Fluid Oscillation in the drop tower
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I.
INTRODUCTION
OBSERVATIONS of interfluid menisci formed in an axisymmetric container during a step reduction in acceleration are presented. Evidence of a wetting transition was sought by analyzing meniscus characteristics. Fluid wetting, pressure, and container geometry determine meniscus shape. A reduction of gravity reduces the pressure in the fluids. This was done by using the Marshall Space Flight Center (MSFC) Drop Tower facility. By unloading the hydrostatic pressure in this controlled way, interface oscillations are started and interfacial forces are no longer masked by earth gravity. Damping of the oscillations occurs during the period of microgravity as a result of viscosity and of the dissipation of energy at the contact line where the menisci meet the container. This will be discussed later. Microgravity conditions extending up to three seconds (of the 4.5 second drop) are generated for the experiment.
II.
CRITICAL W E T T I N G T H E O R Y
In 1977, Cahn uJ introduced the Critical Point Wetting (CPW) theory. Verification of the CPW theory is the main purpose of the experiments. Cahn predicted an abrupt transition from partial wetting to complete wetting could occur between two fluid phases against a third, inert phase during heating to the consolute temperature. Between a wetting temperature (Tw) below the consolute (or critical) temperature (To) and the Tc of a two-fluid system, one fluid phase is expected to preferentially wet another inert phase (e.g., container or vapor phase). The non-wetting phase will lose contact with the inert phase completely. Cahn showed CPW has universal applicability to any immiscible system. To describe fluid wetting a solid surface, Young's equation is used. In this equation, the interfacial free energies, o-, of the two fluids, 1 and 2, and the solid surface S, are related through a contact (or wetting) angle O. 0"1, 2 COS 0 = 0"1,S -- 0"2, S
[1]
WILLIAM F. KAUKLER is Associate Research Professor, Department of Chemistry, The University of Alabama in Huntsville, Huntsville, AL 35899. This paper is based on a presentation made in the s y m p o s i u m "Experimental Methods for Microgravity Materials Science Research" presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.
METALLURGICAL TRANSACTIONS A
A result of the full wetting condition when above Tw, from the solution of Young's equation, tlJ is the zero contact angle formed by the interfluid interface and the noncritical surface (glass). Below the Tw, partial wetting and thus, nonzero contact angles are expected. This phenomenon is difficult to observe in earth gravity due to the sedimentation effect which causes the more dense phase to lie under the less dense phase, often with an essentially flat meniscus. Contact angle evidence of the wetting transition is obscured by gravity. Other factors such as Tw existing below the melting point and some T~'s being very close to Tc hav
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