la0c01689_si_005.mp4 (186.59 kB)
Substrate Wettability Influences Internal Jet Formation and Mixing during Droplet Coalescence
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posted on 2020-08-06, 22:21 authored by Thomas C. Sykes, David Harbottle, Zinedine Khatir, Harvey M. Thompson, Mark C. T. WilsonThe internal dynamics
during the axisymmetric coalescence of an
initially static free droplet and a sessile droplet of the same fluid
are studied using both laboratory experiments and numerical simulations.
A high-speed camera captured internal flows from the side, visualized
by adding a dye to the free droplet. The numerical simulations employ
the volume of fluid method, with the Kistler dynamic contact angle
model to capture substrate wettability, quantitatively validated against
the image-processed experiments. It is shown that an internal jet
can be formed when capillary waves reflected from the contact line
create a small tip with high curvature on top of the coalesced droplet
that propels fluid toward the substrate. Jet formation is found to
depend on the substrate wettability, which influences capillary wave
reflection; the importance of the advancing contact angle subordinated
to that of the receding contact angle. It is systematically shown
via regime maps that jet formation is enhanced by increasing the receding
contact angle and by decreasing the droplet viscosity. Jets are seen
at volume ratios very different from those accepted for free droplets,
showing that a substrate with appropriate wettability can improve
the efficiency of fluid mixing.