Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

20010024921 Stanford Univ., Dept. of Chemical Engineering, Stanford, CA USA
Stability of Thermocapillary Return Flows Under Vertical Gravity Modulation
Suresh, V. A., Stanford Univ., USA; Homsy, G. M., Stanford Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics
and Transport Phenomena Conference; December 2000, pp. 747-769; In English; See also 20010024890; No Copyright; Avail:
CASI; A03, Hardcopy; A10, Microfiche
G-jitter convection is of general interest to NASA, and the time-dependence of the body force opens the possibility of new
physical effects such as resonance, streaming, and parametric instability. The hydrodynamic stability of fluids in a non-uniform
temperature field subject to constant body forces has been extensively studied in the literature, Rayleigh-Benard convection being
the classical example. Later work has shown that time dependent acceleration can significantly affect the stability of such flows.
This work considers the effect of the interaction between g-jitter and thermocapillarity in determining the stability characteristics
of a model flow.
Author (revised)
Thermocapillary Migration; Capillary Flow; Gravitation; Modulation; Temperature Distribution
20010024922 City Coll. of the City Univ. of New York, Dept. of Physics, NY USA
Molecular Dynamics of Fluid-Solid Systems
Koplik, Joel, City Coll. of the City Univ. of New York, USA; Banavar, Jayanath, Pennsylvania State Univ., USA; Proceedings
of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp. 770-799; In English; See also
20010024890; No Copyright; Avail: CASI; A03, Hardcopy; A10, Microfiche
We will discuss the results of molecular dynamics (MD) simulations of polymer melts. The idea is to specify the interactions
between the individual atoms or monomers of a polymer chain in the form of classical two-body potentials, and integrate Newton’s
equations of motion. These simulations intrinsically study small regions of fluid over microscopic time intervals, usually in the
tens of nanometers and nanoseconds, but this often suffices to exhibit continuum behavior. A further limitation here is the restriction
to relatively short molecules, lengths up to 100 or so monomers, but fortunately this exceeds the estimated entanglement
length. The specific computational polymers studied here are freely jointed FENE chains, whose properties have been determined
by previous simulations. The ”canonical” rheological properties (shear-thinning etc.) have been measured by Hess, Loose and
collaborators, while ”chemical physics” properties (entanglement, reptation, etc.) have been studied by Binder, Grest, Kremer et
al., as well as other groups. The approach taken in this work is the simulation of complete micro-experiments of fluid mechanical
phenomena: putting microscopic regions of fluid into motion with appropriate forces, taking full account of bounding surfaces,
temperature control and so on. We employ standard molecular dynamics simulation techniques which have been successfully
applied to Newtonian fluid flows for some time, and lately polymeric fluid flow as well. In addition to the binding FENE force,
any two monomers interact with the familiar cut-off Lennard-Jones potential. Bounding surfaces consist of solid atoms attached
to lattice sites by a confining potential and interacting with the fluid through a Lennard-Jones potential. A useful generic configuration
is that of a liquid drop on a solid substrate, a starting point for several calculations. Since the molecular configurations are
crucial, it is important to initialize the system so as to avoid any bias. We begin with FENE chains alone, whose monomers are
placed on adjoining regular lattice sites, and randomize them by cooking. The chains are confined to a box with impermeable
boundaries (using a simple repulsive force field) at low density and a high temperature. After all spatial distribution functions have
become isotropic, the fluid is cooled by kinetic energy rescaling and squeezed by a central force field to produce a constrained
spherical drop, and then the force field is gradually removed. The resulting drop is stable if the previous operations are slow
enough. The solid is separately equilibrated, and the drop is placed near the solid and allowed to interact with it. If the Lennard-
Jones attractive term is 0.8 times the standard value, the liquid is partially wetting, and forms a drop (for chains of lengths 2-100,
at least). Higher values, 1.0 or more, will give complete wetting, where the drop will spread until it runs out of molecules. One
easy application of this configuration is to solidify the drop by cooling the substrate. If the temperature of the solid atoms is reduced
the liquid above will attempt to come to thermal equilibrium by losing heat to the substrate, and eventually solidifying. The interest
in this process stems from the recent suggestion that if the liquid completely wets its own solid thermal state will be a spherical
cap (neglecting gravity), but if instead it partially wets there will be a small dimple on top. In fact, for these interactions and chains
of length 2, 10, 30 and 100, there is no dimple, and as the drop cools it conducts heat sufficiently well as to produce only weak
gradients in temperature and mean-square atomic displacements internally. We are now exploring other choices of interaction.
A more striking simulation which can be readily carried out from the same initial state is coalescence. The drop plus substrate
system, i.e., the atomic positions and velocities, are duplicated and flipped over to give two drops opposing each other. In this case
the drops are initially separated by a distance just less than the interaction range, so that a few nearby monomers in the separate
drops are mutually attracted, and pull their respective molecules along with them. These molecules in turn pull their neighbors
along, and the two drops smoothly coalesce. The process is qualitatively the same for molecules of any length, even if surrounded
by a second fluid. A third application of this technique concerns liquid bridge dynamics. to study the elongation properties of non-
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