From Wang, Y. and Bourouiba, L. (2018) Non-isolated drop impacts. Journal of Fluid Mechanics. 835:24-44.

Abstract: Upon impact on a solid surface, a drop expands into a sheet, a corona, which can
rebound, stick or splash and fragment into secondary droplets. Previously, focus
has been placed on impacts of single drops on surfaces to understand their splash,
rebound or spreading. This is important for spraying, printing, and environmental
and health processes such as contamination by pathogen-bearing droplets. However,
sessile drops are ubiquitous on most surfaces and their interaction with the impacting
drop is largely unknown. We report on the regimes of interactions between an
impacting drop and a sessile drop. Combining experiments and theory, we derive
the existence conditions for the four regimes of drop–drop interaction identified, and
report that a subtle combination of geometry and momentum transfer determines a
critical impact force governing their physics. Crescent-moon fragmentation is most
efficient at producing and projecting secondary droplets, even when the impacting
drop Weber number would not allow for splash to occur on the surface considered
if the drop were isolated. We introduce a critical horizontal impact Weber number
Wec that governs the formation of a sheet from the sessile drop upon collision with
the expanding corona of the impacting drop. We also predict and validate important
properties of the crescent-moon fragmentation: the extension of its sheet base and the
ligaments surrounding its base. Finally, our results suggest a new paradigm: impacts
on most surfaces can make a splash of a new kind – a crescent-moon – for any
impact velocity when neighbouring sessile drops are present