Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

toward one another causes a macroscopic helixing at low T and causes thick regions to transiently appear at high T, a 1D analog
of island formation on a rapidly compressed film.
Author (revised)
Ferroelectric Materials; Chirality; Liquid Crystals; Ferroelectricity
20010024903 Colorado Univ., Dept. of Chemical Engineering, Boulder, CO USA
Thermocapillary-Induced Phase Separation with Coalescence
Davis, R., Colorado Univ., USA; Rother, M., Colorado Univ., USA; Zinchenko, A., Colorado Univ., USA; Proceedings of the
Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp. 341-357; In English; See also
20010024890; No Copyright; Avail: CASI; A03, Hardcopy; A10, Microfiche
Understanding the behavior of dispersions of one liquid immersed in a second, immiscible liquid is important in both traditional
engineering applications, such as separations and molten materials processing, as well as more fundamental problems in
the general sciences. This work examines the interactions of two drops under conditions that viscous forces dominate inertia,
focusing on the role of interfacial deformation. The pairwise information may then be used in the study of dilute dispersions, where
the probability of three-body interactions is low. Two regimes of deformation are considered: small deformation, where the drops
remain spherical except for a small flattening or dimpling between the drops, and moderate or large deformation, where the interfaces
of both drops distort globally. The effects of deformation are significant, because small deformation inhibits drop coalescence,
while global deformation promotes alignment of the drops, which may lead to coalescence, break-up of the smaller drop,
or even more complicated coalescence-breakup phenomena. This paper is focused on the interaction of two drops of different size
which experience buoyancy and/or thermocapillary relative motion. Small deformations are considered first, followed by moderate
and large deformations. Using methodology from matched asymptotic expansions and a local boundary-integral approach,
coupling the lubrication flow in the gap to the internal flow within the drops, the critical horizontal offset demarcating trajectories
which lead to coalescence or separation of the drops with small deformations is found. The resulting collision efficiencies for
slightly deformable (solid curves) and spherical (dashed curves) drops of ethyl salicylate (ES) in diethylene glycol (DEG) in gravitational
motion is shown. The collision efficiency of slightly deformable drops is approximately the same for spherical drops
until a particular value of the average radius at which the collision efficiencies for spherical and slightly deformed drops rapidly
diverge. With a further increase in the average radius, the collision efficiency for slightly deformed drops quickly approaches zero,
as the flattening and dimpling in the near-contact region slows the film drainage and reduces the coalescence rate. Population
dynamics simulations of droplet growth due to coalescence are shown for the same ES/DEG system, but now in thermocapillarydriven
motion, at volume fraction phi(sub 0) = 0.05. Although the two distributions for deformable drops in frames a and b begin
with different initial conditions, they become nearly indistinguishable later as a result of the retardation of coalescence by small
deformations. Coalescence may also be inhibited by appropriated anti-parallel alignment of the gravity vector and temperature
gradient. Turning to larger deformations, computational results have been complemented by experiments with glycerol/water
drops undergoing gravitational motion in castor oil. A particularly interesting phenomenon observed in the experiments is cyclic
capture-breakup, in which the head of a smaller drop does not coalesce with a larger drop after breaking. Instead, the head of the
smaller drop passes through the larger drop, moves back around it and is again captured and breaks. A typical cyclic process, which
we have called ’suckthrough,’ for viscosity ratio unity is shown.
Author (revised)
Capillary Flow; Deformation; Drops (Liquids); Thermocapillary Migration; Phase Separation (Materials); Coalescing
20010024904 Illinois Univ., Dept. of Mechanical Engineering, Chicago, IL USA
Fluid Dynamics and Solidification of Molten Solder Droplets Impacting on a Substrate in Microgravity
Megaridis, C. M., Illinois Univ., USA; Poulikakos, D., Swiss Federal Inst. of Technology, Switzerland; Boomsma, K., Illinois
Univ., USA; Nayagam, V., National Center for Microgravity Research on Fluids and Combusiton, USA; Proceedings of the Fifth
Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp. 358-371; In English; See also
20010024890; No Copyright; Avail: CASI; A03, Hardcopy; A10, Microfiche
This research program investigates the fluid dynamics and simultaneous solidification of molten Sn/Pb solder droplets
impacting on flat, smooth, unyielding substrates. The problem of interest combines a fundamental investigation of fluid transport
and heat transfer, with the development of the novel technology of on-demand dispension (printing) of microscopic solder deposits
for the surface mounting of microelectronic devices. This technology, known as solder jetting, features on-demand deposition of
miniature solder droplets (30 to 120 microns in diameter) in very fine, very accurate patterns using techniques analogous to those
developed for the ink-jet printing industry. After ejection, the molten metal droplets collide, spread, recoil and eventually solidify
on the substrate. This solder application technology has shown great promise in microelectronic packaging and assembly, therefore,
the development of a good understanding of the pertinent fundamental fluid dynamics and solidification phenomena is essen-
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