Exploration of Sweeping Effect: Droplet Coalescence Jumping of a Rolling and Static Droplet

The sweeping effect of merged droplets
plays a key role in enhancing
application performance due to the continuing coalescence caused by
the horizontal jumping velocity. Most studies focused on static droplet
coalescence jumping, while moving droplet coalescence is poorly understood.
In this work, we experimentally and numerically study the coalescence
of a rolling droplet and a static one. When the droplet radius ratio
is larger than 0.8, as the dimensionless initial velocity increases
and the vertical jumping velocity first decreases and then increases.
The critical dimensionless initial velocity Vc* corresponding
to the minimum vertical jumping velocity could be estimated as 0.9(rs2rm2). When the droplet radius ratio is smaller
than 0.8, the dimensionless initial velocity has a positive effect
on the vertical jumping velocity. The mechanism of the vertical jumping
velocity can be attributed to two parts: liquid bridge impact and
retraction of the merged droplet. The squeezing effect generated by
the initial velocity between the two droplets promotes the growth
of the liquid bridge and enhances the impact effect of the liquid
bridge but weakens the upward velocity accumulation caused by the
retraction of the merged droplets. However, different from the vertical
jumping velocity, the horizontal jumping velocity is approximately
proportional to the dimensionless initial velocity. The outcome of
our work elucidates a fundamental understanding of a rolling droplet
coalescing with a static one.