| dc.description.abstract |
<p>Fiber-based coalescers are widely used to accumulate droplets from aerosols and
emulsions, where the accumulated droplets are typically removed by gravity or shear.
This thesis investigate self-propelled removal of droplets from a hydrophobic fiber,
where the surface energy released upon drop coalescence overcomes the drop-fiber adhesion
toward the spontaneous departure. The self-propelled removal occurs above a threshold
drop-to-fiber radius ratio, disrupting the power-law accumulation on a fibrous coalescer.
The departure velocity approaches the capillary-inertial one at large radius ratios.</p><p>In
experiments, the condensation process including self-propelled removal phenomenon
was captured on Teflon-coated fibers with radius of 13~$mu$m and 40~$mu$m. The power
law of condensation is obtained by plotting time and averaged radius of droplets condensed
on fibers in 2-dimensional (2D) image at that time. Then, to better understand the
mechanism resulting such self-propelled removal, droplet-pairs with equal size were
manipulated on different radius of fibers (on cones with slowly varied radius). To
simplify analysis, two droplets were aligned so that their center connection line
was perpendicular to the axis of fiber. By using two high-speed cameras, two views
of this removal process were captured simultaneously. Based on information obtained
in those video-pairs, the velocity immediate after removal and drop-to-fiber radius
ratio were extracted for every case. In plotting those velocity against the radius
ratio, data-sets of different size of fibers were collapsed on a single curved, implying
critical radius-ratio (at which the removal starts) and asymptotic removal velocity
(when radius ratio is very large). In understanding the fluid field in this dynamic
process, a 2D phase-field simulation were qualitatively compared with experimental
observation, which help to explain why such self-propelled removal can happen on highly
curved hydrophobic surface (micro-fiber), but not on flat hydrophobic surface. Understanding
to this phenomenon can be useful in chemical industry, ventilation system, and oil
separation, in all of which fibrous beds are used to separate aerosol from immiscible
flow.</p>
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