Author:
MACHU GUNTHER,MEILE WALTER,NITSCHE LUDWIG C.,SCHAFLINGER UWE
Abstract
The motion and shape evolution of viscous drops made from a dilute suspension of
tiny, spherical glass beads sedimenting in an otherwise quiescent liquid is investigated
both experimentally and theoretically for conditions of low Reynolds number. In
the (presumed) absence of any significant interfacial tension, the Bond number
[Bscr ] = (Δρ)gR2/σ is effectively infinite.
The key stages of deformation of single drops and
pairs of interacting drops are identified. Of particular interest are (i) the coalescence of
two trailing drops, (ii) the subsequent formation of a torus, and (iii) the breakup of the
torus into two or more droplets in a repeating cascade. To overcome limitations of the
boundary-integral method in tracking highly deformed interfaces and coalescing and
dividing drops, we develop a formal analogy between drops of homogeneous liquid
and a dilute, uniformly distributed swarm of sedimenting particles, for which only
the 1/r far-field hydrodynamic interactions are important. Simple, robust numerical
simulations using only swarms of Stokeslets reproduce the main phenomena observed
in the classical experiments and in our flow-visualization studies. Detailed particle
image velocimetry (PIV) for axisymmetric configurations enable a mechanistic analysis
and confirm the theoretical results. We expose the crucial importance of the initial
condition – why a single spherical drop does not deform substantially, but a pair
of spherical drops, or a bell-shaped drop similar to what is actually formed in the
laboratory, does undergo the torus/breakup transformation. The extreme sensitivity
of the streamlines to the shape of the ring-like swarm explains why the ring that
initially forms in the experiments does not behave like the slender open torus analysed
asymptotically by Kojima, Hinch & Acrivos (1984). Essentially all of the phenomena
described above can be explained within the realm of Stokes flow, without resort to
interfacial tension or inertial effects.
Publisher
Cambridge University Press (CUP)
Subject
Mechanical Engineering,Mechanics of Materials,Condensed Matter Physics
Cited by
92 articles.
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