A Shear-Induced Instability in Freely Suspended Smectic-ALiquid-Crystal Films

Abstract

This experiment examines the stability of a smectic-A liquid-crystal film subject to a uniform shear flow with velocity and velocity gradient in the plane of the smectic layers. The experiment is made possible by virtue of a freely suspended liquid-crystal film which is sheared at its edge by a Couette apparatus. It was found that while there was no shear-induced phase transition, line defects were nucleated in the film when the shear rate ᠨ g attained a certain threshold. The critical shear rate ᠨ g c was measured as a function of temperature and film thickness.[S0031-9007 (97)03898-2] PACS numbers: 61.30.Gd, 47.20.Hw, 61.30.Jf, 64.70.MdElongated molecules are capable of forming various mesophases with different translation and rotation symmetries. Smectic-A (Sm-A) is unique among these phases in that it has a layered structure with molecules aligned parallel to the surface normal. Since there is no translational order, molecules within the layers behave like a twodimensional liquid. The purpose of this investigation is to explore the effects of shear on this simply structured fluid, with emphasis on defect generation and evolution of these defects in the presence and absence of the flow field.That Sm-A is a strongly anisotropic fluid raises interesting questions concerning stability of layer orientations, characterized by their surface normal, with respect to an imposing velocity field. This question was addressed experimentally by Safinya et al. for a sheared thermotropic liquid crystal (LC) using synchrotron x-ray scattering [1]. Specifically, they examined how molecular orientations, which were well known in the nematic phase under shear flow [2], transformed when the smectic phase appeared. It was found that while the average orientation of the molecules, called the directorn, was shear aligned along the velocity direction deep in the nematic phase, it switched to a new configuration withn parallel to the vorticity direction in the vicinity of the nematic to SmA transition. The sudden change took place when the shear rate ᠨ g became comparable to the incipient smectic-order fluctuation time t j of domain size j. More recently simultaneous rheological and structural measurements have been carried out by Panizza et al. [3]. These measurements seemed to suggest that smectic layers could also form multilamellar cylinders oriented along the flow at low shear rates, whereas the layer's normaln became parallel to the vorticity direction at high-shear rates similar to that found by Safinya et al. Though it is intuitively reasonable that this so-called a configuration, i.e., with the velocity and velocity gradient in the plane of smectic layers, has a relatively high stability compared to other configurations due to minimal distortions of the smectic order as well as the director fieldn, it gained theoretical support only recently [4,5].We have observed a striking effect that seems to indicate that even in the a configuration, shear can cause an instability which is not captured by linear theory [4,5]. The a configu...

Bifurcations in annular electroconvection with an imposed shear

“…This was comparable to the film width d, so that the air drag on the film was negligible. [9] All experiments were performed at the ambient room temperature of 23 ± 1…”

“…Uniform suspended smectic films are always an integer number of smectic layers thick and while they flow readily, they seldom change thickness; they are robustly two-dimensional. Unlike soap solutions [9,10], smectic liquid crystals have very small vapor pressures, hence the film can be enclosed in an evacuated environment. The reduction in ambient pressure leads to a proportional reduction in the air drag to which the film is subjected.…”

“…The reduction in ambient pressure leads to a proportional reduction in the air drag to which the film is subjected. [9] This system thus has several attractive features for the study of bifurcations in simple nonlinear systems that may be described by amplitude equation models. Our study greatly extends and complements previous work on electroconvection in isotropic liquids [11][12][13][14][15][16][17], nematics [18,19], smectic A [3,6,7,[20][21][22][23][24][25] and higher smectic phases [26][27][28][29].…”

Observations of the Collapse of Dilute Lyotropic Lamellar Phases under Shear Flow

“…Our work is possible using a recently developed Couette cell which allows for shearing thin films of material in the above mentioned flow configuration. So far the effect of shearing fluids in such a geometry has focused on smectic-A liquid crystals and soap films [3,9]. Experiments using smectic-A liquid crystal films [3] have reported a strong increase of turbidity above a certain shear rate; this transition is accompanied by the appearance of several defects in the film.…”

Surfaces and Interfaces of Free-Standing Smectic Films

“…Because of the coupling of the c-director to shear flow in tilted smectic phases, dynamic structures can be generated by mechanical excitation as well [122][123][124]. Even the first-order phase transition between SmA-and SmC * -like structures in freely suspended films can be induced by transient changes of the film tension [125].…”