Abstract

  Isotropic liquid droplets are formed by different mechanisms near the smectic-isotropic phase transition in smectic freely suspended films. Because of the layered structure of the smectic films, the interplay of tensions that determines the droplet geometry is essentially different from that of liquid droplets on solid or isotropic liquid surfaces. By means of interferometric measurements, the three-dimensional droplet geometry is determined. A model is proposed that describes the droplet shapes, and the relevant parameters are discussed on the basis of experimental data. We derive surface and interface tensions of low molecular mass and polymeric mesogens and provide a molecular interpretation. In particular, the small difference between surface tensions in the smectic and isotropic phases is accessible with the method; it can be related to the structure of the isotropic liquid to air interface of smectogens. Capillary forces causing a lateral motion of droplets in inhomogeneously thick films are discussed. A trapping mechanism is described that lines up the droplets at film thickness steps.

Introduction

  Smectic free-standing films represent thin ordered fluid layers that can be prepared with macroscopic lateral dimensions (up to several square centimeters film area), and a thickness between two and more than thousand molecular layers. The smectic A phase is characterized by a two-dimensional isotropic liquid disorder within the layers, where the preferred orientation of the mesogens is normal to the free-standing film plane.
  In particular, these films are well suited to study influences of decreasing dimensionality on phase transitions and surface properties of liquid and liquid crystalline materials. At the phase transition to the nematic or isotropic phase, the smectic layer order melts, and under certain conditions, one can observe the formation of nematic or isotropic liquid inclusions in the film. Although these droplets do not necessarily represent thermodynamic equilibrium structures, their lifetimes between several minutes and hours allow us to study their shape and dynamics, which provides valuable information on surface and interface tensions of the involved liquid and liquid crystalline phases. The stratified structure of the smectic films has important consequences for the geometrical appearance of the droplets.
  Equilibrium shapes of droplets in various liquid crystalline bulk phases, including the "inverse" problem of smectic droplets in an isotropic liquid bulk, and their relations to surfaces and interface tensions have been studied theoretically as well as experimentally. Similar to colloids, the isotropic droplets can interact and self-organize themselves to ordered structures. Whereas it has been shown that droplets of isotropic liquids embedded in a nematic bulk phase interact via long-range but comparably weak elastic forces of the nematic director field, the much stronger long-range forces on particles (or liquid inclusions as considered here) or between them in thin free-standing films are primarely of capillary origin. We will show that they provide a trapping mechanism that pins the droplets to film thickness steps. Because the dynamics of droplet motion is mainly determined by the supporting film and not by their inner structure, there is a direct relation to forces on solid particles suspended in liquid films, which have been described and measured, e.g., by Sur et al. . Interactions and self-organization of particles on thin organic films have been discussed in the context of biological membranes, where interactions of membrane inclusions are mediated via deformations of the membrane.
  The high stability and well-defined discrete thickness of smectic membranes makes them ideal model systems for quantitative studies of the involved viscous and capillary forces. The formation of liquid droplets in smectic films has been observed earlier, but their actual three-dimensional geometry and the interplay of forces balancing the stationary droplet shape have been disregarded so far. We will show that, from their shapes and their arrangement on thin smectic films, surface and interface tensions are accessible.   The paper is organized as follows: After a description of the preparation and experimental setup, we present experimental data of isotropic droplets for different mesogenic materials and analyze them qualitatively. Section 4 introduces models for the droplet shape, which are evaluated on the basis of the experimental results. In the fifth section, the arrangement of droplets on films with discrete, inhomogeneous thickness is described, and the role of capillary forces for the motion of droplets in the film plane is discussed. The final section gives a molecular ...