Imaging of Surface-Tension-Driven Convection Using Liquid Crystal Thermography

Abstract

Surface-tension forces can drive fluid motion within thin liquid layers with a free surface. Spatial variations in the temperature of the free surface create surface tractions that drive cellular motions. The cells are most commonly hexagonal in shape and they scale on the thickness of the fluid layer. This investigation documents the formation of cells in the liquid film in the presence of a uniform-heat-flux lower boundary condition. Liquid crystal thermography was used to image the cells and measure the temperature distribution on the lower surface of the liquid layer. A 1.1 mm deep pool of silicone oil was supported on a 50 μm thick electrically heated metal foil. The oil was retained inside an independently heated acrylic ring mounted on the top surface of the foil and a dry-ice cooling plate served as the low-temperature sink above the free surface of the oil. Color images of hexagonal convection cells were captured using liquid crystal thermography and a digital image acquisition and processing system. The temperature distribution inside a typical cell was measured using thermographic image analysis. Experimental issues, such as the use of an independently heated retaining ring to control the height of the liquid film and the utility of a flux-based Marangoni number are discussed.

Conjugated liquid layers driven by the short-wavelength Bénard–Marangoni instability: experiment and numerical simulation

“…Rough estimates based on a convection-free system suggest that the deformation of the polymer film leads to temperature variations at the liquid-gas interface in the order of 20 − 100 millikelvins. This is well within the regime of temperature variations observed in conventional BM-convection in a single liquid layer (Dutton et al 2010). No matter which of the presumed mechanisms dominates, given that the polymer film is a solid after UV-treatment, the complete alignment between the convection cells and the polymer film structure must indeed be caused by the deformations of the polymer.…”

“…While the convection rolls in the upper liquid layer shear and deform the liquid-liquid interface, the resulting deformations, albeit small compared to h 02 , in turn significantly affect the temperature distribution at the liquid-gas interface. This occurs because the temperature differences along a convection cell at the liquid-gas interface are within millikelvins only (Dutton et al 2010) and are sensitive to the deforming liquid-liquid interface. To demonstrate this, the regular multi-layer setup was first used to induce deformations of the liquid-liquid interface, similar to what is shown in figures 3(b) and 4.…”