We studied orientational fluctuations in a smectic-C Langmuir monolayer. Our measurements of orientational correlations are in excellent agreement with theoretical predictions. In addition, we present the first measurements of orientational elasticity and viscosity in a two-dimensional system whose density can be varied. The orientational viscosity strongly depends on temperature and density, changing by more than an order of magnitude with a 2.5% increase in temperature or a 20% change in density. The orientational elasticity is only weakly dependent on temperature and density.[S0031-9007 (97)03853-2] PACS numbers: 61.30.EbLangmuir monolayers, i.e., monolayers of amphiphilic molecules at an air-water interface, have long been studied as model two-dimensional systems [1]. As a function of temperature and surface pressure, Langmuir monolayers display a variety of phases; x-ray scattering [2-4] and microscopy [5][6][7][8] experiments have revealed the microscopic structure of many of these phases. Recently an amphiphilic substance was identified that exhibits a 2D smectic-C phase [8] characterized by quasilong-range orientational order and short-range positional order. The molecules in this phase are tilted from the surface normal; their projection onto the interface defines a 2D director fieldn͑r͒ ͓sin w͑r͒, cos w͑r͔͒, where w is the angle that the projection makes with an arbitrarily defined x axis. One interesting property of a 2D smectic-C liquid crystal is that there are strong orientational fluctuations inn͑r͒. Earlier studies have used freely suspended liquid crystal films as a model for 2D smectics [9][10][11][12][13]. Compared to freely suspended liquid crystal films Langmuir monolayers have the advantage that their density can be varied.The orientational fluctuations in a 2D nematic phase were studied theoretically by de Gennes [14] and by Van Winkle and Clark [10]. In particular, they obtained an expression for the mean-square difference s 2 ͑r, t͒ in orientation angle w, where s 2 ͑r, t͒ ϵ ͗jw͑0, 0͒ 2 w͑r, t͒j 2 ͘. In the absence of disclinations, the change in the free energy of a 2D nematic due to nonuniformities in n͑r͒ is given by [10]

We studied capillary wave damping in heterogeneous monolayers of triglyceride at the air-water interface over a range of surface wavelengths (70-300 pm) using a light scattering technique. In addition, we studied the monolayer morphology using a Brewster angle microscope. We found that the morphology has a strong effect on the capillary wave damping. In the gaslliquid expanded (GLE) coexistence region the monolayer forms a two-dimensional foam structure, where "bubbles" of gas phase are separated by regions of liquid expanded phase. If the width of the LE regions is smaller than the wavelength of the capillary wave, the monolayer has no measurable effect on the damping of the capillaq wave. When the width of the LE regions is larger than the wavelength, the capillary wave damping constant increases from its value for a clean water surface. We attribute this increase to a rise in the dynamic dilational elasticity of the heterogeneous monolayer.