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surfaces. The splash threshold concerned here is for drop impact on metal surfaces,
and therefore only for the corona and prompt splash.
Figure 2.8.: Outcomes of the drop impact on solid surfaces, reported by Rioboo et
al. [114].
The distinct morphologies of the corona splash and the prompt splash indicate
different underlying mechanisms for the two types of splash. Therefore the splash
threshold, which is a theoretical model that predicts the inception of the splash,
should comprise two modes which are responsible for each phenomenon specifically.
It is therefore of critical importance to answer the question, what creates the
corona splash, and what induces the prompt splash.
Xu et al. observed the corona splash created by φ3.4 mm ethanol drop impacting
on a dry smooth surface at various surrounding gas pressures [168]. By decreasing
the gas pressure, the corona splash was weakened effectively. This fact suggests
that the existence of the surrounding gas plays an indispensable role on the
corona splash. This finding is further supported by Mishra et al. who conducted
drop impacts under gas pressures up to 12 bar [94]. Although no corona splash
was observed because of the limited range of the impact velocity, it was identified
that higher pressures enhanced the undulations at the periphery of the spreading
lamella.
2.2. Drop Impact on Dry Surfaces 19