesting the possibility of shear alignment. The dependence of the alignment on the molecular polar tilt proves that the mechanism is different than in nematic liquid crystals. We argue that the degenerate lattice symmetry lines of the underlying pseudohexagonal lattice align in the flow direction, and we explain the observed alignment angle using geometrical arguments. PACS numbers: 68.15. + e, 83.20.Hn, 83.70.Jr The structure of fatty acid monolayers at the air-water interface has been extensively studied during the past decade [1]. Of particular interest are the condensed phases in which the polar heads form a hexatic lattice, while the alkane tails that protrude into the vapor phase may be either tilted or untilted with respect to the surface normal. Observation using Bewster angle microscopy (BAM) [2][3][4] of the tilted phases shows a polydomain structure revealing long range correlations of orientational order. X-ray diffraction studies have contributed to the significant knowledge we currently have concerning the nature of both the ordering of the underlying lattice and the ordering of the alkane tails (polar tilt, azimuth͞lattice-bond coupling, and herringbone order).Classical studies of flow in monolayers focused on measurements of effective surface viscosity, with little knowledge of the effects of flow on the underlying structure. The use of recent observation techniques, such as BAM, has allowed a closer analysis of the coupling between an imposed flow and the structure of Langmuir monolayers. In recent work, Fuller's group [5][6][7] showed that a strong coupling between external flow and the orientational order of the monolayer may exist, and that the nature of the coupling differs for different tilted condensed phases. By studying the L 2 , L 0 2 , and S phases, they found that, depending on the phase, the initially rich mosaic structure may be annealed by a shear flow, so that only two reflectivities are observed, which they relate to the tails being oriented either along or against the flow. They also reported the observation of propagating fronts across domains, which they described as shear bands. By analogy with some bulk nematic phases [9], it is generally expected that the chains forming the Langmuir monolayer may align under shear like a two-dimensional nematic.In this Letter, we present experiments where shear is applied to monolayers in the Ov phase [4], which is characterized by a lack of herringbone order in the backbone plane, by an azimuthal orientation towards next-nearestneighbor (NNN) bonds, and by a lattice structure that is hexagonal in the plane perpendicular to the tails [1,8]. We find that these monolayers undergo a flow-induced alignment of the alkane tails, observable at all surface pressures and for a wide range of shear rates....

Shear-induced molecular precession in a hexatic Langmuir monolayer

“…This picture assumes that the degenerate lattice lines align with thē ow. This is the same lattice alignment that was observed as a result of shear alignment in the Ov phase 8 and is consistent with the observations reported here. The¯ow ®eld (horizontal clockwise shear) relative to the reference site (1) is illustrated by the arrows in a. a, A region that initially satis®es the next nearest neighbour (NNN) lattice-bond orientation is considered.…”

Comparison of Steady-State Shear Viscosity and Complex Shear Modulus in Langmuir Monolayers

“…The flow behavior of liquid crystal phases of monolayers of simple long-chain fatty acids confined to the air−water interface (Langmuir monolayers) is highly nonlinear. , This raises the important question of how to characterize the rheological response of these materials, as standard linear viscoelastic relationships may not apply. Recent studies of the flow behavior of these phases have focused on measuring flow profiles in channels, − domain orientation, − and the complex shear modulus, ,− G *(ω) = G ‘ (ω) + iG ‘ ‘ (ω). The complex shear modulus is defined by the relationship σ(ω) = G *(ω)γ(ω), where σ(ω) is the stress in response to a strain γ(ω).…”

Relaxation processes in Langmuir films under lateral compression

“…In particular, the MDC and SHG investigations of the 4-n-pentyl-4Ј-cyanobiphenyl (5CB) monolayers on the air-water interface during monolayer compression have been reported [1][2][3], and therefore a large data set is available for comparison with results from the theoretical routes [4,6]. In turn, the past experimental investigations of flow in monolayers during the lateral compression [7] or under shear rate [8][9][10] has been focused mainly on measurements of effective surface viscosity, with little knowledge of the effects of flow on the underlying structure. The nature of the relaxation process of the director n to its equilibrium orientation n eq in the LC film on the water surfaces is also discussed in the separate investigations.…”