Modeling of the Fluid Merging Viscosity Measurement (FMVM) International Space Station experiment with COMSOL MultiPhysics

Abstract

‡The purpose of FMVM was to measure the rate of coalescence of two highly viscous liquid drops and correlate the results with the liquid viscosity and surface tension. The experiment takes advantage of the low gravitational force free floating conditions in space allowing the unconstrained coalescence of two nearly spherical drops. The merging of the drops is accomplished by deploying them from a syringe and suspension on Nomex threads. An astronaut’s slow manipulation of one of the drops toward a stationary droplet till there is contact initiates the droplet coalescence. Coalescence and merging occurs due to shape relaxation and reduction of surface energy, being resisted by the viscous drag within the liquid. Experiments were conducted onboard the International Space Station in July of 2004 and subsequently in May of 2005. The coalescence was recorded on video and down-linked near real-time. When the coefficient of surface tension for the liquid is known, the increase in contact radius can be used to determine the coefficient of viscosity for that liquid. The viscosity is determined by fitting the time to achieve contact neck diameter equal to half of the initial droplet diameter. This time is compared with a relaxation time scaling coefficient to arrive at the liquid viscosity. Recent fluid dynamical numerical simulations with COMSOL MultiPhysics of the coalescence process will be presented. The results are important for a better understanding of the coalescence process. The experiment is also relevant to liquid phase sintering, free form in-situ fabrication, and as a potential new method for measuring the viscosity of viscous glass formers at low shear rates.