Rheological studies of surfactant-coated drops in microgravity.

Abstract

Measurement of the rheological properties (viscosity, tension, elasticity) of liquid surfaces and interfaces is a task that is complicated by configuration and unwanted contamination. Acoustic levitation can partially alleviate the configuration and contamination problems by isolating the test fluid as a drop in air or another host liquid. However, the drops are unavoidably deformed from a spherical shape by the intense ultrasonic radiation force required to balance the gravitational force. Use of sonic fields to position drops in a microgravity environment can provide a spherical drop in a contact-free environment, making experiments easier to perform, and enabling comparisons with the solution of theoretical models in which a simple geometry can be assumed. The First United States Microgravity Laboratory (USML-1) mission, which will utilize the shuttle Columbia for a 13-day flight devoted to science, will include experiments designed by our Yale acoustics group to be performed in a dedicated acoustic positioning apparatus called the drop physics model (DPM). The goals of these experiments are: (1) to determine the rheological properties of liquid drops in the presence or absence of surface active materials by exciting single drops into their quadrupole resonance and observing their free decay; and (2) to investigate the mechanisms for coalescence of droplets with and without surfactants using a variety of techniques for perturbation of the interface between the drops. Results from current ground-based experiments and theoretical modeling performed to support the microgravity effort will be presented. Additionally, the flight experiments’ design, implementation, and execution will be discussed, as well as ideas for possible future experiments that would exploit the unique environment that the DPM provides. [Work supported by NASA through JPL Contract 958722.]