Surface Tension Driven Flow in Glass Melts and Model Fluids
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SURFACE TENSION DRIVEN FLOW IN GLASS MELTS AND MODEL FLUIDS
THOMAS J. MCNEIL, ROBERT COLE AND R. SHANKAR SUBRAMANIAN Department of Chemical Engineering, Clarkson College of Technology, York 13676
Potsdam,
New
ABSTRACT Experiments on surface tension driven flow in nominally cylindrical columns of a glass melt and silicone oils are described and results are presented. Predictions from an appropriate theoretical model are included. Conclusive proof has been obtained for the dominance of surface tension driven flow in these systems under the conditions of the present investigation.
INTRODUCTION The term "Thermocapillary Convection" refers to flow induced by gradients of interfacial tension on a free liquid surface when such gradients are the result of temperature variations. While surface tension variations also can be caused by composition and/or electrical charge density gradients, a temperature gradient is more convenient to induce and maintain in a system than one of the latter kinds. Surface tension driven flows have been recognized for many years as playing an important role in a number of applications. Excellent review articles on this subject have been written by Scriven and Sternling [1] and Levich and Krylov [2]. While traditionally the interest has been in systems involving liquid dispersions and gas bubbles such as extraction and distillation operations, in recent years new impetus for the study of such flows has come from crystal growth and space-processing applications. For instance, the theoretical calculations of Chang and Wilcox [3,4J have demonstrated the role of surface-tensiondriven flows in the floating-zone technique for the growth of silicon crystals. Experimental confirmation in model zones of sodium nitrate has been provided by Schwabe et al. [5,6] while Chun and coworkers [7,8] have performed studies in silicone oil half-zones. The processing of materials in orbit is another area in which surface-tension forces will be of paramount importance. Under the near free fall conditions in orbit, due to the reduced gravity levels (on the order of 10- 3 g or lower), buoyancy effects are expected to be overshadowed by surface tension effects in the presence of free liquid surfaces. It has been suggested that the facility in orbit to process materials without a container can be used to produce high technology glasses which are difficult or impossible to make on earth (9,10]. Such materials undergo heterogeneous nucleation on the container walls leading to devitrification thereby making it impossible to form glasses. Examples include certain rare earth oxide optical glasses [11i and laser glasses. Also, it has been suggested that ultrapure materials such as optical waveguides be processed in orbit to avoid contamination by container walls. In the manufacture of glasses, buoyancy makes a significant contribution both in the mixing and homogenization of the melt and in the elimination of gas bubbles (fining). To perform these operations in orbit, it is necessary to seek alternative mechanisms. At
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